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LECTURE NOTES
Course : MIB 404; PLANT PATHOLOGY (3 Credits /Compulsory)
Course Duration : 30hrs Teaching and 45hrs Practical
Lecturer: BELLO, Omolaran Bashir
Ph.D, M.Sc. (Ilorin), B.Sc. (Ibadan), OND (Computer Studies)
Course: PST 204 Plant Morphology (2 Credits /Compulsory)
Course Duration: 15hrs Teaching and 45hrs Practical
E-mail : [email protected],
obbello [email protected]
[email protected]
Office Location: Department of Biological Sciences
Consultation Hours; 2.30-4.00 pm Monday-Thursdays.
1. HISTORY OF PLANT DISEASE
Plant disease is any condition in a plant caused by living and non-living agents that interferes
with its normal growth and development. Diseases or plant health problems can impact plants in
many ways since all parts of a plant can be affected including flowers, leaves, fruits, seeds,
stems, branches, growing tips, and roots. A plant disease is also abnormal growth and/or
dysfunction of a plant. Diseases are the result of some disturbance in the normal life process of
the plant. Many different factors can cause plant health problems. These factors can be divided
into two groups based on whether they are living or non-living. Non-living disease agents, often
called abiotic agents, include factors such as environmental stress or cultural care. Living disease
agents, called biotic agents or plant pathogens, include microorganisms such as fungi and
bacteria.
How diseases occur
In order for disease to occur, three factors must be present. Because of this, disease is often
pictured as a triangle having three equal sides. Each side of the triangle is necessary in order for
disease to occur. One side of the triangle represents the Host Plant, the second side represents the
Causal Agent or Factor, and the third side represents the Environmental Conditions that are
necessary in order for the other two sides to interact (Figure 1). When one or more sides of the
triangle are missing, the triangle collapses and disease will not occur (Figure 2). For example,
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Scab of Crabapple is a very common disease in the Connecticut landscape. The “disease
triangle” for this disease consists of the host plant, a susceptible variety of crabapple, the causal
factor, the fungus Venturia inaequalis, and the proper environment, typically a cool, wet spring
during which the young, emerging leaves stay wet for extended periods of time. When all three
of these factors are present, scab will develop. If one component is missing, perhaps the spring
weather is hot and dry and not favorable for disease development, disease will not occur since
one side of the triangle is not present and the triangle collapses.
Figure 1. The Disease Triangle: All components are present and disease occurs.
Figure 2. The Disease Triangle: One of the components is missing (Causal Agent) so disease
does not occur.
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Diseases That Altered History
The history of plant pathology is divided into different five eras:
1. Ancient era: Ancient to 5th Century (476 A.D)
2. Dark era: 5th to 16th Century (476 A.D. to 1600)
3. Pre-modern era: 17th Century to 1853 (1600 to 1853)
4. Modern era: 1853 to 1906
5. Present era: 1906 onwards
1.
Ancient Era {Ancient to 5th Century (476 A.D)}: Diseases in plant have been known
since ancient times. Rust, blight, mildews, smuts, were familiar to Hebrews, Greeks, Romans,
Chinese and Indians. Plant disease was recorded in Vedas (Rugveda, Athavaeda) as early as
1200 B.C. Symptoms and control of disease have been mentioned in “VRIKSHAYURVED” by
Surapal in ancient India. Even mention of plant disease has been made in Buddhist literature of
500 B.C.
Theophrastus (300 B.C); a great botanist noted occurrence of crop disease and suggested some
remedies to control them. He also wrote about plant disease in this era.
Lord Pliny (100 A.D) described plant diseases and suggested some remedies. He believed that
disease originates from the plants or from the environment.
2.
Dark Era (476 A.D to 1600):
Plant pathology made very little progress during this era. Some Arabians like Ibnal-awan
described symptoms and control measures for some plant disease.
1440: Printing was introduced in Europe and this reflected interest in learning science.
i.
ii.
iii.
iv.
v.
vi.
vii.
3.
Pre-modern Era {18th century to 1853 (1600 to 1853)}:
Robert Hooke 1665: The father of cell theory. He had developed or invented first compound
microscope. He reported that plant tissues are made up minute units called as cells.
Anton Van Leeuwenhoek 1676: A Dutch worker from Holland. He invented first simple
microscope with home ground lenses between two metal plates. He described different types of
protozoa and bacteria as “Little animalcules”. All unicellular microorganisms (Protozoa , algae,
and bacteria) were firstly recorded by him.
P.A Micheli 1729: An Italian Botanist studied several fungi and described their morphology for
first time. He studied that fungi originates from spores. Father of Founder of Mycology.
John Needham 1743: Reported plant parasitic nematodes in wheat galls.
Carlous Linnaeus 1753: Established Latin Binomial system of Nomenclature of Plants and
animals in his book “Species plantarum”.
Tillet 1755: Proved that Bunt of wheat is contagious or infectious and can be controlled by seed
treatment.
Prevost 1807: A French Botanist suggested CuSO4 seed treatment for bunt of wheat.
This is known as autogenic or physiologic period, since plant diseases were distinctly
physiologic with tendency towards the mycology. At the end of the period it was clear that fungi
were very closely associated with diseases.
In 1845, late blight of potato appeared in Ireland, over one million people died and one and half
million got migrated and the history entered the next era.
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iv.
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vii.
viii.
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4.
Modern Era 1853 to 1906
This known as pathogenic period which was devoted the study of role of fungi causing plant
diseases.
Anton de Bary 1853: He proved that late blight of potato was caused by Phytophorn infestans.
Founder or father of plant pathology.
T.J. Burril 1873: American plant pathologist. He proved, Bacterial Nature of Fire Blight of
Apple and pear.
Robert Koch 1876: Bacterial nature of Anthrax disease in animals ( 1881) Gelatin is used as
solidifying agent in culture media which is replaced by Agar-Agar. He described the theory
called “KOCH’S POSTULATES”.
P.A. Millardet 1882-85: Use of Bordeaux mixture (CuSO4 + Lime) for control of Downey
Mildew of grapes.
Adolf Mayer 1886: Described TMV and proved that TMV should be transmitted from diseased
plant to healthy plants.
Jenson 1887: Hot water treatment for loose smut of wheat.
E.F Smith 1890: Father of phyto-bacteriology. He worked on bacterial wilt of cucurbits and
crown gall diseases.
Iwanowski 1892: Demonstrated that Tobacco Mosaic Virus (TMV) can pass through bacteria
proof filters and proved filterable nature of viruses.
Cragie 1827: Showed function of Puccinia in rust fungi.
Biffen 1905: Pioneers in Genetic of Plant diseases resistance.
5.
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iii.
iv.
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Present Era 1906 Onwards:
The present or current era commencing from 1906 has since remarkable discoveries.
J.C Luthra 1931: Solar heat treatment for loose smut of wheat.
W.M. Stanley 1935: He proved crystalline nature of virus. He got Nobel Prize.
F.C Bowden and Pierie 1936: Nuckeo-protenous nature of virus.
G.H. Flor 1955: Gene for gene theory hypothesis.
Doi and Asuyama: Discovered Mycoplasma like organism (MLO) responsible for yells type of
disease.
For the past 50 years, the ability to combat plant diseases through the use of modern farm
management methods, fertilization of crops, irrigation techniques, and pest control have made it
possible for the United States to produce enough food to feed its population and to have
surpluses for export. However, the use of pesticides, fungicides, herbicides, fertilizers and other
chemicals to control plant diseases and increase crop yields also poses significant environmental
risks. Air, water, and soil can become saturated with chemicals that can be harmful to human and
ecosystem health.
While early civilizations were well aware that plants were attacked by diseases, it was not until
the invention of the first microscope that people began to understand the real causes of these
diseases. There are references in the Bible to blights, blasts, and mildews. Aristotle wrote about
plant diseases in 350 BC and Theophrastus (372-287 BC) theorized about cereal and other plant
diseases. During the Middle Ages in Europe, ergot fungus infected grain and Shakespeare
mentions wheat mildew in one of his plays.
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After Anton von Leeuwenhoek constructed a microscope in 1683, he was able to view
organisms, including protozoa and bacteria, not visible to the naked eye. In the eighteenth
century, Duhumel de Monceau described a fungus disease and demonstrated that it could be
passed from plant to plant, but his discovery was largely ignored. About this same time,
nematodes were described by several English scientists and by 1755 the treatment of seeds to
prevent a wheat disease was known.
In the nineteenth century, Ireland suffered a devastating potato famine due to a fungus that
caused late blight of potatoes. At this time, scientists began to take a closer look at plant diseases.
Heinrich Anton DeBary, known as the father of modern plant pathology, published a book
identifying fungi as the cause of a variety of plant diseases. Until this time, it was commonly
believed that plant diseases arose spontaneously from decay and that the fungi were caused by
this spontaneously generated disease. DeBary supplanted this theory of spontaneously generated
diseases with the germ theory of disease. Throughout the rest of the nineteenth century scientists
working in many different countries, including Julian Gotthelf Kühn, Oscar Brefeld, Robert
Hartig, Thomas J. Burrill, Robert Koch, Louis Pasteur, R. J. Petri, Pierre Millardet, Erwin F.
Smith, Adolph Mayer, Dimitri Ivanovski, Martinus Beijerinck, and Hatsuzo Hashimoto, made
important discoveries about specific diseases that attacked targeted crops.
During the twentieth century advances were made in the study of nematodes. In 1935 W. M.
Stanley was awarded a Nobel Prize for his work with the tobacco mosaic virus. By 1939, virus
particles could be seen under the new electron microscope. In the 1940s fungicides were
developed and in the 1950s nematicides were produced. In the 1960s Japanese scientist Y. Doi
discovered mycoplasmas, organisms that resemble bacteria but lack a rigid cell wall, and in
1971, T. O. Diener discovered viroids, organisms smaller than viruses.
Plant diseases have impacted our daily lives in many ways. Yield losses are a direct effect of
plant diseases that farmers and producers face every year, and the effects of plant diseases
reaches far beyond this. A plant disease was said to have caused the deaths of thousands during
the Middle Ages and incited the Salem witch trials. It was also said to have contributed to the
French Revolution. An historical icon “Peter the Great halted in his efforts to capture a warm
water port at Constantinople (Istanbul) by the same plant disease.
Ergot of rye, caused by Claviceps purpurea (an ascomycete fungus), is the plant disease
interacting with humanity in the historical events mentioned above. C. purpurea infects the
ovary of rye (or other cereal grains) while the plant is blooming. The fungus colonizes the ovary
and begins to replace the plant tissue with hard black structures called Sclerotia (Figures 1 and
2.). In the field, some of the sclerotia drop to the ground as the grain ripens and function as a
winter survival mechanism for the fungus. During the following spring, sclerotia produce tiny
stalks called Stromata (Figure 3). In the stromata, perithecia form and produce ascospores (a type
of sexual spore produced in an ascus or sac). These ascospores are disseminated by wind, infect
flowers of new rye plants, and initiate the disease cycle again. However, the problems with ergot
for humans and animals begin when the grain is harvested for use in flour and feed. Ergot
sclerotia are unintentionally harvested along with the grain. If these sclerotia are not removed,
they will be ground into flour along with the grain. Toxic alkaloids contained in the sclerotia are
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distributed throughout the flour during grinding. Baking does not destroy the ergot alkaloids.
When people eat bread or baked goods made with flour containing ergot, they also consume the
alkaloids and can develop the symptoms of ergotism. Symptoms of ergotism include:
hallucinations, burning or crawling sensations under the skin, miscarriage, gangrene, loss of
limbs, and death. Animals are affected by ergotism when they consume grain or hay containing
ergot sclerotia.
Figure 1- Sclerotia of wheat
Figure 2- Sclerotia of rice
Figure 3- Stromata produced by Sclerotia
Today, ergot is controlled by crop rotation, sound management practices, grain standards in rye
and wheat, and milling standards. However, ergot and endophytic fungi related to ergot remain
disease problems that require careful management as localized outbreaks of ergotism (primarily
in livestock) continue to be recorded.
Plants in landscapes, gardens, production fields, forests, and interior scapes are subject to a wide
variety of problems that threaten their health. These problems can affect the aesthetics of the
plant or can pose more serious consequences which result in plant disfigurement, crop loss due to
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reductions in yield and quality, and plant death. Throughout time, plant diseases have had
profound effects on the history of human civilization and culture and plant health problems
continue to impact our daily lives. Plant diseases affect food, fiber, and ornamental plants as well
as those in natural areas.
Notable among these diseases is Potato Late Blight. This disease, caused by Phytophthora
infestans, was responsible for widespread epidemics throughout Ireland and Europe in the
1840’s. Many agricultural economies focus on a particular crop, so a single disease could be a
big threat—and a major historic force. This devastating disease not only resulted in famine, but
was also responsible for the emigration of 1.5 million people from Ireland to the United States
and Canada. Late blight continues to threaten potato production in many regions of the U.S. as
new strains of the fungus develop.
Another interesting but less dramatic example which illustrates the impact of a plant disease on
human culture is the disease Coffee Rust. This fungal disease, caused by Hemileia vastatrix,
appeared in coffee plantations in Ceylon (presently Sri Lanka) in the 1860’s and subsequently
destroyed the coffee industry in that nation. As a part of the British Empire, Ceylon was one of
the primary sources of coffee for England, a nation whose citizenry drank coffee as their favorite
boiled beverage. However, as a result of the coffee rust epidemic, tea was planted in its place and
coffee became very difficult to obtain. Thus, the English had to modify their drinking habits and
as a result of a plant disease, England became known as a nation of tea drinkers.
Plant diseases have also changed the composition of the forest and landscape. Until the early
1900’s, the American chestnut was one of the most dominant and important hardwood tree
species in the forests of the eastern United States. It was prized for its commercial value as a
source of lumber, pulpwood, poles, tannins, railroad ties, and edible nuts. With the introduction
of the fungus Cryphonectria parasitica, a species which was not native to the U.S., chestnut trees
became infected with the chestnut blight fungus and the tree was almost completely eliminated
from the forest. Today, sprouts continue to grow from old stumps although they usually succumb
to disease.
Dutch Elm disease is another example of a disease that changed our city and town streets and
greens. The fungus Ophiostoma ulmi, along with one of the insects that transmits it, were
introduced to the U.S. on logs imported from Europe. The American elm, Ulmus americana, was
highly susceptible to these exotic pests and quickly succumbed to infection. Since many of the
elm trees were planted in rows along city streets and parks, the fungus easily spread from tree to
tree through root grafts and feeding activities of the beetle.
In 1879, a new disease, downy mildew of grape, was introduced into Europe from the United
States, spread rapidly, and threatened to ruin the vineyards of Europe. A mixture of copper
sulfate and lime, used initially to deter pilferers, was discovered to control the disease. This
discovery became known as the “Bordeaux mixture” and stimulated the study of the nature and
control of plant diseases.
Because of the diversity of plant health problems and causal factors, it is important to learn to
recognize them, understand what causes them, and why and when they occur. It is also helpful to
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understand their importance or relative impact. This information is helpful in order to prevent the
problems from occurring or when they do occur, to properly manage them.
Plant Disease Control
1. Soil Management- Control of plant disease begins with good soil management. The best soil for
most plants is loamy, with good drainage and aeration. This minimizes diseases that attack the
roots and allows the roots to feed nutrients from the soil to the rest of the plant. Organic methods,
such as the addition of compost, can improve soil quality, and fertilizers can be added to the soil
to enrich the nutrient base. Soil pH measures the degree of acidity or alkalinity of the soil.
Gardeners and farmers must be aware of the pH needs of their plants, since the right pH balance
can help reduce susceptibility to disease, especially root diseases like club root or black root rot.
2. Disposal of infected plants- Plant diseases can be spread by seeds, and by transplants and
cuttings; careful attention to the presence of disease in seeds, transplants, and cuttings can avoid
the spread of pathogens.
3. Cleaning of Farm tools- This is important in the control of diseases. It involves the careful
maintenance of tools and equipment used in farming and gardening. Many plant diseases can
easily be spread by hand and by contact with infected tools, as well as by wind, rain, and soil
contamination.
4. Crop Rotation- Crop rotation is an important part of reducing plant diseases. Pathogens that
favour a specific crop are deprived of their preferred host when crops are rotated. This reduces
the virulence of the pathogen and is a natural way to reduce plant disease.
5. Other important factors in the control of plant disease are the selection of disease-resistant
plants (cultivars), proper watering, and protection of plants from extreme weather conditions.
Reading lists
The Epidemiology of Plant Diseases, edited by B.M. Cooke, D.G. Jones, and B. Kaye. New
York: Springer, 2006.
Lucas, G.B., C L. Campbell, and L.T. Lucas. Introduction to Plant Diseases. Westport, CT: AVI
Publishing, 1985.
Rappaport, Rosalynn. Controlling Crop Pests and Diseases. Warwickshire, U.K.: Practical
Action Publishing, 2005.
Strange, Richard N. Introduction to Plant Pathology. New York: John Wiley & Sons, 2003.
2. CLASSIFICATION OF PLANT DISEASES; (BACTERIA, FUNGI, VIRUSES,
NEMATODES)
I.
II.
III.
IV.
Types of Plant Diseases
Plant diseases are classified into various classes;
Diseases caused by fungi
Diseases caused by bacteria
Diseases caused by viruses
Diseases caused by nematodes.
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Classification of Disease Caused by Plant Pathogenic Fungi
Generally, fungi are organisms that are classified in the Kingdom “Fungi”. They lack chlorophyll
and conductive tissue. Until a few years ago, fungi were considered lower forms of plants, but
today are classified as a group by themselves. Because fungi cannot manufacture their own food,
(due to lack of chlorophyll) they must obtain it from another source as either a saprophyte or
parasite.
1.
Diseases caused by plant pathogenic fungi are of various types which belong to various
classes are;
I.
II.
III.
I.
II.
III.
a) Class Ascomycetes; under this class, there are several species that causes plant disease are;
Fusarium specie (causal agent of Fusarium wilt disease)
Thielaviopsis specie(causal agent of :canker rot, black root rot, thielaviopsis)
Verticillium species
IV.
Magnaporthe grisea (causal agent of blast rice and gray leaf spot in turf grasses).
b) Class Basidiomycetes; under this class, there are several species that causes plant
disease are:
Rhizoctonia specie
Phakospora pachyrhizi (causal agents of soybean rust)
Puccina specie (causal agents of severe rusts of virtually all cereal grains and cultivated grasses)
Diseased caused by flagellated fungi and fungal-like protozoa:
a) Plasmodiophoromycetes (currently in the kingdom protoctista): This class contains what many
interpret as entophytic slime molds, because they produce amoeboid cells and plasmodia within
the cells of their host.
b) Chytridiomycota: Several different groups of chytrids parasitize plants. Olpidium spp. infects
pollen, algae, other fungi, and several different groups of higher plants. Physoderma maydis
causes brown spot or streak of corn leaves and species of Synchytrium cause wart of potatoes. In
cool, moist climates, chytrids can cause extensive damage to crop plants.
c) Oomycetes: (currently in the Kingdom Stamenopila): There are four orders of Oomycetes, all
characterized by producing heterokont zoospores, (with one whiplash and one tinsel flagellum)
asexually and oospores sexually. The most economically important group of Oomycetes is the
Peronosporales that contain the late blight of potato fungus Phytophthora infestans and relatives
such as Peronospora, Bremia, Plasmopara and others that cause “downy mildews”, the
“damping off” fungi, Pythium spp., and the white rust fungi, Albugo spp
Disease caused by non flagellated fungi:
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a) Zygomycota: The most important order of Zygomycetes that cause diseases of plants and decay
of plant products is the Mucorales. Members of the bread mold genus Rhizopus causes soft rots
of vegetables and fruits. Species of a related fungus, Choanephora, causes blossom blight and
decay of squash and similar vegetables.
b) Ascomycota: This is one of the largest and most complex groups of fungi, there are various
groups of Ascomycetes recognized by the way they produce their asci.
c) Deuteromycetes: This is an artificial group of fungi that is made up of the conidial (asexual)
states of various fungi, but largely Ascomycetes. Species of Alternaria, Bipolaris, Botrytis,
Cercospora, Diplodia, Dreschlera, Exerohilum, Fusarium, Phoma, Phomopsis, Rhizoctonia, and
Verticillium are among the most common groups that cause molds, blights, cankers, leaf spots,
root rots and other maladies.
d) Basidiomycetes: Among plant diseases caused by Basidiomycetes rusts, smuts, felt fungi, root
rots, heart rots, and thread-blights.
Various infections on plants caused by fungi
a)
A.
S
hows fungal infection on maize plant.
B.
S
hows the gradual process on fungal infection on a leaf.
Classification of diseases caused by plant pathogenic virus:
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Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate
intracellular parasites that do not have the molecular machinery to replicate without a host. Plant
viruses are pathogenic to higher plants.
There are many types of plant virus, and some are even asymptomatic. Under normal
circumstances, plant viruses cause only a loss of crop yield. Therefore, it is not economically
viable to try to control them, the exception being when they infect perennial species, such as fruit
trees. Most plant viruses have small, single-stranded RNAgenomes. These genomes may encode
only three or four proteins: a replicase, a coat protein, a movement protein, in order to allow cell
to cell movement though plasmodesmata, and sometimes a protein that allows transmission by a
vector. Plant viruses must be transmitted from plant to
plant by a vector. This is often by an insect (for
example, aphids), but some fungi, nematodes, and
protozoa have been shown to be viral vectors.
Viral infection caused on fruits and plants

Classification of plant pathogenic bacteria
Bacteria are single-celled microorganisms, generally ranging from 1-2 µm in size that cannot be
seen with the unaided eye. Plant associated bacteria may be beneficial or detrimental. All plant
surfaces have microbes on them (termed epiphytes), and some microbes live inside plants
(termed Endophytes). Plant pathogenic bacteria cause many serious diseases of plants throughout
the world, but fewer than fungi or viruses, and they cause relatively less damage and economic
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cost. Most plants, both economic and wild, have innate immunity or resistance to many
pathogens. However, many plants can harbor plant pathogens without symptom development
(asymptomatic).
Most bacteria that are associated with plants are actually saprotrophic, and do no harm to the
plant itself. However, a small number, around 100 known species, are able to cause disease.
Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world.
Most plant pathogenic bacteria are rod-shaped (bacilli). In order to be able to colonize the plant
they have specific pathogenicity factors. Five main types of bacterial pathogenicity factors are
known:
1. Cell wall-degrading enzymes: These are used to break down the plant cell wall in order to
release the nutrients inside. Used by pathogens such as Erwinia, to cause soft rot.
2. Toxins: These can be non-host-specific, which damage all plants, or host-specific, which
cause damage only on a host plant.
3. Effector proteins: These can be secreted into the extracellular environment or directly into
the host cell, often via the Type three secretion systems. Some effectors are known to suppress
host defense processes.
4. Phytohormones: For example, Agrobacterium changes the level of auxins to cause tumours.
5. Exopolysaccharides: These are produced by bacteria and block xylem vessels, often leading
to the death of the plant.
The significant plant pathogenic bacteria
1. Burkholderia:
Burkholderia is a genus of proteobacteria probably best known for its pathogenic members:
Burkholderia mallei, responsible for glanders, a disease that occurs mostly in horses and related
animals; Burkholderia pseudomallei, causative agent of melioidosis; and Burkholderia cepacia,
an important pathogen of pulmonary infections in people with cystic fibrosis (CF). The
Burkholderia (previously part of Pseudomonas) genus name refers to a group of virtually
ubiquitous gram-negative, motile, obligately aerobic rod-shaped bacteria including
animal/human (see above) and plant pathogens as well as some environmentally important
species. In particular, B. xenovorans (previously named Pseudomonas cepacia then B. cepacia
and B. fungorum) is renowned for their ability to degrade chlororganic pesticides and
polychlorinated biphenyls (PCBs). The use of Burkholderia species for agricultural purposes
(such as biodegradation, biocontrol and as plant-growth-promoting rhizobacteria) is subject to
discussions because of possible pathogenic effects in immuno-compromised people (especially
CF-sufferers), e.g., hospital acquired infections. Due to their antibiotic resistance and the high
mortality rate from their associated diseases Burkholderia mallei and Burkholderia pseudomallei
are considered to be potential biological warfare agents, targeting livestock and humans.
The genus was named after Walter H. Burkholder, plant pathologist at Cornell University.
I.
Xanthomonas:
Xanthomonas is a genus of Proteobacteria, many of which cause plant diseases. Xanthomonas
can infect a wide variety of species including pepper, rice, citrus, cotton, tomato, and soybeans.
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Some types of Xanthomonas cause localized leaf spot or leaf streak while others spread
systemically and cause black rot or leaf blight disease. They inject a number of effector proteins,
including TAL effectors, into the plant via their type iii secretion system.
II.
Proteobacteria:
The Proteobacteria are a major group (phylum) of bacteria. They include a wide variety of
pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable
genera. Others are free-living, and include many of the bacteria responsible for nitrogen fixation.
In 1987, Carl Woese established this grouping, calling it informally the "purple bacteria and their
relatives". Because of the great diversity of forms found in this group, the Proteobacteria are
named after Proteus, a Greek god of the sea, capable of assuming many different shapes, and it is
therefore not named after the genus Proteus.
All proteobacteria are Gram-negative, with an outer membrane mainly composed of
lipopolysaccharides. Many move about using flagella, but some are nonmotile or rely on
bacterial gliding. The last include the myxobacteria, a unique group of bacteria that can
aggregate to form multicellular fruiting bodies. There is also a wide variety in the types of
metabolism. Most members are facultatively or obligately anaerobic, chemoautotrophs, and
heterotrophic, but there are numerous exceptions. A variety of genera, which are not closely
related to each other, convert energy from light through photosynthesis. These are called purple
bacteria, referring to their mostly reddish pigmentation.
III.
Pseudomonas:
Pseudomonas is a genus of gammaproteobacteria, belonging to the family Pseudomonadaceae
containing 191 validly described species. P. syringae is a prolific plant pathogen. It exists as over
50 different pathovars, many of which demonstrate a high degree of host plant specificity. There
are numerous other Pseudomonas species that can act as plant pathogens, notably all of the other
members of the P. syringae subgroup, but P. syringae is the most widespread and best studied.
Although not strictly a plant pathogen, P. tolaasii can be a major agricultural problem, as it can
cause bacterial blotch of cultivated mushrooms. Similarly, P. agarici can cause drippy gill in
cultivated mushrooms.
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14
a) Shows cycles of bacterial infection b) bacterial in pops c) infection on watermelon
Control of Plant Diseases
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Breeding practices have been perfected over centuries, but with the advent of genetic
manipulation even finer control of a crop's immunity traits is possible. The engineering of food
plants may be less rewarding, however, as higher output is frequently offset by popular suspicion
and negative opinion about this "tampering" with nature.
Chemical Control of Plant Diseases
Many natural and synthetic compounds that could be employed to combat the above threats
exist. This method works by directly eliminating disease-causing organisms or curbing their
spread; however, it has been shown to have too broad an effect, typically, to be good for the local
ecosystem. From an economic standpoint, all but the simplest natural additives may disqualify a
product from "organic" status, potentially reducing the value of the yield.
Biological Control of Plant Diseases
Crop rotation may be an effective means to prevent a parasitic population from becoming wellestablished, as an organism affecting leaves would be starved when the leafy crop is replaced by
a tuberous type, etc. Other means to undermine parasites without attacking them directly may
exist.
Integrated Control of Plant Diseases
The use of two or more of these methods in combination offers a higher chance of effectiveness.
Reading Lists
i. Jackson RW (ed). (2009). Plant Pathogenic Bacteria: Genomics and Molecular Biology.
Caister Academic Press.
ii. Bird, A.F. and J. Bird. 1991. The Structure of Nematodes, Second edition. Academic Press
Inc. London.
iii. Plant pathogenic diseases Wikipedia.
4. BASIC PROCEDURES IN DIAGNOSIS OF PLANT DISEASES
Rapid and accurate diagnosis of disease is necessary before proper control measures can be
suggested. It is the first step in the study of any disease. Diagnosis is largely based on
characteristic symptoms expressed by the diseased plant. Identification of the pathogen is also
essential to diagnosis. Three steps involved in diagnosis include careful observation and
classification of the facts, evaluation of the facts, and a logical decision as to the cause.
Diagnosis is best done in the presence of the growing plant. Disease is suspected when, for
example, part or all of a plant begins to die. Disease also is indicated when blossoms, leaves,
stems, roots, or other plant parts appear abnormal—i.e., misshapen, curled, discoloured,
overdeveloped, or underdeveloped. Examination of leaves is usually considered to be the best
starting point in diagnosis. The colour, size, shape, and margins of spots and blights (lesions) are
often associated with a particular fungus or bacterium. Many fungi produce “signs” of disease,
such as mold growth or fruiting bodies that appear as dark specks in the dead area.
Early stages of bacterial infections that develop on leaves or fruits during humid weather often
appear as dark and water-soaked spots with a distinct margin and sometimes a halo—a lightercoloured ring around the spot. Examination of stems, shoots, branches, and trunk follows a
16
thorough leaf examination. Sunken, swollen, or discoloured areas in the fleshy stem or bark may
indicate canker infection by a fungus or bacterium or injury caused by excessively high or low
temperatures, hail, tools, equipment, vehicles, or girdling wires. To diagnose a plant disease, it is
necessary to first determine whether the diseases are caused by pathogens or environmental
factors.
The pathogens that cause plant diseases include Bacteria, Fungi, Nematode, Mollicutes
Plant disease caused by bacteria
A. Bacterial leaf spots or blights
Bacterial diseases caused by Xanthomonas campestris pv. dieffenbachiae (formerly called X.
dieffenbachiae), X. c. pv. syngonii, Pseudomonas cichorii, Erwinia carotovora subsp. carotovora
and E. chrysanthemi are all important pathogens of these foliage plants. These bacterial diseases
may destroy leaves, petioles and stems rendering infected plants unsightly and unsalable.
I.
Xanthomonas Leaf Spots or Blight
Xanthomonas leaf spot or blight on dieffenbachias initially appears as yellowish, translucent or
water-soaked specks usually first at the margins, later anywhere on the leaf blade except the
midrib. The lesions enlarge to about half an inch (1.25 centimetres) and turn tan, yellow, orangeyellow, or reddish. The center is a dull watery-green, surrounded by an orange-brown border.
Sometimes the roundish-to-elongate lesions are restricted to the leaf margins when infections
occur through hydathodes. Older lesions are irregular and distinctly yellow with a reddish center.
The lesions may coalesce to destroy the entire leaf. When the air is very dry, the lesions remain
as small, dry, reddish brown specks. Under moist air conditions (high relative humidity) the
spots enlarge and merge to cover rather large irregular areas that soon turn yellow, wilt, and dry.
If the lesions are numerous, affected leaves finally turn yellow, wilt, and die. Dead leaves are a
dull tan to light brown and tough, but not brittle. All parts of the leaf blade except the midrib are
susceptible.
A bacteria ooze usually appears on the lesions on the lower leaf surface. When the lesions are
wet, the mounds of exudates are slimy; when dry, the exudates is thin, waxy, silvery-white, and
easily peeled off the leaf.
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Xanthomonas leaf spot or blight of dieffenbachia
Red edge of heartleaf philodendron caused
by Xanthomonas campestris pv dieffenbachiae
Diagnosis
Xanthomonas leaf spot on the heart-leaf philodendron (Philodendron oxycardium formerly P.
cordatum) is called “red edge” due to the conspicuous coloration of most infections which are
confined to the leaf margins. Under warm, moist conditions large areas of the leaf blade can
become infected, but lesions are generally confined to interveinal areas. On anthuriums small,
irregular, water-soaked, translucent spots are first seen at the leaf margins following infections
through hydathodes. Lesions resulting from foliar infections are usually restricted to the leaf
margins although large areas may be adjacent tissue is chlorotic. The bacterium can become
systemic in anthuriums causing a general chlorosis of older leaves. The petioles often break off
revealing a tan to brown discoloration of the vascular system. Systemic infections can also result
in leaf spots which form anywhere on the leaf blades.
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Disease Cycle
The Xanthomonas bacteria enter a leaf through hydathodes on the leaf margins, stomates on the
lower surface, or wounds. Symptoms appear 7 to 18 days after infection. The newly developing
leaves on an infected plant do not necessarily become infected, since the bacteria are rather slowgrowing and are favoured by warm (70° to 90°F or 21° to 32°C), moist conditions. The
organisms are easily spread from leaf to leaf and plant to plant by splashing water, contaminated
tools, insects, handling infected plants, and by propagating infected plants. The bacteria can even
be in tissue-cultured plants. Other ornamental plants infected by Xanthomonas campestris pv.
dieffenbachiae include Chinese evergreen, Silver Queen and other Aglaonema species, fancyleaved caladiums (Caladium spp.), and cocoyam (Xanthosoma caracu).
Erwinia Leaf Spot, Blight, Stem Rot, and Soft Rot
The most common bacterial pathogens of these four foliage plants are Erwinia carotovora subsp.
carotovora and subspecies of E. chrysanthemi. Both organisms cause soft rots or leaf spots and
blight. The first symptom is usually soft, mushy, watersoaked areas at the base of the stem at or
below the soil line. These lesions are gray to tan or pale brown, irregular in shape, with a distinct
line separating diseased and healthy tissue. Stem and cane lesions below the soil surface usually
go unnoticed until infection causes severe decay. Diseased plants commonly produce terminal
leaves that are pale yellow and small. Under warm, moist conditions the stem lesions develop
rapidly, causing the lower leaves to turn yellow, become soft and mushy, collapse, and die
prematurely. Stem rot of cuttings is common; the lower leaves wilt and turn yellow and an entire
cutting may turn into a rotted mass. The stem rot phase may develop so rapidly that entire plants
or cuttings die before the bacterium has advanced into the leaves. Diseased tissue commonly has
a foul, dead fish odor due to secondary microorganisms colonizing dead tissue.
Erwinia stem rot on a cultivar of dieffenbachia maculate
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Erwinia leaf blight on Philodendron selloum.
Diagnosis
Leaf infections start as pinpoint spots that are water-soaked and yellow to pale brown. The
lesions are sometimes surrounded by a diffused yellow halo. When the humidity is high and
temperatures are warm to hot, the spots expand rapidly, becoming slimy, irregular, and sunken
with light tan centers, darker brown borders, diffused yellow margins, and may involve the entire
leaf in a few days. An invasion of the midrib and larger veins by the casual bacterium often
results in advancement into the petiole and stem. The result is yellowing and total collapse of the
leaf.
Disease Cycle
The two Erwinia species, like the Xanthomonas and Pseudomonas leaf spot and blight organism,
enter primarily through hydathodes and wounds. Disease development is favoured by the
presence of moisture and temperatures of 71° to 93°F (22° to 34°C). The bacteria are transmitted
from an infected to a healthy plant by splashing water, insects, and by contaminated knives,
tools, gloves, and carrying trays. The bacteria may invade the vascular (xylem) system and
become systemic, causing the leaves and stems to turn soft and rot (Figure 5). Plants of all ages
are susceptible to infection, with young leaves being more susceptible than older ones. Erwinia
chrysanthemi can survive in greenhouse potting media with or without a host plant for a year or
more and in the leaves of host or nonhost plants in a greenhouse for 5 to 6 months. The same
bacteria have been associated with bacterial soft rot diseases of a majority of foliage plants and
numerous flowering plants (Table 1). Many fleshy vegetables are also susceptible.
Bacterial Wilt
This is an example of a tomato disease.
Symptoms
Wilting first appears on the youngest leaves of plants during hot daytime temperatures. The
infected plants may recover, temporarily, in the evening, when temperatures are cooler. A few
days later, a sudden and permanent wilt occurs. The roots and lower portion of the stem have a
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browning of their vascular system. The invaded roots may rot due to infection from secondary
bacteria. Diseased stems that are cut and placed in a small container of water will show
yellowish or grayish bacterial ooze coming from the cut end.
When conditions are less favorable for disease development (for example, cool and dry), the
infected plants may only show signs of stunting, and adventitious roots may develop on the main
stems. The lower leaves will turn yellow before wilting symptoms occur. Symptoms of this
disease are distinguished from those of bacterial canker, which causes leaf chlorosis, stem
cankers, and “bird’s eye” spots on fruits. Bacterial wilt symptoms are distinguished from those of
Fusarium wilt because of the rapidity of the wilt, under favorable conditions, for the former, and
the drier, firmer stem rot of the latter.
Bacterial wilt
General Control of Bacterial Diseases
1. When possible, start with culture-indexed, pathogen-free plants from a reputable commercial
propagator. Plant immediately into a sterilized growing medium.
2. Water only the soil surface. Avoid overhead irrigation and splashing water on the leaves. Water
early in the day to promote rapid drying of the foliage.
3. Avoid crowding plants, heavy shade, poor air circulation, over-watering, mechanical injuries to
plants, and high humidity. These factors all favour disease development.
4. Separate infected plants from apparently healthy plants when disease is first noticed.
5. Carefully remove and destroy seriously infected plants and plant parts. Handle these plants with
throw-away plastic gloves to avoid contaminating other plants.
6. Sterilize contaminated pruning knives, tools, carrying trays, and the like between plants by
dipping or swabbing in a solution of 70 percent rubbing alcohol, liquid household bleach (1part
bleach in 4 parts water), or 38 to 40 percent formaldehyde solution.
7. Where feasible, lower the temperature at which plants are grown to 70°F (21°C). This will slow
down multiplication and spread of the causal bacteria.
8. For bacterial leaf spots and blights apply several sprays of a streptomycin formulation (such as
Agrimycin) at 200 parts per million of active ingredient, 4 to 7 days apart during damp weather,
starting when disease first appears. There is always the potential of getting resistance to
streptomycin compounds if they are used repeatedly. Copper materials provide moderate control
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of Xanthomonas and Pseudomonas leaf spots and blights. When applying any spray, carefully
follow all the manufacturer’s directions and precautions on the container label.
9. For bacterial soft rot or stem rot, dip cuttings in a streptomycin formulation (200 parts per
million) for 20 minutes. Carefully follow the manufacturer’s directions. Immerse dieffenbachia
cane pieces in a hot-water bath held at exactly 120°F (49°C) for 40 to 60 minutes, depending on
the cane diameter (40 minutes for canes 1 inch in diameter and 60 minutes for canes of 1 ½
inches).
10. Extreme care is required in handling the cuttings and cane pieces after treatment to avoid
recontamination. Treated plant materials should always be planted into a sterilized potting
medium.
11. Use only disinfected trays, tools, gloves, and soil mix. Wash hands with soap and hot running
water before and after contacting suspected diseased plants.
12. Control insects and mites following recommendations of University of Illinois Extension
Entomologists.
Plant Disease Caused by Mollicutes
Phytoplasmas are prokaryotes lacking cell walls that are currently classified in the class
Mollicutes (Agrios, 1997). Phytoplasmas are associated with plant diseases, and are known to
cause more than 600 diseases in several hundred plant species (Kirkpatrick, 1992; McCoy et al.,
1989).
Phytoplasmas in plant cell (left) and insect cell (right).
Symptoms
The symptoms shown by infected plants include: yellowing or reddening of the leaves,
shortening of the internodes with stunted growth, smaller leaves, excessive proliferation of
shoots resulting in a witches' broom, phyllody, virescence, sterile flowers, necrosis of the phloem
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tissues, dieback of the branches of woody plants, and the general decline and death of the plant
(Agrios, 1997; Kirkpatrick, 1992; McCoy et al., 1989).
Life Cycle of Phytoplasma
Phytoplasmas are transmitted from plant to plant by insect vectors, mainly leafhoppers and
psyllids (Ploaie, 1981). They traverse the wall of the intestinal tract, multiply in the hemolymph,
and pass through the salivary glands, in which they multiply further. Then, the insect vectors
introduce phytoplasmas along with salivary fluids into the phloem of a new host plant (Agrios,
1997). Usually these insect vectors do not transmit phytoplasmas transovarially, although two
exceptions have been reported: aster yellows and mulberry dwarf phytoplasmas (Alma et al.,
1997; Kawakita et al., 2000)
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Phylogenetic relationships among Mollicutes before molecular phylogenetical analysis
Classes of Mollicutes
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Plant Disease Caused by Fungi
Fungi cause the great majority, an estimated two-thirds, of infectious plant diseases. They
include all white and true rusts, smuts, needle casts, leaf curls, mildew, sooty molds, and
anthracnoses; most leaf, fruit, and flower spots; cankers; blights; scabs, root, stem, fruit, and
wood rots; wilts; leaf, shoot, and bud galls; and many others. All economically important plants
apparently are attacked by one or more fungi; often many different fungi may cause disease in
one plant species.
Symptoms
In general, a fungal infection can cause local or extensive necrosis. It can also inhibit normal
growth (hypotrophy) or induce excessive abnormal growth (hypertrophy or hyperplasia) in a
portion of or throughout an entire plant. Symptoms associated with necrosis include leaf spots,
blight, scab, rots, damping-off, anthracnose, dieback, and canker. Symptoms associated with
hyperplasia include clubroot, galls, warts, and leaf curls. In some instances, the fungus infecting
the plant may produce growth or structures on the plant, stems, or leaves such as masses of
mycelium or aggregates of spores with a characteristic appearance. These developments are
referred to as signs of infection, in contrast to symptoms, which refer specifically to the plant or
plant tissue.
Transmission:
Fungi are spread primarily by spores, which are produced in abundance. The spores can be
carried and disseminated by wind currents, water (splashing and rain), soil (dust), insects, birds,
and the remains of plants that once were infected. Vegetative fungal cells that exist in dead plant
material also can be transmitted when they come in contact with a susceptible host. The survival
of vegetative cells of plant pathogenic fungi in nature depends on climatic conditions,
particularly temperature and moisture. Vegetative cells can survive temperatures from 5° to 45°
C (23° to 113° F); fungal spores are considerably more resistant. The germination of spores,
however, is favoured by mild temperatures and high humidity.
Control:
Because many thousands of fungal species can infect a broad range of plants and because each
fungal species has different characteristics, a variety of practices are available to control fungal
diseases. The principal control measures include the use of disease-free seed and propagating
stock, the destruction of all plant materials that may harbour pathogenic fungi, crop rotation, the
development and use of resistant plant varieties, and the use of chemical and biological
fungicides.
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Sclerotium rolfsii: (a) in pathogenicity test (note hyphal runners), (b) on decayingwatermelon, (c)
basal rot with the formation of brown spherical sclerotia.
Rhizoctonia species: There are many Rhizoctonia species and strains in Vietnam. These species
are quite variable in their distribution and host range. Morphological identification to species
level is difficult.
A variety of diseases are caused by Rhizoctonia species in Vietnam (Figure 10.4).
Some species grow on plant stem and leaf surfaces in warm, wet or humid conditions causing
infection and disease of these plant parts. For example, one Rhizoctonia species infects maize
leaves and causes distinctive patterning
It is thought that the same species, or a similar species, causes head rot of cabbage. These fungi
may produce irregular brown sclerotia on diseased plant surfaces. Rhizoctonia oryzae causes
sheath blight of rice, a well-known disease.
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a)
b)
d)
Examples of Rhizoctonia diseases: (a) spear point symptoms on diseased roots, (b) Rhizoctonia
sheath blight on rice, (c) sclerotia of Rhizoctonia on diseased cabbage, (d) Rhizoctonia disease
on maize hull.
Pythium species: Pythium species belong to the class Oomycetes. They are not true fungi as this
class is in the kingdom Chromista.
Motile spores called zoospores are important spores formed by Pythium (and Phytophthora)
species, and distinguish these fungi from the true fungi in the kingdom Fungi (Mycota). The
asexually produced zoospores enable these fungi to disperse in wet soil and irrigation water.
Figure 10 shows diseases of peanuts caused by Pythium
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a)
b)
c
Pythium diseases on peanuts: (a) Pythium rootlet rot and stem rot of peanut seedlinggrown under
very wet conditions, (b) comparison of two mature peanut plants, healthy plant (left),
stunted plant with severe Pythium root rot (right), (c) severe Pythium pod and tap root rot of
peanuts.
Plant Disease Caused by Nematode
Nematodes are simple, worm-like parasites that come in friendly varieties as well as pathogenic
types that attack plants. These nematodes cause diseases by attacking all parts of the plant,
including stems, leaves, flower blossoms and roots. Nematodes attack with mouths called
"stylets" that stab through plant tissue, causing disease infection of a variety of plants.
Root Knot Nematode
Root knot nematodes (Meloidogyne species) create the disease of plants referred to as "root
knot." Caused by over 40 different species, root knot consists of swelled areas called galls on
roots and parts of plants that live underground in soil.
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Lesion Nematode
Lesion nematodes (Pratylenchus species) affect over 400 different plants. This nematode causes
a lesion disease which, as the name suggests, creates lesions on root surfaces.
Nematode feeding in a plant root
Sting Nematodes
Sting nematode disease is caused by the pathogen Belonolaimus longicaudatus and other
pathogens of the Belonolaimus species. These nematodes attack a number of hosts including
trees, grass, vegetables and other crops. Sting nematodes parasitically feed on the tips of roots.
Plants may suffer partial destruction of roots that renders the plant incapable of absorbing
necessary water, the plant above ground may suffer from overall decline, the plant may
experience stunted growth and deformed plant parts and in severe cases, the entire root system
may die, according to the American Phytopathological Society. For control, the best option is
nematicides and cultural control. For cultural management, make sure you plant clean,
uninfected plants; consider adding organic amendments to soil to increase potential for nematode
resistance. A chemical option includes nematicide application; use carbamate or
organophosphate to decrease sting nematode infestations. Additionally, a biological control
29
method includes the use of Pasteura usgae, a soil-inhabiting bacterium that helps control sting
nematode populations.
Environmental Factors that Cause Plant Disease
Important environmental factors that may affect development of plant diseases and determine
whether they become epiphytotic include temperature, relative humidity, soil moisture, soil pH,
soil type, and soil fertility.
i. Temperature: Each pathogen has an optimum temperature for growth. In addition, different
growth stages of the fungus, such as the production of spores (reproductive units), their
germination, and the growth of the mycelium (the filamentous main fungus body), may have
slightly different optimum temperatures. Storage temperatures for certain fruits, vegetables, and
nursery stock are manipulated to control fungi and bacteria that cause storage decay, provided
the temperature does not change the quality of the products. Little, except limited frost
protection, can be done to control air temperature in fields, but greenhouse temperatures can be
regulated to check disease development. Knowledge of optimum temperatures, usually combined
with optimum moisture conditions, permits forecasting, with a high degree of accuracy, the
development of such diseases as blue mold of tobacco (Peronospora tabacina), downy mildews
of vine crops (Pseudoperonospora cubensis) and lima beans (Phytophthora phaseoli), late blight
of potato and tomato (Phytophthora infestans), leaf spot of sugar beets (Cercospora beticola),
and leaf rust of wheat (Puccinia recondita tritici). Effects of temperature may mask symptoms of
certain viral and mycoplasmal diseases, however, making them more difficult to detect.
ii. Relative humidity: Relative humidity is very critical in fungal spore germination and the
development of storage rots. Rhizopus soft rot of sweet potato (Rhizopus stolonifer) is an
example of a storage disease that does not develop if relative humidity is maintained at 85 to 90
percent, even if the storage temperature is optimum for growth of the pathogen. Under these
conditions, the sweet potato root produces suberized (corky) tissues that wall off the Rhizopus
fungus. High humidity favours development of the great majority of leaf and fruit diseases
caused by fungi and bacteria. Moisture is generally needed for fungal spore germination, the
multiplication and penetration of bacteria, and the initiation of infection. Germination of
powdery mildew spores occurs best at 90 to 95 percent relative humidity. Diseases in greenhouse
crops—such as leaf mold of tomato (Cladosporium fulvum) and decay of flowers, leaves, stems,
and seedlings of flowering plants, caused by Botrytis species—are controlled by lowering air
humidity or by avoiding spraying plants with water.
iii. Soil moisture: High or low soil moisture may be a limiting factor in the development of
certain root rot diseases. High soil-moisture levels favour development of destructive water mold
fungi, such as species of Aphanomyces, Pythium, and Phytophthora. Excessive watering of house
plants is a common problem. Overwatering, by decreasing oxygen and raising carbon dioxide
levels in the soil, makes roots more susceptible to root-rotting organisms. Diseases such as takeall of cereals (Ophiobolus graminis); charcoal rot of corn, sorghum, and soybean
(Macrophomina phaseoli); common scab of potato (Streptomyces scabies); and onion white rot
(Sclerotium cepivorum) are most severe under low soil-moisture levels.
30
Water mold fungi
iv. SOIL pH: Soil pH, a measure of acidity or alkalinity, markedly influences a few diseases,
such as common scab of potato and club root of crucifers (Plasmodiophora brassicae). Growth
of the potato scab organism is suppressed at a pH of 5.2 or slightly below (pH 7 is neutral;
numbers below 7 indicate acidity, and those above 7 indicate alkalinity). Scab is not normally a
problem when the natural soil pH is about 5.2. Some farmers add sulphur to their potato soil to
keep the pH about 5.0. Club root of crucifers (members of the mustard family, including
cabbage, cauliflower, and turnips), on the other hand, can usually be controlled by thoroughly
mixing lime into the soil until the pH becomes 7.2 or higher.
vi.
Soil type:
Certain pathogens are favoured by loam soils and others by clay soils. Phymatotrichum root rot
attacks cotton and some 2,000 other plants in the south-western United States. This fungus &it is
serious only in black alkaline soils—pH 7.3 or above—that are low in organic matter. Fusarium
wilt disease, which attacks a wide range of cultivated plants, causes more damage in lighter and
higher (topographically) soils. Nematodes are also most damaging in lighter soils that warm up
quickly.
vii.
Soil fertility:
Greenhouse and field experiments have shown that raising or lowering the levels of certain
nutrient elements required by plants frequently influences the development of some infectious
diseases—for example, fire blight of apple and pear, stalk rots of corn and sorghum, Botrytis
blights, Septoria diseases, powdery mildew of wheat, and northern leaf blight of corn. These
diseases and many others are more destructive after application of excessive amounts of nitrogen
fertilizer. This condition can often be counteracted by adding adequate amounts of potash, a
fertilizer containing potassium.
Reading List
Agrios, G. N. (1997). Plant diseases caused by Mollicutes: phytoplasmas and spiroplasmas. In
Plant Pathology.
Alma, A., Bosco, D., Danielli, A., Bertaccini, A., Vibrio, M., and Arzone, A. (1997).
Identification of phytoplasmas in eggs, nymphs, and adults of Scaphoideus titanus Ball reared on
healthy plants.
31
Gundersen, D.E., Lee, I.-M., Rehner, S.A., Davis, R.E. and Kingsbury, D.T. (1994). Phylogeny
of mycoplasmalike organisms (phytoplasmas): a basis for their classification.
Kirkpatrick, B. C. (1992). Mycoplasma-like organisms: plant and invertebrate pathogens. In The
prokaryotes, 2nd ed., pp. 4050-4067. Edited by Balows, A., Truper, H. G., Dworkin, M., Harder,
W. and Schleifer, K. H. New York: Springer-Verlag. Seemüller, E., Schneider, B., Maurer, R.,
Ahrens, U., Daire, X., Kison, H., Lorenz, K.H., Firrao, G., Avinent, L., Sears, B.B. and et, al.
(1994).
Phylogenetic classification of phytopathogenic mollicutes by sequence analysis of 16S ribosomal
DNA.
Disease Cycle of Two Named Economic Crops
Downy Mildew of Maize
Mildews caused by Peronosclerospora and Sclerospora spp. are often referred to the "true"
downy mildews of maize. Peronosclerospora spp. produces conidia that germinate by germ
tubes. Sclerospora spp. produces sporangia that germinate by producing zoospores, although
direct germination occasionally occurs. Sporangia are operculate, and conidia have no apical
modification. Conidia and sporangia are produced at night. They are disseminated primarily by
wind. Germination of conidia and sporangia usually occurs in less than an hour, and free water is
required. Leaf penetration occurs approximately 1-2 hours after germination and always through
stomata.
Symptoms
Infection of maize plants at the seedling stage (less than 4 weeks old) results in stunted and
chlorotic plants and premature plant death. Leaves on older plants display characteristic
symptoms of downy mildews which include mottling, chlorotic streaking and lesions, and white
striped leaves that eventually shred. Infected plants have leaves that are narrower and more erect
compared to healthy leaves. Infected plants are often stunted, tiller excessively and have
malformed reproductive organs (tassels and ears).
Disease Life Cycle
At the onset of the growing season, at soil temperatures above 20°C, oospores in the soil
germinate in response to root exudates from susceptible maize seedlings. The germ tube infects
the underground sections of maize plants leading to characteristic symptoms of systemic
infection including extensive chlorosis and stunted growth. When oospores initiate infection, the
first leaf generally remains disease free as it is able to outgrow the fungi. However, the whole
plant will show disease symptoms if the pathogen was seed-borne. Oospores are reported to
survive in nature for up to 10 years.
Once the fungi has colonised host tissue, sporangiophores (conidiophores) emerge from stomata
and produce sporangia (conidia) which are wind and rain splash disseminated and initiate
secondary infections. Depending on the species, sporangia germinate directly or release
zoospores that initiate infection. Germination of sporangia is dependent on the availability of free
water on the leaf surface. If sufficient water is available, sporangia germinate and infect the plant
through stomata on the leaf, sheaths, or stems in a couple of hours.
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Control
1. Use resistant varieties.
2. Do not rotate or simultaneously cultivate maize with alternate hosts of downy mildew.
3. Remove infected crop debris if possible.
4. Treat seed or crop with systemic fungicide.
5. Plant when soil temp is below 20°C which is unfavorable for oospore germination.
6. Ensure seed has low moisture content (below 9%) before planting.
7. Control weeds to increase aeration within the crop and reduce moisture levels in the soil.
8. Reduce crop density to increase aeration.
Powdery Mildew of Soyabeans (Cowpea)
Powdery mildew of soybeans is caused by the fungus Microsphaera diffusa.. Powdery mildew of
soybeans also infects some members of two other plant families (Caprifoliaceae and
Solanaceae).
Symptoms
White, powdery, talcum-powder-like patches form on all plant parts, but primarily the leaves.
These patches continue to enlarge and merge until plant parts are covered. Some varieties show
33
chlorosis, or yellowing, scorching of the leaves, rusty patches on the under leaf surfaces, and
premature defoliation. Heavily infected pods usually contain shriveled, deformed, undeveloped,
and flattened green seeds.
Disease Life Cycle
Infection occurs when microscopic asexual spores (conidia) germinate and penetrate the
epidermal cells. The conidia form several germ tubes, with the first attaching itself to the cells
via an anchorage structure (appressorium). A thin filament (infection peg) forms under the
appressorium and penetrates the host epidermis. This gives rise to the first feeding structure
(haustorium), the only fungus structure found inside the host cells. The rest of the fungus body,
or mycelium, grows superficially over the epidermal cells. Conidiophores (asexual fruiting
structures) soon develop, giving rise to chains of conidia.
Wind-borne conidia start new infections and repeat the disease cycle continuously until soya
bean plants mature. Cool weather of 65° to 76°F (18° to 24°C) favors disease development,
while temperatures above 86°F (30°C) arrest the growth and reproduction of the fungus. During
rainy periods, conidia are washed away, temporarily delaying the secondary spread of the
fungus.
Mycelia growing over a soybean leaf surface with erect conidiophores, chains of conidia, and
haustoria within the epidermal cells.
Control
34
The only control is to plant resistant soybean varieties. Certain varieties are susceptible in the
seedling stage and express resistance about flowering time while others are resistant throughout
their lifetime.
Disease Cycle of Cabbage
Clubroot of Cabbage (Plasmodiophora brassicae). Clubroot is a very serious disease of cabbage
and closely related crops. The most susceptible crops include cabbage, Chinese cabbage,
Brussels sprouts and some cultivars of turnip.
Symptoms and Signs
The symptoms first noticed will be a decline of the plant including yellowing of leaves, and a
tendency to wilt during hot days. Examination of the roots will reveal swollen, club-shaped roots
instead of the normal fine network of roots. In severe cases, most roots will be affected. The
swollen roots will begin to decay and eventually disintegrate.
Slightly infected plants may show few symptoms above ground other than slow growth and will
have very small knots on roots. Young infected plants may not show severe enough symptoms to
be detected.
Disease Cycle
Club root is caused by the fungus Plasmodiophora brassicae. The important features of its life
history include its longevity in soil, means of spread, and its reaction to soil pH. After the disease
has occurred, the fungus can survive from seven to ten years without any susceptible plant ever
being grown there. If any susceptible crops or weeds grow during this period, the fungus may
become more prevalent. Resting spores of the fungus are produced in the swollen club roots and
released into the soil when these disintegrate.
Club root symptom on Cabbage
35
Close-up of the club shaped roots
Control
1 Avoid purchasing infected transplants.
2 Control weeds to avoid potential build-up of the disease on them.
3 Do not grow cabbage for more that one year in a row.
Leaf Blight or Spot of Carrot
Cercospora leaf spot or blight disease is caused by the fungi Cercospora carotae. Cercospora
blight generally occurs earlier in the season. It is also more severe on young leaves and increases
as the plant grows. In warm moist weather, entire carrot fields may appear bronzed or scorched
by these blights.
Symptoms
Primary lesions usually form along the margins of the leaflets causing them to curl. The lesions
may merge into large blotches that shrivel, blacken, and kill entire leaflets, closely resembling
symptoms of Alternaria blight. The petioles may be girdled, causing the leaves to die. Floral
parts of carrots grown for seed shrivel and die if infected early.
Disease Cycle
The spores (conidia) of Cercospora are borne on the surface of the leaflets and petioles Hyphae
from germinated spores penetrate through stomata and infect the carrot foliage. Infection occurs
over a temperature range of 60° to 92°F (16° to 33°C) with an optimum of about 73° to 82°F
(23° to 28°C). Symptoms can appear three to five days after infection has occurred and a new
crop of microscopic conidia is formed on the new lesions. The conidiophores bear the elongated,
cylindrical conidia successively at their tips.
36
Cercospora leaf blight or spot of carrot
Cercospora carotae, the fungus that causes Cercospora leaf blight or spot of carrot, as it would
appear under high power of a light microscope (Katharine A. Golasyn-Wright).
Reading Lists
Compendium of Brassica Diseases, APS Press.
Frederiksen, R. A., and Odvody, G. (1979). Chemical control of sorghum downy mildew.
Sorghum Newsl. Pg. 22-129.
Frederiksen, R. A., and Renfro, B. L. (1977). Global status of maize downy mildew. Annu. Rev.
Phytopathol. 15:249-275.
37
Gladders, P., and Paulus, A.O. (1997) Academic Press.
Handoo, M.I., Renfro, B. L., and Payak, M.M. (1970). On the inheritance of resistance to
Sclerophthora rayssiae var. zeae in maize. Indian Phytopathol. 23:231-249.
Kaneko, K., and Aday, B. A. 1980. Inheritance of resistance to downy mildew of maize,
Peronosclerospora philippinensis. Crop Sci. 20:590-594.
Kenneth, R. G. (1981). Downy mildews of grarnineous crops. The Downy Mildews. D. M.
Spencer, ed. Academic Pg. 367-394. Press, New York.
Lal, S., Bhargava, S. K, and Upadhyay, R. N. (1979). Control of sugarcane downy mildew of
maize with metalaxyl. Plant Dis. Rep. 63:986-989
Plant Disease Diagnostic Clinic (2000). Cornell University, Department of Plant Pathology and
Plant-Microbe Biology. Retrieved http://www.pathology.cornelluniversity.plantmicrobial
activity.
Smith and Renfro_Compedium of Corn diseases pdf
Vegetable Diseases: A Colour Handbook, S.T. Koike,
Zitter,
T.A.
(2001).
Vegetable
MD
Clubroot
of
Crucifers,
Retrieved
http://vegetablemdonline.ppath.cornell.edu/factsheets/Crucifers_Clubroot.htm.
5. EPIPHYTOTICS
Epiphytotic is an outbreak of a plant disease that suddenly and rapidly affects large numbers of
susceptible plants in a specific area.
The following diseases are often epiphytotic: rusts and smuts of bread grains, phytophthoric
disease of potatoes, apple scab, cotton wilt, and snowy and common leaf cast. When the number
of individuals a disease affects increases dramatically, it is said to have become epidemic
(meaning “on or among people”). A more precise term when speaking of plants, however, is
epiphytotic (“on plants”); for animals, the corresponding term is epizootic. In contrast, endemic
(enphytotic) diseases occur at relatively constant levels in the same area each year and generally
cause little concern.
Epiphytotics affect a high percentage of the host plant population, sometimes across a wide area.
They may be mild or destructive and local or regional in occurrence. Epiphytotics result from
various combinations of factors, including the right combination of climatic conditions. An
epiphytotic may occur when a pathogen is introduced into an area in which it had not previously
existed. Examples of this condition include the downy mildews (Sclerospora species) and rusts
(Puccinia species) of corn in Africa during the 1950s, the introduction of the coffee rust fungus
into Brazil in the 1960s, and the entrance of the chestnut blight (Endothia parasitica) into the
United States shortly after 1900. Also, when new plant varieties are produced by plant breeders
without regard for all enphytotic diseases that occur in the same area to some extent each year
(but which are normally of minor importance), some of these varieties may prove very
susceptible to previously unimportant pathogens. Examples of this situation include the
development of oat varieties with Victoria parentage, which, although highly resistant to rusts
(Puccinia graminis avenae and P. coronata avenae) and smuts (Ustilago avenae, U. kolleri),
proved very susceptible to Helminthosporium blight (H. victoriae), formerly a minor disease of
grasses. The destructiveness of this disease resulted in a major shift of oat varieties on 50 million
acres in the United States in the mid-1940s. Corn (maize) with male-sterile cytoplasm (i.e.,
plants with tassels that do not extrude anthers or pollen), grown on 60 million acres in the United
38
States, was attacked in 1970 by a virulent new race of the southern corn leaf blight fungus
(Helminthosporium maydis race T), resulting in a loss of about 700 million bushels of corn.
More recently the new Helminthosporium race was widely disseminated and was reported from
most continents. Finally, epiphytotics may occur when host plants are cultivated in large
acreages where previously little or no land was devoted to that crop.
Epiphytotics may occur in cycles. When a plant disease first appears in a new area, it may grow
rapidly to epiphytotic proportions. In time, the disease wanes, and, unless the host species has
been completely wiped out, the disease subsides to a low level of incidence and becomes
enphytotic. This balance may change dramatically by conditions that favour a renewed
epiphytotic. Among such conditions are weather (primarily temperature and moisture), which
may be very favourable for multiplication, spread, and infection by the pathogen; introduction of
a new and more susceptible host; development of a very aggressive race of the pathogen; and
changes in cultural practices that create a more favourable environment for the pathogen.
Causes of Epiphytotics
i.
ii.
iii.
iv.
v.
vi.
Consequent severe drought: A severe drought affected the region in spring time (March-April),
aggravating the physiological damage of rust infection (synergetic effect of evapo-transpiration
in diseased plants
Unorganized irrigation (side-effects of sprinkler irrigation)
Mono-culturing of A Particular crops
Unpreparedness for chemical intervention (fungicidal treatment). Syrian wheat farmers usually
apply pesticides in very few cases which make their crops prone to epiphtytotic infection:
absence of Pre- or post- emergence herbicidal applications
The arrival and establishment of a new pathotype(s) Of diseases which are highly virulent
Absence of functioning forecasting systems for pest and diseases
Plant disease forecasting: this is a management system used to predict the occurrence or change
in severity of plant diseases. Plant disease forecasting systems have been developed to help plant
pathologist to make economic decisions about disease management (Agrios 2004). Plant disease
forecasting systems may support a producer's decision-making process with regard to the costs
and benefits of pesticide applications,


which propagation material or seed stock to purchase, and
Whether to plant a specific crop in an area (Agrios 2004).
The principle behind plant disease forecasting systems is to determine the risk that a disease will
occur, or that the intensity of the disease will increase (Campbell and Madden 1990).
At the field scale, these systems are used by growers to make economic decisions about disease
treatments for control. Often the systems ask the grower a series of questions about the
susceptibility of the host crop, and incorporate current and forecast weather conditions to make a
recommendation. Typically a recommendation is made about whether disease treatment is
necessary or not. Usually treatment is a pesticide application.
39
Forecasting systems are based on assumptions about the pathogen's interactions with the host and
environment, the disease triangle. The objective is to accurately predict when the three factors host, environment, and pathogen - all interact in such a fashion that disease can occur and cause
economic losses. In most cases the host can be suitably defined as resistant or susceptible, and
the presence of the pathogen may often be reasonably ascertained based on previous cropping
history or perhaps survey data. The environment is usually the factor that controls whether
disease develops or not. Environmental conditions may determine the presence of the pathogen
in a particular season through their effects on processes such as overwintering. Environmental
conditions also affect the ability of the pathogen to cause disease, e.g. a minimum leaf wetness
duration is required for grey leaf spot of corn to occur. In these cases a disease forecasting
system attempts to define when the environment will be conducive to disease development.
Good disease forecasting systems must be reliable, simple, cost-effective and applicable to many
diseases. As such they are normally only designed for diseases that are irregular enough to
warrant a prediction system, rather than diseases that occur every year for which regular
treatment should be employed.
History of Disease Forecasting
During the last 15 to 20 years considerable research in plant pathology has been dedicated
toward developing mathematical models of plant disease development which can be used to
forecast disease outbreaks. Most often these forecasting systems are driven by environmental
factors such as temperature, leaf wetness, rainfall, and relative humidity. To most effectively use
the forecasting models, the weather conditions near the fields need to be monitored regularly,
and then the data need to be run through the computer models. In the past, data collection has
been a time consuming and expensive process. Recent innovations in weather data collection, the
use of personal computers, and the development of the Internet have allowed weather data
collection for disease forecasting models to become affordable. There are two types of data
available to farmers - data that is collected on electronic instruments placed in fields and
retrieved by phone lines and data which is extrapolated to local conditions using mathematical
equations from central collection sites. In order to make use of the first type of data, growers
must obtain software for their own computer or subscribe to a service which connects to their
weather instrument and interprets the data into disease predictions for them. In order to make use
of the second type of data, growers must subscribe to a service which owns the mathematical
models for extrapolation. In many cases, the use of the models will result in fungicide
application savings which pay for the data acquisition costs.
Among the forecasting programs available are potato and tomato early and late blight, onion
programs for Botrytis leaf blight, Alternaria and downy mildew; and white mold of snap beans.
In addition, weather monitoring information can be useful for analysis of crop growth and yields.
Network for Environment and Weather Applications site (NEWA) makes daily disease forecasts
for a number of pests of vegetable crops in New York based on weather data collected in
growers' fields.
Characteristics of a good forecasting system
40







What defines a successful plant disease forecasting system? Campbell and Madden (1990)
outlined several attributes, including:
reliability (use of sound biological and environmental data),
simplicity (the simpler the system, the more likely it will be applied and used by
producers),
importance (the disease is of economic importance to the crop, but sporadic enough that
the need for treatment is not a given),
usefulness (the forecasting model should be applied when the disease and/or pathogen
can be detected reliably),
availability (necessary information about the components of the disease triangle should
be available),
multipurpose applicability (monitoring and decision-making tools for several diseases
and pests should be available), and
cost effectiveness (forecasting system should be cost affordable relative to available
disease management tactics).
Plant disease forecasting systems often provide information about how a grower's management
decisions can help to avoid initial inoculum or to slow down the rate of an epidemic. These two
concepts are important because they often differentiate the risk for a monocyclic disease (having
only one cycle of infection) versus polycyclic disease, where there are multiple infection cycles,
and a forecasting system can be used to time appropriate management tactics, such as a foliar
fungicide application (Madden et al. 2007). It should be noted that some plant disease forecasts
focus both on avoiding initial inoculum and also on reducing the rate of the epidemic during the
season (see below)
Plant disease forecasting systems are emphasized based on the following principles
(Campbell and Madden 1990):

Forecasts based on measures of initial inoculum or disease, example: Stewart's disease of
corn

Forecasts based on favorable weather conditions for development of secondary inoculum,
example: Late blight of potato (description and simulation)

Forecasts based on both initial and secondary inoculum, example: Apple scab
(description and simulation)

An example of a multiple disease/pest forecasting system is the Epidemiology,
Prediction, and Prevention (EPIPRE) system developed in the Netherlands for winter wheat that
focused on multiple pathogens (Reinink 1986).
Current examples of plant disease forecasting providing daily information on-line are available
for two important plant diseases: Fusarium head blight of wheat and Asian soybean rust. Both
systems provide background information on the disease, current management recommendations,
as well as disease forecast information based on host, pathogen, and environmental factors
important for making an accurate forecast. The successful development of a plant disease
forecasting system also requires the proper validation of a developed model. There is increased
interest among plant disease modelers and researchers to improve producer profitability through
41
validation based on quantifying the cost of a model making false predictions (positive and/or
negative). Yuen discusses this issue in his article. As pointed out by Yuen, this methodology is
not necessarily a new concept, as historical systems, such as the Mills rules for apple scab, or
those used for potato late blight and Alternaria leaf blight of carrots, were developed using
prediction rules for plant disease management. An economic validation of a plant disease
forecasting system requires the examination of two false predictions:
1. False positive predictions, in which a forecast was made for a disease when in fact no disease
was found in a location, and
2. False negative predictions, in which a forecast was made for a disease not to occur when in fact
the disease was found.
3. These two types of false predictions may have different economic effects for producers (Madden
2006).
4. Lastly, the range of disease forecasting models has expanded to include a Bayesian statistical
approach. This information is beyond the scope of this exercise and the interested reader is
referred to the discussions of Bayesian approaches found in Mila et al. (2003), Yuen (2006), and
Madden (2006).
Throughout the rest of this document, an introduction to plant disease forecasting is presented
through examples, many of which use the R programming environment (Garrett et al. 2007; R
Development Core Team). A brief introduction to some of the mathematical/statistical
approaches that have been used for developing plant disease forecasting systems is presented,
followed by an introduction to how using rainfall and temperature may be applicable for
developing a forecast model, and finally, four case studies are presented that highlight the
following:
1. Modeling the generic risk of infection due to a plant pathogen,
2. Use of differential equations to model the population dynamics of sugar beet cyst nematodes in
order to understand long-term changes in nematode density, and
3. Developing a model that increases the accuracy of an existing forecast model through new
information on the percentage of mature pseudothecia.
Examples of Disease Forecasting Systems
Forecasting systems may use one of several parameters in order to work out disease risk, or a
combination of factors. One of the first forecasting systems designed was for Stewart's Wilt and
based on winter temperature index as low temperatures would kill the vector of the disease so
there would be no outbreak. An example of a multiple disease/pest forecasting system is the
EPIdemiology, PREdiction, and PREvention (EPIPRE) system developed in the Netherlands for
winter wheat that focused on multiple pathogens. USPEST.org graphs risks of various plants
diseases based on weather forecasts with hourly resolution of leaf wetness. Forecasting models
are often based on a relationship like simple linear regression where x is used to predict y. Other
relationships can be modeled using population growth curves the growth curve that is used will
depend on the nature of the epidemic. Polycyclic epidemics such as potato late blight are usually
best modeled by using the logistic model, whereas monocyclic epidemics may be best modeled
using the monomolecular model. Correct choice of a model is essential for a disease forecasting
system to be useful.
42
Plant disease forecasting models must be thoroughly tested and validated after being developed.
Interest has arisen lately in model validation through the quantification of the economic costs of
false positives and false negatives, where disease prevention measures may be used when
unnecessary or not applied when needed respectively. The costs of these two types of errors need
to be weighed carefully before deciding to use a disease forecasting system.
Future Developments
In the future, disease forecasting systems may become more useful as computing power
increases and the amount of data that is available to plant pathologists to construct models
increases. Good forecasting systems also may become increasingly important with climate
change. It will be important to be able to accurately predict where disease outbreaks may occur,
since they may not be in the historically known areas. Diverse modeling approaches viz. neural
networks and multiple regressions have been followed to date for disease prediction in plant
populations. However, due to their inability to predict value of unknown data points and longer
training times, there is need for exploiting new prediction softwares for better understanding of
plant-pathogen-environment relationships. Further, there is no online tool available which can
help the plant researchers or farmers in timely application of control measures.
Weather-based forecasting systems reduce the cost of production by optimizing the timing and
frequency of application of control measures and ensures operator, consumer and environmental
safety by reducing chemical usage. A major aim of many forecasting systems is to reduce
fungicide use, and accurate prediction is important to synchronize the use of disease control
measures to avoid crop losses. A prediction model based on the relationship between
environmental conditions at the time of management and late-season disease severity could be
used to guide management decisions. Thus, if a sound forewarning system is developed, the
explosive nature of the disease could be prevented by timely application of the control measures.
Various techniques of computer modeling and simulation viz. machine learning techniques like
artificial neural networks and the conventional multiple regression approaches are being used to
help synthesize and develop scientists' understanding of this complex plant-pathogenenvironment relationship. The resultant models enable exploration of the factors that govern
disease epidemics and the design of control systems that minimize yield losses. The same models
have potential to guide breeding programs and work to develop strategies that will prolong the
usefulness of disease-resistance genes. These brought about a case study on rice blast disease
forecasting by following a new prediction approach, SUPPORT VECTOR MACHINE and
compared its performance with the existing artificial neural networks-based and multiple
regression-based prediction approaches.
Support Vector Machine (SVMs) are universal approximators based on statistical learning and
optimization theory which supports both regression and classification tasks and can handle
multiple, continuous and categorical variables. To construct an optimal hyperplane, SVM
employees an iterative training algorithm, which is used to minimize an error function. For the
application of SVM, the complete theory can be found in Vapnik's monographs.
Reading lists
43
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
Agrios, George (2005). Plant Pathology. Academic Press. ISBN 978-0120445653.
APS Education Centre - Stewart's wilt of corn
Campbell, C. L.; Madden, L. V. (1990). Introduction to Plant Disease Epidemiology.
New York: Wiley and Sons.
Esker, P. D.; A.H. Sparks, L. Campbell, Z. Guo, M. Rouse, S.D. Silwal, S. Tolos, B.
Van Allen, and K.A. Garrett. "Ecology and Epidemiology in R: Disease Forecasting".
The Plant Health Instructor (APS Press).
Madden, Laurence; Gareth Hughes, Frank Van Den Bosch (2007). Study of Plant.
Reinink, K (1986). "Experimental verification and development of EPIPRE, a supervised
disease and pest management system for wheat". European Journal of Plant Pathology
(SpringerLink) 92.
Taylor MC, Hardwick NV, Bradshaw NJ, Hall AM: Relative performance of five
forecasting schemes for potato late blight (Phytophthora infestans) I. Accuracy of
infection warnings and reduction of unnecessary, theoretical, fungicide applications.
Vapnik VN: Statistical Learning Theory. New York: John Wiley and Sons; 1998.
Vapnik VN: The nature of statistical learning theory. New York: Springer; 1995.
44