Download Diseases Part 1

Focus of this course
• First half - diseases
• Second half - weeds
• Many important plant diseases are
caused by pathogens and this will be
the main emphasis of our studies
Plant Disease defined:
• It is a disruption of normal plant function in
a host plant due to a causal agent
• The disruption results in a morphological
change in the host plant, this is what we
term symptoms.
• The causal agent may be biological or
physiological (ie. Chemical or physical)
Disease vs. injury
• Disease is a continous (chronic) or
persistent condition.
• Injury is acute and/or short lived
Physiological agents
• These are non-living and include
things such as cold, chemicals,
elements, pollution, and etc.
• they can’t reproduce
• are not infectious
• and are called physiopaths
Biological agents
• These are living organisms such as
fungi, bacteria, viruses, etc.
• they can reproduce
• they are infectious and can spread
through a host population
• they are called pathogens
Physiological agents
• Symptoms usually uniformly distributed
over entire plant
• typically occur as tip or marginal leaf
scorch
• more likely to occur on older leaves
Biological agents
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Leaf lesions scattered or sporatic
lesions usually have halo but not always
spread readily to surrounding plants
margins of leaf spots are typically smooth
Host plant disease relationship
• All plants are susceptible to one or
more diseases
• plant diseases have always occurred
and continue to occur in nature but are
not a problem due to diversity
• man made ecosystems tend to be
monocultures
Epiphytotics
• Plant epidemics
• monocultures pave
the way for this to
occur
• pythium in
turfgrass is an
example
History of plant disease
• 1500 BC - first description of cultural
controls, especially planting dates
• 950 BC - first use of burning as a cultural
control
• 470 BC - Pliney the Elder uses olive oil for
control of blight in grapes
• 1660 AD - connection between barberry and
wheat rust discovered
History of diseases continued:
• 1824 - sulfur recommended for powdery
mildew on grapes
• 1845 - Irish potato famine
• 1858 - “Diseases of Cultivated Plants”
published
• 1868 - deBary - proved association of
fungus with specific plant disease
Still more to come!
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1880 - first commercial sprayer
1882 - Bordeaux mix & Paris green
1883 - Koch’s postulate published
1885 - USDA creates mycology section
1940 - synthetic organic fungicides for
protecting military clothing
• 1946 - tomato blight
Koch’s Postulate
• Acceptable proof of causation
– organism must be associated with all
cases of the disease
– organism must be isolated from diseased
plant
– organism must then be able to produce
disease in a healthy innoculated plant
– organism must be re-isolated from the
“artificially” infected plant
Symptoms and Signs
• Symptom is defined as any visible evidence
of disease expressed by the plant.
• Sign is any visible evidence manifested by
the pathogen itself.
• Symptoms are usually specific for a
particular plant.
Classifying symptoms
• Local vs. systemic
• Local occurs in only in one plant organ or
tissue
• Systemic occurs in two or more plant
organs or tissues
Classifying symptoms
• Lesional vs. habital
• Lesional - a specific structural change,
obvious damage or wounds
• Habital - an abnormal growth habit
Classifying symptoms
• Primary vs. secondary
• Primary symptoms occur at the point of
infection
• Secondary symptoms occur away from the
point of infection
Classifying symptoms
• Necrotic vs. chlorotic
• Necrotic symptoms cause eventual death of
the tissue
• Chlorotic symptoms involve a loss of or
failure to produce color & may or may not
result in death of the tissue
Classifying symptoms
• Hypoplastic vs. hyperplastic
• Hypoplastic symptoms are the results of
underdeveloped growth
• Hyperplastic symptoms are the result of
over developed growth
Types of necrotic symptoms
• Hydrosis - water soaked appearance due to
rupturing of cell membranes and resultant
“leaking” of the cytoplasm (cell sap)
• wilt - loss of turgor pressure, usually a
secondary symptom due to plugging of
zylem tissue by an organism
• dieback - death of the branch system of
trees or shrubs
Leaf showing hydrosis
Types of necrotic symptoms
• Scorch - sudden death of the tissue due to
physiological agents
• blight - sudden death of the tissue due to
pathogen(s)
• spot - lesions which vary in size, shape, and
color
• rot - slow decay/mushiness ending in
mummification
blight
scorch
Leaf spot
More leaf spot
Seeing spots yet?
Fungal leaf spot and perithecium
Healthy cherry on right, rot on
left
Mummification following rot
Types of necrotic symptoms
• Canker - oblong lesion with a sunken center
usually occurring on stems
• damping-off - seeding rot often occurring
on the stem near the soil line
• scald - blanching of foliage due to high light
intensities and standing water
stem canker
Types of chlorotic symptoms
• Chlorosis - failure or partial failure of green
color.
• Yellowing - loss of green color after it has
developed
• (difference is academic to some extent)
Chlorosis/mottling
• Camellia mosaic virus
Types of hypoplastic symptoms
• Albication - albinism
• suppression - failure of organs to develop
when and where they usually develop
• etiolation - spindly growth with poor color
• dwarfing - stunting of the whole plant or
specific organ(s)
• rosetting - failure of internodes to elongate
Rosetting caused
by bermudagrass
mites
Types of hyperplasitic symptoms
• Prolepsis - premature organ development
• abscission - premature drop of organ(s)
• russeting - roughening of the epidermis of
organ(s) due to excess suberin deposits
• discoloration - expression of pigments other
than chlorophyll (bronzing, purpling)
• tumefactions - gall formation
• fasciculation - clustering of organs, witches
broom
Discoloration/purpling
gall
• Cedar-apple rust
• Gall on left
• Telial horns (i.e. fruiting body) protruding
from gall on right
Crown gall
Types of hyperplastic symptoms
• Callus - bulging growth of stem cambium
due to lesions
• curling\cupping - overgrowth of tissue on
one side of a leaf or petal
• scab - raised, rough, cracked lesion
• fasciation - flat, fan-shaped, over
development of stems, leaves, flowers
Apple scab on pyracantha
Exudates
• Ooze - liquid around lesions
• slimeflux - fermenting ooze
• gummosis\resinosis - thich, viscous
secretions around lesions
• odors - many diseases have a distinctive
odor associated with their exudates