Download ABIOTIC DISEASES OF PLANTS Helen Ogle

10
OF PLANTS
ABIOTICDISEASES
HelenOgle
Contents
1 0 . 1 lntrodtrctinn........,....
10.2 Adu er s e w eather conditions
Temperature...,........
Humiditg
Atmospheric gases,.
Light........
1O.3 Aduerse properties of the grotutng medtum
PhgsicaL properties
ChemicaLproperties
1O.4 Chemical and phgsicaLifiuries
1O.5 Further reading
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10.1 Introduction
Diseases caused by abiotic or non-pathogenic factors cannot be transmitted from
affected plants to healthy plants. Some abiotic diseases are the result of genetic
changes that occur in the meristematic cells of plants, resulting in a flower, fruit
or branch differing in appearance from those on the rest of the plant. The extent
of the disorder depends on how early in the plant's development the change
occurs. Citrus plants are particularly prone to such changes which are referred
to as chimeras. Leaves on affected twigs may become variegated, fruit may have
corrugated or differently-coloured sectors, branches may develop a willowy
growth habit, stems may be twisted or flattened or galls or rough bark may be
produced on the stems. If the cell change takes place at a very early stage of
development, the whole plant may be affected. Generally these changes are
undesirable, but occasionally a mutant with desirable properties such as early or
late maturing fruit, high productivity or special fruit characteristics occurs. A
new, improved cultivar may result. The Washington Navel orange cultivar, many
strains of Red Delicious apple as well as other fruits such as plum, peach,
nectarine and European pear have arisen in this way.
More often, abiotic diseases are caused by unfavourable environmental
conditions. For example, a lack, excess, imbalance or deleterious interaction of
the physical or chemical factors necessary for healthy plant growth may cause
abnormal development. Environmental factors that affect plant growth include
temperature, moisture, light and the properties of the medium in which the plant
is growing. For each environmental factor, there is an optimum range within
which plants grow best. Plants can tolerate some variations either side of the
optimum for each environmental factor. However, they may exhibit growth
abnormalities if the level of an environmental factor varies greatly from its
optimum, or severely impacts on another factor.
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t57
Plant species have different optima and abilities to tolerate deviations from
optimum levels. Furthermore, various plant processes (photosynthesis, nutrient
uptake, storage of food reserves, leaf growth, etc.) may have different optima for
the same environmental factor. Similarly, a particular process may have different
optima for an environmental factor at various stages of plant development.
Tissues or organs on the same plant may also vary in their optima for a
particular environmental factor. With respect to tolerance of environmental
factors, three groups of plants have been recognised on the basis of the
biochemical pathways used in photosynthesis: C3 or Calvin cycle plants, Ca or
dicarboxylic acid cycle plants and CAM or crassulacean acid cycle plants. Ca and
CAM plants have lower transpiration rates than C3 plants. They also tolerate high
light intensity and high temperature better than C3 plants and photosynthesise
more efficiently. These attributes make them more able to tolerate semi-arid
subtropical and tropical conditions.
Sometimes non-pathogenic diseases show characteristic symptoms and the
causal factor(s) can be readily recognised by matching the symptoms with
prevailing weather conditions, cultural practices or soil properties. However,
symptoms of some abiotic diseases may closely resemble those of biotic diseases.
In such cases it is necessary to establish that a pathogen is not associated with
the disease syndrome and then relate the disease to a specific environmental
factor. Restoring the relevant abiotic factor to a range more favourable for plant
development should result in a return to normal growth. For example, sunflower
plants showing symptoms of phosphorus deficiency resume healthy growth after
the application of phosphorus ferlilisers. Similarly, in wheat, zinc deficiency can
be corrected by foliar or soil applications of zinc salts.
Abiotic diseases can be caused by adverse weather conditions, adverse
properties of the growing medium as well as chemical or physical injury. These
factors can affect all plants, but are usually more important for cultivated plants
subjected to cultural practices such as fertilisation, irrigation or chemical
spraying, especially those grown in artificial environments such as greenhouses
or indoors.
10.2 Adverse weather conditions
Various forms of severe weather such as drought, flood, high winds, frost, hail,
snow and lightning may damage or kill plants or provide access sites for
pathogens. Variations in 'normal' weather conditions can also affect growth
resulting in reduced yield or even death of plants. For convenience, weather
conditions are often discussed as independent variables such as temperature,
moisture, light, composition of the atmosphere, etc. In reality, these factors
interact with each other.
Temperature
Plants generally grow at temperatures ranging from about l"C to 4O"C, with the
optimum range usually being 2O-35"C. However, Ca plants such as many tropical
grasses, have an optimum range of 3l-37"C and can grow at temperatures up to
45-55'C. Many plants can withstand temperatures of 2-7'C if they are protected
from wind, but these temperatures can be damaging at wind speeds above 16
km/h because of both dehydrating and cooling (wind chill) effects. Perennial
plants and dormant organs such as seeds and corms can tolerate more extreme
temperatures than annual plants or actively growing organs.
Temperature directly affects physiological processes such as photosynthesis,
respiration, membrane permeability, water and nutrient absorption,
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Helen OgLe
transpiration, enzyme activity and denaturation of proteins. Because the
minimum, optimum and maximum temperatures for each process in an
individual plant can differ (and also differ for the same process in different
plants), plant reactions to temperature can be highly variable. Generally, plants
growing at temperatures close to either the upper or lower end of their range will
grow poorly and produce fewer and smaller flowers and fruits.
Low temperatures cause a wide range of damage to plant tissue. The severity
of the damage depends on how quickly the temperature drops and also, it is
thought, on how quickly tissues thaw out after freezing. Late frosts can damage
young meristematic tips, entire herbaceous plants and the buds, flowers, young
fruit, new twigs of fruit trees and other deciduous trees. Low temperatures may
even kill the young roots of trees or cause tree bark to split, allowing canker
development. Indoor or tropical plants exposed to low temperatures may turn
yellow and drop their leaves or buds. Fleshy tissues, such as potato tubers, show
varying degrees of low temperature damage depending on how low and for how
long the temperature drops.
Low temperature damage to plant tissues results from the formation of ice
crystals inside plant cells. Ice forms first on the surface of plants and spreads
rapidly in the continuous film of moisture on plant cells to the intercellular
spaces. Eventually ice crystals form within cells. If the growth of the ice crystals
ruptures membranes or other vital cellular structures, the affected cells die.
Freezing injury does not normally occur until temperatures drop below -5 to
-6oC. However, some species of bacteria in the genera Pseudomonas,
Xanthomonas and Ertuinia act as nuclei for ice formation allowing ice to form on
plants at temperatures between -l and -soc. These bacteria grow epiphytically
on plant surfaces in temperate zones around the world and one effective icenucleating bacterial cell on a leaf is sufficient to cause ice formation in that entire
leaf.
Chilling injury occurs at much higher temperatures than frost damage. It is
not related to the formation of ice crystals, but to the disruption of enzyme
systems which causes a build-up of plant by-products which essentially 'poison'
the plant. Symptoms of chilling, depending on severity, can appear immediately
after chilling has occurred, or be delayed for some time. Chilling injury occurs
commonly in tropical foliage plants such as Helicon[a as far north as Cairns in
north Queensland (Latitude 17"S). Subtropical plants are damaged at 2-B'C
while tropical plants may be injured at 12-13"C. Minor chilling damage in plants
usually results in water soaking, yellowing and browning along the edges of
leaves. Older leaves show symptoms first. If the damage is severe, plants wilt and
often die. Chilling injury also occurs in a wide range of fruits and vegetables if
they are stored at sub-optimal temperatures after harvest. Tropical fruits may be
damaged at temperatures below lO-15'C while apples can be safely stored at 23oC. Oranges stored below their optimum temperature of 5'C may develop
sunken, brown spots or pits while cucumbers stored at or below 7'C develop
surface pitting or dark-coloured water-soaked areas under the skin.
High-temperature injury results from the coagulation and denaturation of
proteins, the disruption of cell membranes and the possible release of toxic
products into cells. Affected cells may be killed and affected tissues desiccated.
As the synthesis of normal protein slows, new proteins called 'heat shock
proteins' are produced which help plants resist the effects of high temperature
and other environmental stresses. The severity of high temperature damage
depends on the temperature and the duration of exposure. It may take several
10. Abiatic diseases of plants
159
weeks for symptoms to develop fully by which time it is difficult to relate the
symptoms to the stress itself.
Plants show a variety of responses to high temperature. Seedlings may show
'stem girdling' due to radiating heat from the soil scorching young
symptoms of
'heat canker' is formed.
stem tissue. If the stem above the injured zone swells, a
High temperatures during summer may cause many of the vegetative buds of
plants such as almonds to abscise. The buds that survive either grow vigorously
or remain dormant leading to very uneven growth. The flesh of ripening stone
fruit darkens, especially near the stone, when temperatures exceed 39-4O'C. If
high temperatures are combined with high winds, the fruit may shrivel due to
combined heat and water stress.
In enclosed greenhouses, high temperatures create high humidity which
significantly retards transpiration, even though the stomates remain open. Since
plants normally rely on transpiration for cooling, these conditions can result in
heat injury.
Humidity
The moisture content of the atmosphere is usually described by its relative
humidity, the amount of moisture in the air expressed as a percentage of the
amount that would be necessary to fully saturate the air at the same
temperature.
Low humidity increases the evaporative demands on plants to the extent that
moisture stress can occur, even when there is an ample supply of water to the
roots. Normally, the evaporation of water from leaf cells in the process of
transpiration cools leaves, preventing them from reaching temperatures at which
damage occurs. If the supply of water becomes inadequate, the concentration of
abscisic acid rises rapidly, causing the stomates to close. Although closing the
stomates protects the plant from excessive water loss, it may, at the same time,
reduce the degree of cooling of the plant through transpiration. The overall result
is tissue damage.
Lack of moisture in the atmosphere is rarely a problem in outdoor plants. It is
usually only a temporary condition and seldom causes serious damage. However,
if low relative humidity is combined with strong winds, high temperatures and
low soil moisture, plants may wilt temporarily or permanently, leaves may be
scorched or burned and fruit shrivelled and burned due to excessive loss of
water. Plants grown in greenhouses are more prone to damage by low humidity
than plants grown out-of-doors. Misting or fogging is frequently used to maintain
the relative humidity in the range appropriate to the plants under cultivation (7O85olo is suitable for most plants). Misting or fogging also helps to reduce
temperature.
When the relative humidity is high, evaporation from the plant may be
suppressed, inhibiting the uptake of nutrients, particularly calcium, from the
growing medium. The resulting lack of nutrients impairs cell formation and
leaves plants vulnerable to a range of disorders. Excessive moisture in the
atmosphere may also damage plants directly. For example, the skins of young
immature cherries may split during periods of prolonged wet weather, exposing
the fruit to the risk of infection by the brown rot fungus, MonitiniaJructtcoLa.
Atmospheric gases
In the field, concentrations of atmospheric gases are unlikely to limit plant
growth. However, in sealed glasshouses, carbon dioxide can become a limiting
factor for plant growth. Adding carbon dioxide during daylight hours increases
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yields, improves quality and shortens cropping times. On a global scale, the
amount of carbon dioxide in the atmosphere is rising and is expected to double
towards the end of the 2lst century. It is anticipated that this increase in the
carbon dioxide content of the air will favour the growth of C3 plants rather than
Ca plants. However, the rise in temperature (1.5-4.5'C) that will accompany the
increase in carbon dioxide concentration should encourage the growth of Cn
plants. The effects of these changes on the occurrence and severity of damage
caused by pathogens are currently being investigated.
In storage, tissues in the centres of fleshy fruits and vegetables may suffer
from a shortage of oxygen and a build up of carbon dioxide if the storage area is
not correctly ventilated. Blackheart of potato, heart leaf injury in lettuce and
brown heart of apples and pears are all attributed to the excessive accumulation
of carbon dioxide in internal tissues during storage. High levels of carbon dioxide
in storage may also result in external damage which usually takes the form of
numerous small sunken pits on the surfaces of fruits and vegetables. While high
levels of carbon dioxide may damage stored produce, properly maintained carbon
dioxide levels in storage areas delay ripening and senescence, reduce the
sensitivity of stored produce to ethylene-induced changes, help reduce
physiological diseases and decay and control insects. This is the principle
underlying the use of controlled atmosphere (CA) for the long term storage of
produce such as apples and pears. Modified atmosphere (MA), a less precise form
of storage than CA, is used in storage and transport of harvested fruits and
vegetables.
A large number of phytotoxic chemicals, such as ethylene, nitrogen dioxide,
peroxyacetyl nitrates (PANS), ozorre,photochemical smog, hydrogen fluoride and
sulphur dioxide, occur in the air over industrial or urban areas. All these
substances, if sufficiently concentrated, can damage leaves of susceptible plants.
In fact, plants sensitive to particular pollutants may be used to monitor their
levels in the atmosphere. Of Australian native plants, species of EucaLgptus
appear to be most susceptible to sulphur dioxide while Casuarina spp. are most
resistant. Most native species tested are resistant to ozone damage although a
few species of Banksia, Acacia, MeLaleucaand Eucatgptus developed acute foliar
symptoms. Plants of any type growing near industrial installations such as
fertiliser plants and smelters are likely to be damaged by emission gases such as
fluorides and sulphur dioxide.
Oddes of sulphur and nitrogen may be washed out of the atmosphere by rain
as sulphuric and nitric acids, respectively, in the phenomenon known as acid
rain. Acid rain has a pH below that of 'normal' rain (pH 5.6) and can damage
plants and other forms of life over wide areas. Even if plants are not damaged
severely enough to show symptoms, the pollutants may interfere with their
metabolism, reducing growth, productivity and ability to reproduce.
More localised damage may occur in enclosed areas such as transport and
storage areas and homes. Under these circumstances, gases produced by plants
or plant products (e.g. ethylene, a plant growth regulator), by leaks in cooling
systems (e.g. ammonia) or by engines (e.g. ethylene) may damage plants. For
example, flowers of Geraldton Wax (ChamaeLaucium unctnatum) packed in boxes
for transport, frequently fall because ethylene produced by flowers infected by
fungi such as Botrytis spp. or subjected to water stress after picking, stimulates
flower abscission. Similarly, if fruits or vegetables that produce ethylene as they
ripen are stored with those that need ethylene to ripen, considerable losses may
occur as the products ripen, and possibly rot, during transport or storage.
Grapes are shipped in boxes containing chemicals that liberate the fungistatic
1O. Abiotic diseases oJplants
161
gas, sulphur dioxide. If the concentration of sulphur dioxide reaches too high a
level, sunken, bleached areas sharply demarcated from unaffected tissue will
appear on the fruit.
Light
Disease attributed to the effects of 'light' are often difficult to separate from the
effects of other environmental factors, such as temperature. When plants grown
in the shade are exposed to excessive light intensities, abnormal photochemical
reactions which inactivate some enzymes and oxidise chlorophyll occur. These
effects tend to be more pronounced if oxygen is freely available. Such photooxidation processes can lead to chlorosis or bleaching of plant parts and
sometimes death of leaves or even the whole plant. In contrast, potato tubers
exposed to light during growth or after harvest develop a green colouration due to
the formation of chlorophyll in tissues that do not .to.rnitty contain chlorophyll.
Other compounds such as the glycoalkaloid solanine may also form in exposed
tissues. Solanine gives potatoes a bitter taste and is poisonous if consumed in
large quantities.
The contribution of ultraviolet light to photo-oxidation processes is largely
unknown. However, ultraviolet light has been implicated in sunscald of bean
pods at high altitudes, in sunscald or atmospheric scorch of peanuts and in solar
injury of melons. Globally, the reduction in the amount of ozone in the
stratosphere will be accompanied by an increase in the amount of LIV-B radiation
(280-320 nm) reaching the earth. Preliminary studies indicate that increased [JVB radiation may suppress photosynthesis and, consequently, reduce plant
growth.
Insufficient light inhibits chlorophyll formation and stimulates cell elongation
resulting in etiolated plants with abnormally long internodes and poorly
developed, pale green leaves. Etiolated plants are susceptible to lodging and
invasion by parasites. They are not common out-of-doors, but may be found
under enclosed conditions such as seed beds and greenhouses if light is limited.
10.3 Adverse properties of the growing medium
Plants cannot function properly if the physical or chemical properties of the
medium in which they are growing prevent their root systems from obtaining
water, nutrients or oxygen. The components of the medium interact with each
other to produce the conditions under which the plant must grow. Interactions
between various properties of the growing medium may cause some plant
requirements to exceed the tolerance range of the plant for that particular
requirement. Accordingly, plant disease symptoms arise from a deficiency, excess
or imbalance of the requirement.
Physical properties
The texture and structure of soils or potting media influence their aeration,
temperature profiles and capacity to hold water. These factors, in turn, influence
the growth of roots and their ability to absorb water and nutrients from the soil.
Plants growing in soils with an inherent hardpan or traffic-compacted layers may
produce shallow root systems as their roots remain above the compacted layer.
Some soils form surface crusts which may retard or prevent the emergence of
seedlings.
The effects of soil water status on physiological processes in plants, especially
those which can be observed as disease symptoms, are influenced by the
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duration and degree of deficiency or excess of soil water, the physiological
adaptations of the plant in relation to water balance and interactions with other
soil factors such as nutrient availability, microbial activity and diffusion of gases.
Internal moisture stresses inhibit cell division and elongation and the
movement of solutes in vascular tissues. Shortages of soil water, therefore, tend
to be associated with symptoms such as poor germination, poor and delayed
emergence, stunted growth, general chlorosis, small leaves, poor fruit
development, wilting and death. Annual plants are usually more susceptible to
water shortage than perennials. Symptoms on perennial plants include retarded
branch growth, small scorched leaves, die-back, defoliation and wilting. Water
shortage also reduces the uptake of nutrients.
Oxygen diffuses through air about 1O,OO0times faster than it diffuses through
water so excessive soil water due to poor drainage, water-logging or flooding may
reduce the concentration of oxygen in the soil below the minimum level required
for root growth. Under these conditions, root cells are unable to respire and the
permeability of cell membranes is altered. Consequently, root growth is
restricted, root tips are brown to black rather than white and there are few to no
root hairs. Plants wilt rapidly because water is not absorbed (although freely
available). Unless plants can move oxygen from their leaves to their roots as some
swamp-loving plants can, their growth will be adversely affected.
The severity of the effects of waterlogging depends on the plant species and its
growth stage as well as the duration of waterlogging and the temperature. Seeds
are especially severely affected as they have no leaves to supply oxygen.
Flowering of annuals can be seriously restricted by even a few days excessive
moisture just before flowering. If excessively moist conditions continue, plants
become stunted and their older leaves may be mottled or pale in colour, later
yellowing, becoming brown to black around the edges and dying. Plants
eventually die. The effects of waterlogging appear faster and are more severe as
temperature rises so excessive moisture during summer can kill some plants very
quickly.
When the growing medium is too wet, toxic compounds such as methane,
ethylene or hydrogen sulphide may be produced or the availability of mineral
elements may be altered. For example, iron and manganese become readily
available and may reach toxic levels. Nitrates in waterlogged growing media are
quickly converted to oxides of nitrogen and nitrogen gas which plants cannot use.
Nitrogen deficiency may result. Other soluble nutrients may be lost if leaching
accompanies waterlogging. The effects of excess salt in the growing medium are
exacerbated by excessive moisture in growing media because disrupted
membranes cannot control the entrance of sodium into root cells. Excess water
(or poor aeration) can decrease the ability of beneficial microorganisms to benefit
plants and can produce changes in the composition of the soil microflora. Some
of these microorganisms can produce phytotoxic substances such as organic
acids whilst facultative parasites may actively invade dead roots.
Excessive or irregular watering may also result in disease symptoms. For
example, prunes and apples develop cracks in their skins if plants short of water
early in the season are subsequently irrigated. Similarly, potato tubers grown
under dry conditions followed by an abundance of water may develop a hollow
heart. Irregular watering can also cause cracking on the outer surfaces of
potatoes. Corky tissue frequently develops to protect the cracks from further
injury, but microorganisms may invade causing the tuber to rot.
10. Abiotic diseases oJ plants
163
Chemical properties
Chemical properties of soils and potting media that affect plant growth include
the types of minerals present, the cation exchange capacity and the pH. The pH
of the soil is of particular importance as it affects the availability of elements in
the soil. Plants usually grow well between pH 4 and pH 8, provided adequate, but
not excessive, supplies of all essential elements can be maintained. However, this
is not easy to achieve at the extremes of this pH range. At low pH certain
elements may reach toxic levels. For example, depending on the soil type,
manganese can reach toxic concentrations at pH less than 5.5 and aluminium at
pH below 4.2..Lbove pH 8, supplies of micronutrients become inadequate.
Over 6O elements have been identified in plant tissues. However, only 16 of
these are considered to be essential for the growth of most crop plants. Essential
elements needed in relatively large quantities are called macronutrients. The
macronutrients
are carbon, oxygen, hydrogen, nitrogen, phosphorus,
potassium, calcium, magnesium and sulphur. Plants obtain carbon, oxygen and
hydrogen from the atmosphere so these three elements are usually considered
separately from the remaining essential elements which are sometimes called the
mineral nutrients. Iron, zinc, manganese, copper, boron, molybdenum and
chlorine are essential for plant growth but are needed in relatively small
amounts. These elements are known as micronutrients
or trace elements.
Other elements such as sodium, aluminium, silicon and nickel sometimes have
beneficial effects on plant growth although their essentiality has not been proved.
Cobalt is essential for nitrogen fixation in both legumes and non-legumes.
Deficiencies or excesses of any of these elements can result in plant damage of
potential economic importance.
Plants need a proper balance between the various nutrients for optimal growth
and may show symptoms of deficiencies or toxicities if certain nutrients are
present at too low or too high, respectively, a concentration. Plants differ in their
sensitivities to specific nutrients because they differ in their requirements for
particular elements or in their abilities to absorb certain elements from the soil.
They also differ in their ability to tolerate excessive amounts of elements,
depending on their ability to restrict certain elements from uptake at the root
surface, to keep absorbed elements away from sensitive tissues or to render
potentially toxic elements less biologically harmful.
Symptoms of deficiencies or toxicities are more visible on leaves, but may
occur on any part of a plant, including the stem, fruit and roots.
Diseases due to nutrient deficiencies
Nutrient deficiencies occur when the supply of an element is not adequate to
sustain optimum plant performance. Some soils, such as those derived from
shales and sandstones, are naturally low in nutrients. Rain leaches many
nutrients from soils, leaving them with depleted levels of essential elements.
Plants grown in containers are especially liable to nutrient deficiencies as
nutrients are leached from the growing medium during watering and not replaced
by recycling processes as they may be in soil. Sometimes sufficient nutrient may
be present in growing media to support good plant growth but the nutrient is not
in a soluble form that plant roots can absorb. The non-availability of nutrients is
frequently related to the pH of the soil. For example, zinc and iron deficiencies
occur in alkaline soils, while molybdenum deficiency may be a problem in
vegetable crops and pasture legumes grown in acid soils.
If one or more nutrient elements are lacking, seedlings may fail to develop in
severe cases. If plants grow, they may be stunted and slow in maturing. Plant
164
Helen OgLe
tissue may become necrotic and root growth and development may be modified.
These symptoms may be associated with colour changes in the leaves. Chlorosis,
either interveinal or general, is a common symptom of deficiencies of nutrients
involved in the production of chlorophyll. The accumulation of anthocyanin
pigments which give leaves a reddish colouration may indicate the lack of an
element such as phosphorus. Post harvest diseases of many fruits and vegetables
are related to calcium deficiency while boron deficiency results in diseases such
as hollow stem of cauliflowers and broccoli, cracked stems in celery and distorted
fruit in avocado. The overall results of nutrient deficiencies are low yields and
poor quality.
The type of symptom produced by the deficiency of an element depends mainly
on the role of that element in the plant. Diagnosis of deficiencies must therefore
begin with some knowledge of the physiological roles of the various elements in
plants. These are outlined in Table l0.l together with typical deficiency
symptoms associated with each element. The location of symptoms of nutrient
deficiencies depends on the extent and rate of retranslocation of nutrients from
old leaves to new growth. Nutrients such as nitrogen, phosphorus and potassium
are readily translocated from old leaves to new growth, so deficiency symptoms
occur first in older leaves. Other nutrients such as iron. calcium. zinc and boron
are not readily retranslocated from old leaves to new, so deficiency symptoms
occur first in young growing areas of the plant. A quick guide to nutrient
deficiency symptoms is given in Table 10.2.
Deficiency symptoms may be specific for a particular nutrient in a crop but
often they result from interactions between more than one factor. Similar
symptoms can also be caused by herbicides, disease, insects, water-logging,
drought and mechanical or wind damage. To further complicate diagnosis,
similar symptoms may arise from deficiencies of different nutrients, different
plants may develop different symptoms in response to the same nutrient
deficiency, different deficiency symptoms occur at various stages of plant
development and some nutrient deficiencies predispose plants to secondary
attacks by pathogens and pests.
When nutrient deficiencies are suspected from the characteristics of the
symptoms, further evidence can be obtained by plant analyses, field and pot
trials, soil analyses and bioassay methods. Control or prevention of nutrient
deficiencies may involve fertiliser applications, modifications of soil pH, other soil
management practices and reorganisation of cropping systems.
Diseases due to excess nutrients (toxicities)
When excessive concentrations of nutrient elements occur naturally in soils or
result from improper fertiliser applications, plants may be damaged. The two
major causes of toxicities are salinity and soil acidity.
Under saline conditions, chloride and sodium are common toxicities. Saline
soils not only have excess sodium which can lead to calcium deficiency in plants,
but their poor structure may inhibit root growth. Saline soils are also prone to
crusting which may prevent the emergence of seedlings. Symptoms of excess
salinity include reduced germination of seeds, reduced seedling vigour, yellowing
of younger leaves, burning of leaf margins and tips, yellowing and death of older
leaves, slow growth accompanied by stunting and, in severe cases, death. Plants
may show signs of wilting although the soil appears to be adequately moist.
Plants vary in their tolerance of salinity. Lettuces, French beans, strawberries,
roses and white clover have a low tolerance to salinity while beetroot, cotton,
carnations and lucerne tolerate very high levels (>gO0ppm chloride).
1O. Abiottc diseases of plants
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169
In acid soils, manganese and aluminium, may reach toxic levels. High
manganese availability can be a problem in orchard soils which become acidic
after several years of cultivation, in waterlogged soils and in some potting media
after steam sterilisation. In addition, elements such as lead, cadmium, nickel,
strontium, rubidium and tungsten can cause toxicity symptoms when absorbed
by plants.
Table 10.2.
A quickguideto nutrientdeficiency
symptomsin plants.(Basedon Handreck,1978,
published
fromCSIROLandand Water)
by kindpermission
Symptoms appear first in the OLDEST leaves:
Nitrogen
Magnesium
Potassium
Phosphorus
Molgbdenum
CobaLt
(Excess saltJ
general yellowing, stunting, premature maturity
patchy yellowing, brilliant colours especially around edges
scorched margins, spots surrounded by pale zones
yellowing, erect habit, lack-lustre look, blue-green, purple
colours
mottling over whole leaf but litfle pigmentation, cupping of
leaves and distortion of stems
legumes only, as for nitrogen
(marginal scorching, generally no spotting)
Symptoms appear first in either the OLDEST or YOUNGEST leaves:
Manganese
interveinal yellowing, veins pale green, diffuse, water-soaked
spots, worst in dull weather
Symptoms appear first in the YOUNGEST leaves:
Calcium
Sulphur
Iron
Copper
Zinc
Boron
tiphooking, blackening and death
yellowing, smallness, rolled down, some pigmentation
interveinal yellowing, veins sharply green, youngest leaves
almost white if severe
dark blue-green, curling, twisting, death of tips
smallness, bunching, yellow-white mottling
yellow margins, crumpling, blackening, distortion
Nitrogen, potassium, calcium, magnesium, sulphur, iron, sodium and silicon
are seldom, if ever, toxic. However, if they are taken up in excess, they may cause
imbalances with other nutrients resulting in poor plant growth. For example, too
much potassium can reduce the uptake of magnesium or calcium, possibly
resulting in a deficiency of one of these elements. Similarly, excessive phosphorus
fertilisation can induce zinc or iron deficiency if these elements are in short
supply. Nitrogenous fertilisers, especially ammonium sulphate, and even nitrogen
fixed by legumes, acidiff soils and, therefore change the availability of elements.
Phosphorus, although essential for healthy plant growth, can be toxic to
agricultural species such as wheat, subterranean clover and stylo if it reaches too
high a concentration (more than O.8% in dry matter) in tissues. Many Australian
native plants in the family Proteaceae are especially sensitive to excess
phosphorus (and nitrogen). They evolved on nutrient-poor soils. When nutrients
such as phosphorus and nitrogen become limiting for growth, they develop
proteoid roots which are able to extract phosphorus from sources that are too
t70
HeLenOgIe
insoluble for other plants to use. In cultivation, such plants are easily damaged
or killed by applications of fertilisers.
Under certain circumstances, some micronutrients may be present at toxic
levels. For example, sawdust derived from timber treated with boron for pest
control may be toxic when used as a mulch around susceptible plants. Similarly,
toxicities associated with zinc and cobalt are rare except near mineral deposits
containing these elements or where sewage sludge containing excessive amounts
of these elements is used as a growing medium. Galvanised roofing and water
pipes may raise the zinc levels of water that comes in contact with them to toxic
levels. An excess of one micronutrient can also affect the availability of another.
For example, iron may become unavailable to plants if excess copper or
manganese is present.
Symptoms associated with the presence of excess nutrients (Table 10.l) are
not specific for the different elements, but have many features in common. In
most cases, chlorosis and necrosis will develop, starting in general at the tips
and/or margins of older leaves. Damage is usually the result of direct injury by
the element to the cell. For example, the increased solubility of aluminium in
strongly acid soils can severely stunt roots of sensitive plants such as lucerne,
barley, sunflower and tomato, resulting in serious reductions in yield, especially
in dry seasons.
10.4 Chemicaland physical injuries
Many agricultural ecosystems are highly unstable and require constant
manipulation by farmers to sustain their productivity. These activities often
require the addition of fertilisers to supplement microbial nutrient cycling and
the application of pesticides to control weeds, pests and pathogens. Application of
an inappropriate material, improper timing of application or incorrect application
of any agricultural chemicals can damage plants. For example, prolonged
application of fertilisers and fungicidal sprays containing copper has resulted in
copper levels in soil which are high enough to affect the development of some
types of citrus. Herbicides may damage non-target plants, causing distortion of
plant parts, yellowing, browning, drying and shedding of leaves and sometimes
death. Even if plants are not killed by agricultural chemicals, their injured parts
may be more susceptible to attacks by pathogens.
Various agricultural and other chemicals may also pollute soils. Subsequently,
they may be absorbed by plant roots damaging the plant or they may inhibit
nutrient cycling processes in the soil leading to nutrient deficiencies. Where
industrial pollution is severe or where sewage sludge is used as a soil
amendment, heavy metal elements such as cadmium, cobalt, chromium, lead,
nickel and thallium may reach toxic levels.
In regions where agricultural production is highly mechanised, the probability
mechanical
of
injury to planting material, growing plants and crop products may
be high. Injured plants surfaces are then liable to infection by pathogens.
Bruising is common in pome, stone and tropical fruits after picking, packing and
transport. Handling citrus fruits forces oil from glands in the skin which burns
the fruit surface causing rind-oil spotting (oleocellosis).If the fruit are green when
injured, the injured area will remain green when the rest of the fruit ripens.
Mechanically-harvested root crops such as potatoes and carrots may be cut or
bruised by harvesting equipment. One of the major goals of modern plant
breeding programs is to develop cultivars of fruits and vegetables able to
withstand the rigours of mechanical harvesting and long distance transport.
1O. Abiotic di,seases oJ plants
t71
10.5 Furtherreading
Atkinson, D. (ed.) (1993). GIobaL cltmate change-its impLicattonsJor crop protection.
British Crop Protection Council, Farnham, England.
Bennett, W.F. (ed.) (1993). Nutrient defictenciesand toxicittes in crop plants. APS Press, St
Paul, Minnesota.
Bergmann, W. (ed.) (1992). Nutritiona| dtsorders oJ ptants-uisual and analgttcal
dtagnosts. Gustav Fischer Verlag Jena, Stuttgart. A fuller version of the following
publication.
Bergmann, W. (ed.) (1992). Nutrittonal. disorders of plants-uisual
diagnosis: colour atlas. Gustav Fischer Verlag Jena, Stuttgart.
and analgtical
Frankland, J.C., Magar, N. and Gadd, G.M. (eds)(1996). F\tngiandenuironmentalchange.
Cambridge University Press, Cambridge.
Grundon, N.J. (f 987). Hungrg crops: a guide to nutrient deJiciencies in Jield crops.
Queensland Department of Primary Industries Information Series g 187002.
Handreck, K.A. (1978). What's LDrongusith mg sorl? Discovering soils No.4. CSIRO
Division of soils.
Krupa, S.V. ( 1997). Air pollutton,people and plants. APS Press, St Paul, MN.
Treshow, M. and Anderson, F.K. (1989). Plant stress from atr polLution. John Wiley &
Sons, Chichester.
Weir, R.G. and Cresswell, G.C. (1993-5). Plant nutrient dtsorders: 7. Temperate and
subtropicalJnit and nut crops, 2. TroptcalJruit and nut crops, 3. Vegetable crops, 4.
Pastures andfield crops, 5. Ornamental pLants and shrubs. Inkata Press, Melbourne.