Contents

1 0ABIOTIC DISEASES OF PLANTS

Helen Ogle

lntrodtrct inn ... . . . . . , . . . .Adu er s e w e ather conditions

Temperature .. . , . . . . . . . .HumiditgAtmospheric gases,.L ight . . . . . . . .

1O.3 Aduerse properties of the grotutng medtumPhgsicaL propertiesChemicaL properties

1O.4 Chemical and phgsicaLifiuries1O.5 Further reading

10.110.2

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10.1 Introduction

Diseases caused by abiotic or non-pathogenic factors cannot be transmitted fromaffected plants to healthy plants. Some abiotic diseases are the result of geneticchanges that occur in the meristematic cells of plants, resulting in a flower, fruitor branch differing in appearance from those on the rest of the plant. The extentof the disorder depends on how early in the plant's development the changeoccurs. Citrus plants are particularly prone to such changes which are referredto as chimeras. Leaves on affected twigs may become variegated, fruit may havecorrugated or differently-coloured sectors, branches may develop a willowygrowth habit, stems may be twisted or flattened or galls or rough bark may beproduced on the stems. If the cell change takes place at a very early stage ofdevelopment, the whole plant may be affected. Generally these changes areundesirable, but occasionally a mutant with desirable properties such as early orlate maturing fruit, high productivity or special fruit characteristics occurs. Anew, improved cultivar may result. The Washington Navel orange cultivar, manystrains 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 environmentalconditions. For example, a lack, excess, imbalance or deleterious interaction ofthe physical or chemical factors necessary for healthy plant growth may causeabnormal development. Environmental factors that affect plant growth includetemperature, moisture, light and the properties of the medium in which the plantis growing. For each environmental factor, there is an optimum range withinwhich plants grow best. Plants can tolerate some variations either side of theoptimum for each environmental factor. However, they may exhibit growthabnormalities if the level of an environmental factor varies greatly from itsoptimum, or severely impacts on another factor.

1O. Abtottc diseases oJplants

Plant species have different optima and abilities to tolerate deviations fromoptimum levels. Furthermore, various plant processes (photosynthesis, nutrientuptake, storage of food reserves, leaf growth, etc.) may have different optima forthe same environmental factor. Similarly, a particular process may have differentoptima for an environmental factor at various stages of plant development.Tissues or organs on the same plant may also vary in their optima for aparticular environmental factor. With respect to tolerance of environmentalfactors, three groups of plants have been recognised on the basis of thebiochemical pathways used in photosynthesis: C3 or Calvin cycle plants, Ca ordicarboxylic acid cycle plants and CAM or crassulacean acid cycle plants. Ca andCAM plants have lower transpiration rates than C3 plants. They also tolerate highlight intensity and high temperature better than C3 plants and photosynthesisemore efficiently. These attributes make them more able to tolerate semi-aridsubtropical and tropical conditions.

Sometimes non-pathogenic diseases show characteristic symptoms and thecausal factor(s) can be readily recognised by matching the symptoms withprevailing 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 withthe disease syndrome and then relate the disease to a specific environmentalfactor. Restoring the relevant abiotic factor to a range more favourable for plantdevelopment should result in a return to normal growth. For example, sunflowerplants showing symptoms of phosphorus deficiency resume healthy growth afterthe application of phosphorus ferlilisers. Similarly, in wheat, zinc deficiency canbe corrected by foliar or soil applications of zinc salts.

Abiotic diseases can be caused by adverse weather conditions, adverseproperties of the growing medium as well as chemical or physical injury. Thesefactors can affect all plants, but are usually more important for cultivated plantssubjected to cultural practices such as fertilisation, irrigation or chemicalspraying, especially those grown in artificial environments such as greenhousesor indoors.

10.2 Adverse weather conditionsVarious forms of severe weather such as drought, flood, high winds, frost, hail,snow and lightning may damage or kill plants or provide access sites forpathogens. Variations in 'normal' weather conditions can also affect growthresulting in reduced yield or even death of plants. For convenience, weatherconditions are often discussed as independent variables such as temperature,moisture, light, composition of the atmosphere, etc. In reality, these factorsinteract with each other.

TemperaturePlants generally grow at temperatures ranging from about l"C to 4O"C, with theoptimum range usually being 2O-35"C. However, Ca plants such as many tropicalgrasses, have an optimum range of 3l-37"C and can grow at temperatures up to45-55'C. Many plants can withstand temperatures of 2-7'C if they are protectedfrom wind, but these temperatures can be damaging at wind speeds above 16km/h because of both dehydrating and cooling (wind chill) effects. Perennialplants and dormant organs such as seeds and corms can tolerate more extremetemperatures than annual plants or actively growing organs.

Temperature directly affects physiological processes such as photosynthesis,respiration, membrane permeabil i ty, water and nutrient absorption,

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transpiration, enzyme activity and denaturation of proteins. Because theminimum, optimum and maximum temperatures for each process in anindividual plant can differ (and also differ for the same process in differentplants), plant reactions to temperature can be highly variable. Generally, plantsgrowing at temperatures close to either the upper or lower end of their range willgrow poorly and produce fewer and smaller flowers and fruits.

Low temperatures cause a wide range of damage to plant tissue. The severityof the damage depends on how quickly the temperature drops and also, it isthought, on how quickly tissues thaw out after freezing. Late frosts can damageyoung meristematic tips, entire herbaceous plants and the buds, flowers, youngfruit, new twigs of fruit trees and other deciduous trees. Low temperatures mayeven kill the young roots of trees or cause tree bark to split, allowing cankerdevelopment. Indoor or tropical plants exposed to low temperatures may turnyellow and drop their leaves or buds. Fleshy tissues, such as potato tubers, showvarying degrees of low temperature damage depending on how low and for howlong the temperature drops.

Low temperature damage to plant tissues results from the formation of icecrystals inside plant cells. Ice forms first on the surface of plants and spreadsrapidly in the continuous film of moisture on plant cells to the intercellularspaces. Eventually ice crystals form within cells. If the growth of the ice crystalsruptures 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 onplants at temperatures between -l and -soc. These bacteria grow epiphyticallyon plant surfaces in temperate zones around the world and one effective ice-nucleating bacterial cell on a leaf is sufficient to cause ice formation in that entireleaf.

Chilling injury occurs at much higher temperatures than frost damage. It isnot related to the formation of ice crystals, but to the disruption of enzymesystems which causes a build-up of plant by-products which essentially 'poison'

the plant. Symptoms of chilling, depending on severity, can appear immediatelyafter chilling has occurred, or be delayed for some time. Chilling injury occurscommonly in tropical foliage plants such as Helicon[a as far north as Cairns innorth Queensland (Latitude 17"S). Subtropical plants are damaged at 2-B'Cwhile tropical plants may be injured at 12-13"C. Minor chilling damage in plantsusually results in water soaking, yellowing and browning along the edges ofleaves. Older leaves show symptoms first. If the damage is severe, plants wilt andoften die. Chilling injury also occurs in a wide range of fruits and vegetables ifthey are stored at sub-optimal temperatures after harvest. Tropical fruits may bedamaged at temperatures below lO-15'C while apples can be safely stored at 2-3oC. Oranges stored below their optimum temperature of 5'C may developsunken, brown spots or pits while cucumbers stored at or below 7'C developsurface pitting or dark-coloured water-soaked areas under the skin.

High-temperature injury results from the coagulation and denaturation ofproteins, the disruption of cell membranes and the possible release of toxicproducts into cells. Affected cells may be killed and affected tissues desiccated.As the synthesis of normal protein slows, new proteins called 'heat shockproteins' are produced which help plants resist the effects of high temperatureand other environmental stresses. The severity of high temperature damagedepends 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 thesymptoms to the stress itself.

Plants show a variety of responses to high temperature. Seedlings may showsymptoms of 'stem girdling' due to radiating heat from the soil scorching youngstem tissue. If the stem above the injured zone swells, a 'heat canker' is formed.High temperatures during summer may cause many of the vegetative buds ofplants such as almonds to abscise. The buds that survive either grow vigorouslyor remain dormant leading to very uneven growth. The flesh of ripening stonefruit darkens, especially near the stone, when temperatures exceed 39-4O'C. Ifhigh temperatures are combined with high winds, the fruit may shrivel due tocombined heat and water stress.

In enclosed greenhouses, high temperatures create high humidity whichsignificantly retards transpiration, even though the stomates remain open. Sinceplants normally rely on transpiration for cooling, these conditions can result inheat injury.

HumidityThe moisture content of the atmosphere is usually described by its relativehumidity, the amount of moisture in the air expressed as a percentage of theamount that would be necessary to fully saturate the air at the sametemperature.

Low humidity increases the evaporative demands on plants to the extent thatmoisture stress can occur, even when there is an ample supply of water to theroots. Normally, the evaporation of water from leaf cells in the process oftranspiration cools leaves, preventing them from reaching temperatures at whichdamage occurs. If the supply of water becomes inadequate, the concentration ofabscisic acid rises rapidly, causing the stomates to close. Although closing thestomates 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 resultis tissue damage.

Lack of moisture in the atmosphere is rarely a problem in outdoor plants. It isusually only a temporary condition and seldom causes serious damage. However,if low relative humidity is combined with strong winds, high temperatures andlow soil moisture, plants may wilt temporarily or permanently, leaves may bescorched or burned and fruit shrivelled and burned due to excessive loss ofwater. Plants grown in greenhouses are more prone to damage by low humiditythan plants grown out-of-doors. Misting or fogging is frequently used to maintainthe relative humidity in the range appropriate to the plants under cultivation (7O-85olo is suitable for most plants). Misting or fogging also helps to reducetemperature.

When the relative humidity is high, evaporation from the plant may besuppressed, inhibiting the uptake of nutrients, particularly calcium, from thegrowing medium. The resulting lack of nutrients impairs cell formation andleaves plants vulnerable to a range of disorders. Excessive moisture in theatmosphere may also damage plants directly. For example, the skins of youngimmature cherries may split during periods of prolonged wet weather, exposingthe 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 plantgrowth. However, in sealed glasshouses, carbon dioxide can become a limitingfactor for plant growth. Adding carbon dioxide during daylight hours increases

160 Helen Ogle

yields, improves quality and shortens cropping times. On a global scale, theamount of carbon dioxide in the atmosphere is rising and is expected to doubletowards the end of the 2lst century. It is anticipated that this increase in thecarbon dioxide content of the air will favour the growth of C3 plants rather thanCa plants. However, the rise in temperature (1.5-4.5'C) that will accompany theincrease in carbon dioxide concentration should encourage the growth of Cnplants. The effects of these changes on the occurrence and severity of damagecaused by pathogens are currently being investigated.

In storage, tissues in the centres of fleshy fruits and vegetables may sufferfrom a shortage of oxygen and a build up of carbon dioxide if the storage area isnot correctly ventilated. Blackheart of potato, heart leaf injury in lettuce andbrown heart of apples and pears are all attributed to the excessive accumulationof carbon dioxide in internal tissues during storage. High levels of carbon dioxidein storage may also result in external damage which usually takes the form ofnumerous small sunken pits on the surfaces of fruits and vegetables. While highlevels of carbon dioxide may damage stored produce, properly maintained carbondioxide levels in storage areas delay ripening and senescence, reduce thesensit ivity of stored produce to ethylene-induced changes, help reducephysiological diseases and decay and control insects. This is the principleunderlying the use of controlled atmosphere (CA) for the long term storage ofproduce such as apples and pears. Modified atmosphere (MA), a less precise formof storage than CA, is used in storage and transport of harvested fruits andvegetables.

A large number of phytotoxic chemicals, such as ethylene, nitrogen dioxide,peroxyacetyl nitrates (PANS), ozorre, photochemical smog, hydrogen fluoride andsulphur dioxide, occur in the air over industrial or urban areas. All thesesubstances, if sufficiently concentrated, can damage leaves of susceptible plants.In fact, plants sensitive to particular pollutants may be used to monitor theirlevels in the atmosphere. Of Australian native plants, species of EucaLgptusappear to be most susceptible to sulphur dioxide while Casuarina spp. are mostresistant. Most native species tested are resistant to ozone damage although afew species of Banksia, Acacia, MeLaleuca and Eucatgptus developed acute foliarsymptoms. Plants of any type growing near industrial installations such asfertiliser plants and smelters are likely to be damaged by emission gases such asfluorides and sulphur dioxide.

Oddes of sulphur and nitrogen may be washed out of the atmosphere by rainas sulphuric and nitric acids, respectively, in the phenomenon known as acidrain. Acid rain has a pH below that of 'normal' rain (pH 5.6) and can damageplants and other forms of life over wide areas. Even if plants are not damagedseverely enough to show symptoms, the pollutants may interfere with theirmetabolism, reducing growth, productivity and ability to reproduce.

More localised damage may occur in enclosed areas such as transport andstorage areas and homes. Under these circumstances, gases produced by plantsor plant products (e.g. ethylene, a plant growth regulator), by leaks in coolingsystems (e.g. ammonia) or by engines (e.g. ethylene) may damage plants. Forexample, flowers of Geraldton Wax (ChamaeLaucium unctnatum) packed in boxesfor transport, frequently fall because ethylene produced by flowers infected byfungi such as Botrytis spp. or subjected to water stress after picking, stimulatesflower abscission. Similarly, if fruits or vegetables that produce ethylene as theyripen are stored with those that need ethylene to ripen, considerable losses mayoccur 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

gas, sulphur dioxide. If the concentration of sulphur dioxide reaches too high alevel, sunken, bleached areas sharply demarcated from unaffected tissue willappear on the fruit.

Light

Disease attributed to the effects of 'light' are often difficult to separate from theeffects of other environmental factors, such as temperature. When plants grownin the shade are exposed to excessive light intensities, abnormal photochemicalreactions which inactivate some enzymes and oxidise chlorophyll occur. Theseeffects tend to be more pronounced if oxygen is freely available. Such photo-oxidation processes can lead to chlorosis or bleaching of plant parts andsometimes death of leaves or even the whole plant. In contrast, potato tubersexposed to light during growth or after harvest develop a green colouration due tothe formation of chlorophyll in tissues that do not .to.rnitty contain chlorophyll.Other compounds such as the glycoalkaloid solanine may also form in exposedtissues. Solanine gives potatoes a bitter taste and is poisonous if consumed inlarge quantities.

The contribution of ultraviolet light to photo-oxidation processes is largelyunknown. However, ultraviolet light has been implicated in sunscald of beanpods at high altitudes, in sunscald or atmospheric scorch of peanuts and in solarinjury of melons. Globally, the reduction in the amount of ozone in thestratosphere will be accompanied by an increase in the amount of LIV-B radiation(280-320 nm) reaching the earth. Preliminary studies indicate that increased [JV-B radiation may suppress photosynthesis and, consequently, reduce plantgrowth.

Insufficient light inhibits chlorophyll formation and stimulates cell elongationresulting in etiolated plants with abnormally long internodes and poorlydeveloped, pale green leaves. Etiolated plants are susceptible to lodging andinvasion by parasites. They are not common out-of-doors, but may be foundunder enclosed conditions such as seed beds and greenhouses if light is limited.

10.3 Adverse properties of the growing mediumPlants cannot function properly if the physical or chemical properties of themedium in which they are growing prevent their root systems from obtainingwater, nutrients or oxygen. The components of the medium interact with eachother to produce the conditions under which the plant must grow. Interactionsbetween various properties of the growing medium may cause some plantrequirements to exceed the tolerance range of the plant for that particularrequirement. Accordingly, plant disease symptoms arise from a deficiency, excessor 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, influencethe 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 mayproduce shallow root systems as their roots remain above the compacted layer.Some soils form surface crusts which may retard or prevent the emergence ofseedlings.

The effects of soil water status on physiological processes in plants, especiallythose which can be observed as disease symptoms, are influenced by the

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r62 HeLen OgIe

duration and degree of deficiency or excess of soil water, the physiologicaladaptations of the plant in relation to water balance and interactions with othersoil factors such as nutrient availability, microbial activity and diffusion of gases.

Internal moisture stresses inhibit cell division and elongation and themovement of solutes in vascular tissues. Shortages of soil water, therefore, tendto be associated with symptoms such as poor germination, poor and delayedemergence, stunted growth, general chlorosis, small leaves, poor fruitdevelopment, wilting and death. Annual plants are usually more susceptible towater shortage than perennials. Symptoms on perennial plants include retardedbranch growth, small scorched leaves, die-back, defoliation and wilting. Watershortage also reduces the uptake of nutrients.

Oxygen diffuses through air about 1O,OO0 times faster than it diffuses throughwater so excessive soil water due to poor drainage, water-logging or flooding mayreduce the concentration of oxygen in the soil below the minimum level requiredfor root growth. Under these conditions, root cells are unable to respire and thepermeability of cell membranes is altered. Consequently, root growth isrestricted, root tips are brown to black rather than white and there are few to noroot hairs. Plants wilt rapidly because water is not absorbed (although freelyavailable). Unless plants can move oxygen from their leaves to their roots as someswamp-loving plants can, their growth will be adversely affected.

The severity of the effects of waterlogging depends on the plant species and itsgrowth stage as well as the duration of waterlogging and the temperature. Seedsare especially severely affected as they have no leaves to supply oxygen.Flowering of annuals can be seriously restricted by even a few days excessivemoisture just before flowering. If excessively moist conditions continue, plantsbecome stunted and their older leaves may be mottled or pale in colour, lateryellowing, becoming brown to black around the edges and dying. Plantseventually die. The effects of waterlogging appear faster and are more severe astemperature rises so excessive moisture during summer can kill some plants veryquickly.

When the growing medium is too wet, toxic compounds such as methane,ethylene or hydrogen sulphide may be produced or the availability of mineralelements may be altered. For example, iron and manganese become readilyavailable and may reach toxic levels. Nitrates in waterlogged growing media arequickly converted to oxides of nitrogen and nitrogen gas which plants cannot use.Nitrogen deficiency may result. Other soluble nutrients may be lost if leachingaccompanies waterlogging. The effects of excess salt in the growing medium areexacerbated by excessive moisture in growing media because disruptedmembranes cannot control the entrance of sodium into root cells. Excess water(or poor aeration) can decrease the ability of beneficial microorganisms to benefitplants and can produce changes in the composition of the soil microflora. Someof these microorganisms can produce phytotoxic substances such as organicacids whilst facultative parasites may actively invade dead roots.

Excessive or irregular watering may also result in disease symptoms. Forexample, prunes and apples develop cracks in their skins if plants short of waterearly in the season are subsequently irrigated. Similarly, potato tubers grownunder dry conditions followed by an abundance of water may develop a hollowheart. Irregular watering can also cause cracking on the outer surfaces ofpotatoes. Corky tissue frequently develops to protect the cracks from furtherinjury, 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 includethe types of minerals present, the cation exchange capacity and the pH. The pHof the soil is of particular importance as it affects the availability of elements inthe soil. Plants usually grow well between pH 4 and pH 8, provided adequate, butnot excessive, supplies of all essential elements can be maintained. However, thisis not easy to achieve at the extremes of this pH range. At low pH certainelements may reach toxic levels. For example, depending on the soil type,manganese can reach toxic concentrations at pH less than 5.5 and aluminium atpH below 4.2..Lbove pH 8, supplies of micronutrients become inadequate.

Over 6O elements have been identified in plant tissues. However, only 16 ofthese are considered to be essential for the growth of most crop plants. Essentialelements needed in relatively large quantities are called macronutrients. Themacronutrients are carbon, oxygen, hydrogen, nitrogen, phosphorus,potassium, calcium, magnesium and sulphur. Plants obtain carbon, oxygen andhydrogen from the atmosphere so these three elements are usually consideredseparately from the remaining essential elements which are sometimes called themineral nutrients. Iron, zinc, manganese, copper, boron, molybdenum andchlorine are essential for plant growth but are needed in relatively smallamounts. These elements are known as micronutrients or trace elements.Other elements such as sodium, aluminium, silicon and nickel sometimes havebeneficial 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 ofpotential economic importance.

Plants need a proper balance between the various nutrients for optimal growthand may show symptoms of deficiencies or toxicities if certain nutrients arepresent at too low or too high, respectively, a concentration. Plants differ in theirsensitivities to specific nutrients because they differ in their requirements forparticular 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 rootsurface, to keep absorbed elements away from sensitive tissues or to renderpotentially toxic elements less biologically harmful.

Symptoms of deficiencies or toxicities are more visible on leaves, but mayoccur 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 tosustain optimum plant performance. Some soils, such as those derived fromshales and sandstones, are naturally low in nutrients. Rain leaches manynutrients from soils, leaving them with depleted levels of essential elements.Plants grown in containers are especially liable to nutrient deficiencies asnutrients are leached from the growing medium during watering and not replacedby recycling processes as they may be in soil. Sometimes sufficient nutrient maybe present in growing media to support good plant growth but the nutrient is notin a soluble form that plant roots can absorb. The non-availability of nutrients isfrequently related to the pH of the soil. For example, zinc and iron deficienciesoccur in alkaline soils, while molybdenum deficiency may be a problem invegetable crops and pasture legumes grown in acid soils.

If one or more nutrient elements are lacking, seedlings may fail to develop insevere 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 nutrientsinvolved in the production of chlorophyll. The accumulation of anthocyaninpigments which give leaves a reddish colouration may indicate the lack of anelement such as phosphorus. Post harvest diseases of many fruits and vegetablesare related to calcium deficiency while boron deficiency results in diseases suchas hollow stem of cauliflowers and broccoli, cracked stems in celery and distortedfruit in avocado. The overall results of nutrient deficiencies are low yields andpoor quality.

The type of symptom produced by the deficiency of an element depends mainlyon the role of that element in the plant. Diagnosis of deficiencies must thereforebegin with some knowledge of the physiological roles of the various elements inplants. These are outlined in Table l0.l together with typical deficiencysymptoms associated with each element. The location of symptoms of nutrientdeficiencies depends on the extent and rate of retranslocation of nutrients fromold leaves to new growth. Nutrients such as nitrogen, phosphorus and potassiumare readily translocated from old leaves to new growth, so deficiency symptomsoccur first in older leaves. Other nutrients such as iron. calcium. zinc and boronare not readily retranslocated from old leaves to new, so deficiency symptomsoccur first in young growing areas of the plant. A quick guide to nutrientdeficiency symptoms is given in Table 10.2.

Deficiency symptoms may be specific for a particular nutrient in a crop butoften they result from interactions between more than one factor. Similarsymptoms 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, differentplants may develop different symptoms in response to the same nutrientdeficiency, different deficiency symptoms occur at various stages of plantdevelopment and some nutrient deficiencies predispose plants to secondaryattacks by pathogens and pests.

When nutrient deficiencies are suspected from the characteristics of thesymptoms, further evidence can be obtained by plant analyses, field and pottrials, soil analyses and bioassay methods. Control or prevention of nutrientdeficiencies may involve fertiliser applications, modifications of soil pH, other soilmanagement practices and reorganisation of cropping systems.

Diseases due to excess nutrients (toxicities)

When excessive concentrations of nutrient elements occur naturally in soils orresult from improper fertiliser applications, plants may be damaged. The twomajor causes of toxicities are salinity and soil acidity.

Under saline conditions, chloride and sodium are common toxicities. Salinesoils 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 tocrusting which may prevent the emergence of seedlings. Symptoms of excesssalinity include reduced germination of seeds, reduced seedling vigour, yellowingof younger leaves, burning of leaf margins and tips, yellowing and death of olderleaves, slow growth accompanied by stunting and, in severe cases, death. Plantsmay 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 (>gO0 ppm chloride).

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In acid soils, manganese and aluminium, may reach toxic levels. Highmanganese availability can be a problem in orchard soils which become acidicafter several years of cultivation, in waterlogged soils and in some potting mediaafter steam sterilisation. In addition, elements such as lead, cadmium, nickel,strontium, rubidium and tungsten can cause toxicity symptoms when absorbedby plants.

Table 10.2. A quick guide to nutr ient def ic iency symptoms in plants. (Based on Handreck, 1978,publ ished by kind permission from CSIRO Land and Water)

Symptoms appear first in the OLDEST leaves:

NitrogenMagnesiumPotassiumPhosphorus

Molgbdenum

CobaLt(Excess saltJ

general yellowing, stunting, premature maturitypatchy yellowing, brilliant colours especially around edgesscorched margins, spots surrounded by pale zonesyellowing, erect habit, lack-lustre look, blue-green, purple

coloursmottling over whole leaf but litfle pigmentation, cupping of

leaves and distortion of stemslegumes 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-soakedspots, worst in dull weather

Symptoms appear first in the YOUNGEST leaves:

CalciumSulphurIron

CopperZincBoron

tiphooking, blackening and deathyellowing, smallness, rolled down, some pigmentationinterveinal yellowing, veins sharply green, youngest leaves

almost white if severedark blue-green, curling, twisting, death of tipssmallness, bunching, yellow-white mottlingyellow margins, crumpling, blackening, distortion

Nitrogen, potassium, calcium, magnesium, sulphur, iron, sodium and siliconare seldom, if ever, toxic. However, if they are taken up in excess, they may causeimbalances with other nutrients resulting in poor plant growth. For example, toomuch potassium can reduce the uptake of magnesium or calcium, possiblyresulting in a deficiency of one of these elements. Similarly, excessive phosphorusfertilisation can induce zinc or iron deficiency if these elements are in shortsupply. Nitrogenous fertilisers, especially ammonium sulphate, and even nitrogenfixed by legumes, acidiff soils and, therefore change the availability of elements.

Phosphorus, although essential for healthy plant growth, can be toxic toagricultural species such as wheat, subterranean clover and stylo if it reaches toohigh a concentration (more than O.8% in dry matter) in tissues. Many Australiannative plants in the family Proteaceae are especially sensitive to excessphosphorus (and nitrogen). They evolved on nutrient-poor soils. When nutrientssuch as phosphorus and nitrogen become limiting for growth, they developproteoid roots which are able to extract phosphorus from sources that are too

t70 HeLen OgIe

insoluble for other plants to use. In cultivation, such plants are easily damagedor killed by applications of fertilisers.

Under certain circumstances, some micronutrients may be present at toxiclevels. For example, sawdust derived from timber treated with boron for pestcontrol may be toxic when used as a mulch around susceptible plants. Similarly,toxicities associated with zinc and cobalt are rare except near mineral depositscontaining these elements or where sewage sludge containing excessive amountsof these elements is used as a growing medium. Galvanised roofing and waterpipes may raise the zinc levels of water that comes in contact with them to toxiclevels. An excess of one micronutrient can also affect the availability of another.For example, iron may become unavailable to plants if excess copper ormanganese is present.

Symptoms associated with the presence of excess nutrients (Table 10.l) arenot specific for the different elements, but have many features in common. Inmost cases, chlorosis and necrosis will develop, starting in general at the tipsand/or margins of older leaves. Damage is usually the result of direct injury bythe element to the cell. For example, the increased solubility of aluminium instrongly acid soils can severely stunt roots of sensitive plants such as lucerne,barley, sunflower and tomato, resulting in serious reductions in yield, especiallyin dry seasons.

10.4 Chemical and physical injuriesMany agricultural ecosystems are highly unstable and require constantmanipulation by farmers to sustain their productivity. These activities oftenrequire the addition of fertilisers to supplement microbial nutrient cycling andthe application of pesticides to control weeds, pests and pathogens. Application ofan inappropriate material, improper timing of application or incorrect applicationof any agricultural chemicals can damage plants. For example, prolongedapplication of fertilisers and fungicidal sprays containing copper has resulted incopper levels in soil which are high enough to affect the development of sometypes of citrus. Herbicides may damage non-target plants, causing distortion ofplant parts, yellowing, browning, drying and shedding of leaves and sometimesdeath. Even if plants are not killed by agricultural chemicals, their injured partsmay 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 inhibitnutrient cycling processes in the soil leading to nutrient deficiencies. Whereindustrial pollution is severe or where sewage sludge is used as a soilamendment, 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 probabilityof mechanical injury to planting material, growing plants and crop products maybe high. Injured plants surfaces are then liable to infection by pathogens.Bruising is common in pome, stone and tropical fruits after picking, packing andtransport. Handling citrus fruits forces oil from glands in the skin which burnsthe fruit surface causing rind-oil spotting (oleocellosis). If the fruit are green wheninjured, 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 orbruised by harvesting equipment. One of the major goals of modern plantbreeding programs is to develop cultivars of fruits and vegetables able towithstand the rigours of mechanical harvesting and long distance transport.

1O. Abiotic di,seases oJ plants

10.5 Further reading

Atkinson, D. (ed.) (1993). GIobaL cltmate change-its impLicattons Jor crop protection.British Crop Protection Council, Farnham, England.

Bennett, W.F. (ed.) (1993). Nutrient defictencies and toxicittes in crop plants. APS Press, StPaul, Minnesota.

Bergmann, W. (ed.) (1992). Nutritiona| dtsorders oJ ptants-uisual and analgttcaldtagnosts. Gustav Fischer Verlag Jena, Stuttgart. A fuller version of the followingpublication.

Bergmann, W. (ed.) (1992). Nutrittonal. disorders of plants-uisual and analgticaldiagnosis: colour atlas. Gustav Fischer Verlag Jena, Stuttgart.

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 LDrong usith mg sorl? Discovering soils No.4. CSIRODivision 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 andsubtropicalJnit and nut crops, 2. TroptcalJruit and nut crops, 3. Vegetable crops, 4.Pastures andfield crops, 5. Ornamental pLants and shrubs. Inkata Press, Melbourne.

t71