22 General Concepts IPM

8/9/2019 22 General Concepts IPM

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22DISEASE ANAGEMENT:ENERAL ONCEPTS

JohnBrown,HelenOgleandMicheleDaleContents22.1 Introdtrction .......... 4322.2 The pLant disease triangLe ...,. 343ExcLuding r reducing nocuLum.. ........... 44Increasing the host's reststance o disease ......... 344Modi{ying the enutronment .. 34522.3 The epidemiologtcaLpproach.......... .... 345Redtrcing the amount oJ initiat inocuLum(xi) . . . . , 349Redtrcing the time auaiLabLeJor he epidemic to deueLop t) . 349Reducing he rate oJdisease ncrease r)......,......................5O22.4 Factors tnfluenctng disease management strategies ........ .... 35OThe economic duantageoJcontro1............ ........... 5OTheJeasibi l i tg JcontroL ......... ........... 51The optionsauaiLabLe ...........351The disease in perspectiue .. 35222.5 Integrateddiseasemandgement,...,...... . . . . . . . . . . . . . . .5222.6 CropheaLthmanagement. . . . . . . , , . . . . . . . . . . . . 54ConJlicttng and compLementary management strateg es . 3 5 522.7 Conclusion ........... 5722.8 Fbrther reading .... 357

22.1 IntroductionIn undisturbed natural ecosystems, relationships between pathogens and theirhosts tend to attain a level of stability and plant populations are rarely severelydamaged by diseases. However, in artificial or disturbed systems that exist inagriculture, horticulture and silviculture, the development of disease epidemicsoften seriously limits production. Epidemics result from a progressive increase ofdisease in time and space. The development of epidemics depends on a regularchain of events, each link of which is dependent on certain specific requirementsbeing satisfied simultaneously. In this chapter, some of the components ofepidemics will be described together with ways in which they can be manipulatedto retard the progress of epidemics and thereby reduce production losses.22.2 The plant disease riangleOne approach to understanding epidemics and how diseases can be managed isthe application of the plant disease triangle (Fig.l.l and Chapter 18). Allpathogenic diseases result from an interaction between three factors, the host,the pathogen and the environment. Epidemics result if all three factors arefavourable for disease development. In other words, an epidemic will develop ifthere is an abundance of a susceptible host, a virulent and aggressive pathogenand weather conditions that favour the rapid multiplication and spread of thepathogen.


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344 JohnBrotun, Heten OgIe and Michele Dale

If any side of the disease triangle is not favourable, an epidemic cannotdevelop. Thus disease can be managed by excluding or reducing the level ofinoculum of the pathogen, by increasing the host's resistance to disease or bymodiffing the environment so that it is unsuitable for the multiplication andspread of the pathogen and the subsequent development of the disease.Excluding or reducing inoculumIf a particular pathogen is not present in an area, attempts should be made toprevent its introduction. Plant quarantine legislation in most countries protectscommercial crops and native flora from exotic pathogens. Quarantine regulationsprohibit or restrict the introduction of plant material from areas where there ishigh risk of contamination by unwanted pests and pathogens. For example, thereare a number of pathogens of Australian native plants that are not present inAustralia. The rust Pucciniapsidii infects guava (Psidtum guqjaua) in Central andSouth America. It also infects Australian species of CaLListemon, rrcalyptus andMeLaleuca grown in that region. The economic and environmental cost ofintroducing P. psidit into Australia is uncertain, but could be devastating.Quarantine regulations are in force to reduce the risk of pathogens such asguava rust entering Australia.Seed certification schemes also reduce the risk of introducing unwantedpathogens into new areas. For example, certified bean (PhaseoLusuutgaris) seedin Australia is free from the pathogens that cause anthracnose (the fungusCoLtetotrichum Lindemuthianum), halo blight (the bacterium Pseudomonassgringae pv. phaseolicola), bacterial brown spot (Pseudomonas sgringae pv.syringae), common bacterial blight (Xanthomonos campestrts pv . phaseoli),peanut mottle virus and bean common mosaic virus (lolo olerance).Planting material can also be treated to kill inoculum. For example, treatmentwith fungicides can kill inoculum in or on the surface of seeds. Vegetativepropagating material can be treated with chemicals or hot water. Manycommercially available seeds of vegetables, ornamentals and other plants aretreated with fungicides or insecticides before they are sold.If a pathogen is already present in an area, attempts can be made to eliminateit or to reduce the amount of inoculum present. Most attempts to eliminate apathogen once it has become established have been unsuccessful (e.g.coffee rustin Papua New Guinea and South America). Methods used to reduce levels ofinoculum include rotations with non-host species, removal of alternative andalternate host species, chemical (e.9. soil fumigation) and physical (e..solarisation) treatments, removal of diseased plants which act as sources ofinoculum (roguing), the use of decoy or trap crops and destruction of insect andother vectors.lncreasing the host's resistance to diseasePerhaps the best way to protect plants from disease is to select or breed plantsfor genetic resistance to specific diseases. The breeding of rust resistant cereals isa good example of how resistance can be used as a disease management strategr.A plant's resistance to disease can also be increased by the application ofchemicals. Protectant fungicides place a layer of fungicide on the host's surfacewhich prevents fungal spores from germinating. The fungicide does not move intothe plant and therefore has no effect on infections that are already established.Most protectant fungicides provide short term protection and have to be appliedevery 7-15 days to provide adequate protection to the plant.


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22. Disease management: general concepts 345Systemic fungicides can also be used to increase the host's resistance todisease. Systemic fungicides or chemotherapeutants are absorbed by the plantand translocated to various tissues. They are effective against fungi that havealready become established within the plant as well as preventing new infectionsfrom developing. Many systemic fungicides have a narrow spectrum of activity

and are only effect ive against specific fungi. Because many systemic fungicidesinhibit specific biochemical activities, there is a tendency for the target fungus todevelop resistance to the fungicide. Systemic chemicals are also used to controlnematode diseases. However. no chemicals that control virus or bacterialdiseases are currently registered for use in Australia, although antibiotics (e.g.streptomycin) are used to control some bacterial diseases in New Zealand andelsewhere.Mod fyi ng the envi ron mentIn some instances it is possible to alter the environment so that it is suitable forplant growth but unsuitable for the development of plant disease epidemics. Ingreenhouse situations it is possible to control the physical environment(temperature, light intensity and day length, humidity and duration of the dewperiod on leaves) as well as the chemical environment. For example, thequantities and ratios of various nutrients can be controlled, particularly whereplants are grown hydroponically.With field-grown crops it is usually more difficult to manage diseases byaltering the environment. There are however, some instances where this has beenachieved. Collar rot of citrus caused by Phgtophthora citrophthora has beenmanaged by improving soil drainage. Plants used to produce vegetable and flowerseeds are often grown in semi-arid areas where flood irrigation is available, toreduce the incidence of seed-borne diseases. Under these conditions the seedproduced is relatively free from infection and contamination by these pathogens.Time of sowing can also be used as a disease management strategy. In somelocations plants can be sowrr at a time when they will not be at a susceptiblestage of growth when the environment favours disease development. For examplealternaria blight of sunflower caused by ALternaria heLianthi causes significantreductions in yield in the central coast and central highlands of Queensland butnot in other sunflower growing regions of Australia. The conditions required forrapid disease development are (i ) warm temperatures (optimum 26'C), (ii)extended periods of wet weather and (iii) maturing plants after thecommencement of flowering. These conditions are frequently met in spring-sowncrops in central Queensland which mature during the summer months whenmean daily temperatures are between 25 and 3O"C and when the chance of rain-bearing cyclonic depressions is high. If plants are sown so that they matureduring the cooler and drier autumn months, when rainfall is less likely to occur,the probability of epidemics developing is low. The micro-environment within acrop can also be modified by changing cultural practices such as plant density,plant stature, method of irrigation and plant debris retention practices.22.3 The epidemiologicalapproachEpidemiologr deals with the build-up and spread of disease in populations andthe factors that influence the development of epidemics. The South Africanscientist J.E. Van der Plank was among the first to quantitatively andmathematically describe plant disease epidemics. He formulated a number oftheories on epidemiology and in his book published in Ig63 he described two


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346 John Brotun, Helen OgLeand Mtchele Dale

types of pathogens-monocyclic or 'simple interest' pathogens and polycyclic or'compound interest' pathogens.Monocyclic pathogens complete only one infection cycle in a growing seasonof the host. The primary inoculum causes the disease and there is no productionof secondary inoculum to contribute to development of the disease during thegrowing season. Consequently, the intensity of the disease depends on thequantity and quality of the primary inoculum present at the beginning of thegrowing season. Such pathogens can be compared with a simple interestmonetary model in which 'interest' is not added to the 'capital investment' untilthe end of the season (or investment period).Epidemics of simple interest pathogens develop relatively slowly but can beimportant, particularly in perennial crops where pathogen populations build-upover a number of years or in annual crops which are grown continuously in thesame field. The disease progress curve of a typical simple interest disease is oftena straight line that flattens out when host tissue available for infection becomeslimiting (Fig. 22.1,4.).Monocyclic pathogens include the vascular wilt fungiFlrsartum oxAsporum (e.g. F. oxAsporum f. sp. cubense) andVerttctltium dahltae[e.. vertici l l ium (vascular) wil t of cotton], root rotting fungi such asGaeumannomAces gramtnis (the cause of take-all in cereals) as well as the clubroot fungus of crucifers (Plasmodiophora brasstcae) and the flag smut fungus ofwheat (Urocystis agropgrt). Soil-borne nematodes such as the root knot and cystnematodes and bacteria such as the vascular wilt pathogen RalstontasoLanacearum also exhibit simple interest disease epidemiology. Van der Plankconsidered that simple interest diseases nvolved pathogens with a'low birth rate'(i.e. ew propagules are produced)and a'low death rate' (i.e.spores produced hada long survival period in soil).Polycyclic pathogens complete more than one infection cycle in a growingseason. The time between inoculation and the appearance of symptoms, whichmay be as short as 3-4 days for some fungi such as ALternaria helianth| and thenumber of generation cycles that occur in a growing season is influenced by theinteractions that occur between the host, the pathogen and the environment.Many airborne pathogens such as the rusts, downy mildews, powdery mildewsand the potato late blight fungus are polycyclic pathogens.Polycyclic pathogens have a'high birth rate' (many propagules produced) anda corresponding 'high death rate' (short-lived propagules and a high rate ofwastage). The disease progress curves for compound interest diseases aretypically sigmoidal or S-shaped (Fig. 22.18) with the three phases of the curvebeing:. the initial lag phase due to the limited amount of inoculum present at the startof the epidemic (low levels of inoculum, abundant host tissue),. the exponential or logarithmic phase when neither inoculum levels nor theamount of host tissue available for infection is limiting,. the decline or plateau phase due to the limited amount of host tissue availableto become infected (abundant inoculum, little host tissue remaining).Diseases caused by polycyclic pathogens can be described as compoundinterest diseases when using a monetary analogy. The 'interest' (the newinoculum produced) is continuously being added to the 'interest-bearing capital'(previous amount of inoculum). Consequently, during the very early stages of anepidemic, disease progress can be represented by the compound interestequation' X, = Xirt


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where ^ t -

x : =- 1

r -+ -L -

22. Disease management: general concepts 347

the proportion of host tissue that is diseased at time t (o rcapital plus interest at the end of the investment period (t) if amonetary analogy is used)the amount of disease at the start of an epidemic or at theinitial observation (or the interest-bearing capital initiallyinvested)the rate of disease increase (or the interest rate)the time available for the epidemic to develop (or the length ofthe investment period)the exponential constant (2.718).

10 0ad - -

fr3 6 0C)a

20

TimeA

TimeB

Figure 22.1 Idealised diseaseprogresscurves. (A)A simple interest diseaseepidemicshowing a straight line epidemicprogresscurve. (B)A compound nterestdiseaseepidemic showing a sigmoidal or S-shapedepidemicprogresscurve, (a) nitial lag phase,(b)exponentialphaseand (c)declinephase.However, as an epidemic progresses, the amount of healthy host tissueavailable for infection is continually being reduced and logarithmic developmentdoes not continue. Because of this the compound interest equation has to bemodified at disease levels above about 5o/oof host tissue infected. This can bedone in various ways but logit transformation is generally used. This consists of

piacing disease incidence on the basis of log [x/(l-x)]. Some logit transformationsof disease progress curves are shown in Figure 22.2 where it can be seen that thesigmoid curves have been transformed to straight lines.During the early stages of an epidemic, when disease levels do not exceedabout 5%, the average infection rate (r) can be calculated using the equation' =/ ' loe ' f

where the amount of disease at time tr = Xr and the amount of disease at time tris x".


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JohnBrotun , HeLen OgLeandMichele DaLe

However, when disease levels are over about 5o/ohe progress of the epidemicdepends on the amount of host tissue still available for colonisation and thefactor (f - d is introduced into the equation to calculate r giving

'=,,+"e.G1{+=#)O B

A0.40 . 2

I. l r l nlJ t r Ul:-

Iq)o r

-2-3

A/ Bl CY,/DB

AUO. Aug. 6 Sept. Sept. 6 Oct. 1Figuxe 22.2 Epidemicprogress urvesof late blight on potatoescausedby Phgtopttthoratnfestans on different host cultivars and during different seasons.(A)Untransformed data. (B) Logit transformation of data. (FromVan derPlank, 1963.)

The exact nature of the equations need not concerrl us here. What is importantin terms of developing disease management strategies is that the level of infectionpresent in a crop at any particular time depends on the amount of inoculumpresent at the beginning of the epidemic, the rate at which the epidemicincreases and the time available for the epidemic to develop. Obviously, anyreduction in the level of the initial inoculum (xJ, the rate of disease increase (r) orthe time available for disease development (t ) wiil automatically reduce theamount of disease.There is one exceptional situation where losses caused by disease can becontrolled without reducing xi, r or t. This occurs when cultivars are tolerant ofpathogens but not resistant to them. There is much confusion in the literatureabout disease tolerance. The only useful definition of a tolerant cultivar is 'acultivar which does not restrict the development of a pathogen but in which


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damage is restricted'. If this definition is used, there are very few valid examplesof tolerant cultivars. However. there can be little doubt that some cultivars ofvarious crops are tolerant of certain viruses. Following infection the virusmultiplies normally but causes little damage. There are also a few examples ofcultivars tolerant of fungus and nematode pathogens.Reducing the amount of initial inoculum (x1)The initial amount of inoculum (xil can be kept at zero by avoiding the pathogenaltogether. This can be achieved by growing crops in regions where the pathogendoes not occur, by choosing pathogen-free planting sites within a particular area,by choosing planting dates that ensure that the crop is not at a susceptible stageof growth when the pathogen is active or by using disease-free planting material.The amount of initial inoculum can also be kept at zero by excluding thepathogen from a region. Strategies used to prevent the introduction of pathogensinto new areas include quarantine regulations, sanitation, use of plantingmaterial certified to be free from disease, and treatment of planting material tokill any pathogens on or in it. Destruction of vectors, such as flying insects, thatmay carry pathogens into an area will also keep the level of initial inoculum atzero.If the pathogen is already present in an area, the level of inoculum can bedecreased by various forms of sanitation such as crop rotation, destruction ofdiseased debris, removal of alternative and alternate hosts, removal anddestruction of diseased plants or plant parts and soil fumigation. Pathogens maybe eliminated from planting material by heat or chemical treatments.Many of these strategies are used to reduce the amount of inoculum of bananabunchy top virus (see Chapter 3O) n southern Queensland. If disease is found,infected plants and the aphid vectors of the virus must be destroyed. Permitsissued free of charge by the Department of Primary Industries (DPI) are requiredbefore planting. Home gardeners may only grow up to five Lady Finger bananaplants. Cavendish bananas, which are more susceptible to bunchy top, but oftendo not show symptoms, are prohibited on residential land. Only disease-freeplanting material approved by the DPI may be used. Protection of plants frominfection also reduces the initial level of inoculum. Plants can be protected byspraying or dusting with biocides. Plant propagules can be treated withchemicals or micro-organisms to protect against infection. In some virusdiseases, inoculation with a benign or mild strain of the virus protects plantsagainst subsequent infection by more virulent forms of the virus. Specific genesfor resistance in plant populations prevent some or all of the available inoculumfrom infecting a crop. If a new pathotype arises which can attack plantscontaining the resistance gene, the disease epidemic will develop as it did beforethe resistance gene was introduced.Reducing the time available for the epidemic to develop (t)The time available for an epidemic to develop can be reduced in some instancesby using early-maturing cultivars which are harvested before the disease hasdeveloped to epidemic proportions. Similarly, with some fruit crops such asbananas, the fruit is picked early (before it is ripe and before postharvestdiseases associated with ripening have had time to develop) and later artificiallyripened by exposure to ethylene under environmental conditions that suppressdisease development. With pastures and forage crops the plants can be grazed orharvested early (e.g. to make hay) if there is a risk of an epidemic developing. Inmany crops however, it is not possible to alter the time available for epidemicdevelopment.


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350 John Broun, Helen OgLeand MicheLeDaLeReducing the rate of disease increase(r)The rate of disease increase can be reduced in a number of ways. Slow rusting,slow mildewing and slow blighting types of resistance reduce the rate of diseaseincrease, as do multilines or cultivar mixtures in which not all individuals aresusceptible to a given strain of a pathogen (see Chapter 26 for more details).Chemical treatments can also reduce the rate of disease development. Protectantfungicides reduce the level of spore germination which results in fewer lesionsbeing formed and less inoculum being produced. Systemic fungicides whichdestroy established infections can also influence the rate of disease developmentas well as the level of initial inoculum. Controlling plant pathogens with othermicro-organisms will also reduce the rate of development.The rate of disease increase is also influenced by climate. Apart from theexpensive process of growing plants in greenhouses, the environment to which acrop is exposed can be altered by changing planting dates, by changing thedensity of the host population, by modiffing irrigation practices or by growingcrops in different climatic areas. The nutritional status of plants also influencestheir susceptibility to disease. Sanitation practices such as removing diseasedplants or eliminating vectors can also reduce the rate of epidemic development.When implements are involved with spread of soil-borne inoculum (e.9.Phgtophthora ctnnamomi in the Jarrah forests of Western Australia), cleaning andsterilising the instruments reduce the rate of epidemic development.Details of the tactics used in each of the major strategies mentioned above willbe discussed in the chapters that follow. In practice, an integrated approach todisease control is recommended. For maximum effect, the strategies used shouldattempt to reduce simultaneously the amount of initial inoculum, the amount oftime available fo r disease epidemics to develop and the rate of epidemicdevelopment.22.4 Factors influencingdisease managementstrategiesThere are many strategies available to manage or control plant diseases.However, before the most appropriate strategy can be recommended severalpractical considerations must be taken into account. These include the economicadvantage of control, the feasibility of control, the options available and thedisease in perspective.The economic advantage of controlIt is reasonably true to say that most plant diseases can be controlled with theaid of modern knowledge and technologr. However, the economic advantage ofcontrol must be a major consideration before any management strategy isrecommended. There is no point in recommending a control strategy that wouldcost more than the crop is likely to return to the farmer.To determine the economic advantage of control, it is necessary to estimate theIikely economic return from the crop if no action is taken (i.e. if the disease is leftuncontrolled), the likely economic return if the disease is controlled by aparticular method as well as the cost of the various control options including costof materials, labour costs and any damage likely to be caused by the treatment(e.g. mechanical or chemical damage). Having estimated the various costs andbenefits likely to result from various control options, the economic advantage ofany given option can be determined by the following formula:


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Economic Expected return ifadvantage of = disease isdisease control ($) controlled ($)Cost of* controltreatment

It is not always possible to predict the economic advantage of control,particularly in regions such as Australia where the climate is erratic. In suchareas crop yields vary greatly from season to season as does the intensity ofdisease epidemics. It is not possible therefore to predict, with any degree ofaccuracy, the expected returns if no action is taken and to compare this with thelikely returns if the disease is controlled. However, in regions that experiencereliable climates or in situations where the climate can be modified by artificialmeans (e.g. rrigation areas) it is possible to predict crop yields and the pattern ofepidemic development.The feasibility of controlEven though a cost-benefit analysis might indicate an economic advantage if aparticular control option is implemented, one might decide not to recommend anycontrol option. There is no point in recommending a control strategy thatrequires equipment or skills which are not available in the region concerned. Forexample, research in the Solomon Islands in the l97Os showed that theincidence of pod rot of cocoa caused by Phgtophthora paLmiuora could be reducedby harvesting all mature fruits at monthly intervals and destroyrng diseased podsby deep-burying. This practice reduced the amount of inoculum available forinfection and the amount of disease that subsequently developed. However, thepractice was not a feasible option in the Solomon Islands at the time. In thevillages where most of the cocoa was grown, the inhabitants were subsistencefarmers who did not rely on a consistent cash flow. Money only becameimportant in times of celebrations and emergencies. Consequently, cocoa wasonly harvested at irregular intervals when money was required. Similarly,fungicide application was not feasible because although farmers may have hadaccess to hand-operated knapsack sprayers, fungicides were not availableoutside the capital crty.The options availableAny control recommendation should always take into account the specificproblem in question. The options available to growers must be considered. Toillustrate this point the following hypothetical example is given. Suppose aparticular vegetable crop shows about 4Oo/onfection with a specific virus. Alsosuppose that the level of infection would make it unprofitable to continue tocultivate and irrigate the crop and to subsequently harvest and market theproduct at a profit. The question that arises is whether or not to destroy the crop.If the crop is ploughed under it may be possible to grow an alternativeopportunity crop of some other species that does not become infected with theparticular virus in question. What economic return would be expected from thisalternative crop and how would the growing of this opportunity crop interferewith the subsequent use of the land? Should the grower remove and destroy thediseased plants to reduce spread of the disease or let them grow without furtherinputs and perhaps harvest the seed rather than the vegetable product forsubsequent sale? Is the virus seed-borne? These and various other options and

I Expected eturn ifI the diseases left(lrncontrolled (s) J


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352 John Brotun, Helen Oqle and Michele DaLeconsiderations must be taken into account before the most appropriatemanagement action can be taken.The disease in perspectiveThe threat or danger of a disease to an industry as a whole or to native vegetationwill influence the control strategy used. If a disease is caused by a newlyintroduced parasite no t previously found in the area and if ttre diseaslconstitutes a serious threat to an important industry, a control strategyregardless of cost might be instigated. Governmental assistance might benecessary to finance such a control program. For example, when coffee rusttHemtlea uastatrix) was observed in Papua New Guinea in 1965, promptdestruction of trees in the affected area and spraying trees at risk with fungicidesby government authorities eliminated the disease. This exercise cost about$7O,OOO xcluding labour costs. Unfortunately, when the rust was reintroduced20 years later, attempts to eradicate it were unsuccessful.22.5 Integrateddisease managementIntegrated Pest Management (IPM) is a system for managing pests that uses allsuitable techniques in ways that complement one another. It aims to reduce pestpopulations and keep them below threshold levels at which economic injuryoccurs. The concept was developed by entomologists in the late t95Os as a resultof the evolution of strains of insects that were resistant to widely usedinsecticides such as DDT, the resurgence of insect pests that had previously beencontrolled by insecticides and the emergence of secondary pests in situationswhere the major pest had been controlled. In addition, the publication in 1963 ofRachel Carson's book 'Silent Spring' made the public a\Mare hat insecticidessuch as DDT had detrimental effects on cerlain environments.

Plant pathologists have not taken the same level of interest as entomologists inintegrated management of diseases largely because there has always been anelement of integrated control in plant pathology. Disease control has not beenreduced to a 'one shot' approach relying on one control strategy (usually theapplication of chemicals). Multiple approaches have been used to modiff theprogress of epidemics at several points in their development. For example, inmajor grain and forage crops, resistant varieties are often used in conjunctionwith crop sanitation, crop rotations, removal of diseased plants and alternate oralternative hosts and manipulation of the environment. Intensive spray scheduleshave only been developed for fruit and vegetable crops that require anunblemished appearance for acceptable product quality. Even with these crops,spray schedules are combined with other practices such as crop sanitation, useof disease-free planting material and modification of the environment to ensuredisease control.When the disease cycle and the modes of survival and dissemination are notfully understood, multiple approaches to disease control are needed. Oneexample of a disease for which some of this information is still being elucidated ispod rot of cocoa caused by the fungus Phgtophthora paLmiuora (Fig. 22.3). pod rotis the most serious disease affecting cocoa worldwide and reduces yields by about3Oo/o.Symptoms include pod rot, stem and flower cushion cankers (importantbecause flower cushions produce flowers year after year), leaf, main stem andseedling blights and sudden death due to the formation of stem cankers.The fungus survives as chlamydospores and saprophytic mycelium in the soiland as mycelium in flower cushions, young fruit (cherelles), stems and leaves onthe cocoa tree and in plant debris. Traditionally, cocoa pods are harvested, their


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22. Disease management: general concepts 353husks split open and the seeds removed in the field. The unwanted pod husksare discarded under the trees and provide a source of inoculum for furtherinfection. Rainsplash and aerosol dispersal of inoculum from discarded podhusks and surrounding infested soil are imporlant up to a height of about onemetre while ants are responsible for moving inoculum into the higher parts ofplants. Tent-building ants carrlr soil containing propagules of the fungus into thecanopy to build their tents. In the process, they initiate peduncle infections. Tree-dwelling ants construct their tents from plant debris which also containspropagules, although fewer propagules than soil. Pod and bark (nitidulid andscolytid) beet les are consistently observed in large numbers on infected podswithin a day of the appearance of symptoms. Since these beetles breed in the podhusks and visit flower cushions and infected pods, they are likely vectors of thepathogen within the canopy of the piant. They could also fly or be blown from oneplanting to another, spreading the pathogen at the same time. Raindrops ordewdrops falling from diseased tissue can carry propagules of the fungus tohealthy tissue. The pathogen may also be spread within the canopy by rainsplashor direct physical contact between infected and healthy tissue.

Figure 22.3 Diseasecycle of Phgtophthorapalmtuora,he causeof pod rot and canker ofcocoa. FromKonam,Dennis,Saul,Floodand Guest, 996.)In Papua New Guinea, management strategies rely heavily on the applicationof fungicides. Trunk injections of phosphonates provide excellent, durable control


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354 John Brotun, Helen OgIe and Michele DaLe

of pod rot and stem canker and have been the recommended fungicide treatmentsince 1992. Trunk injections are widely and routinely used in plantations.However, few smallholders follow this recommendation. Significant diseasecontrol and corresponding increases in yield can be achieved using simple andinexpensive management practices which also enhance control achieved withphosphonate injections. Rainsplash and aerosol dispersal can be reduced byremoving or burying discarded pod husks and by using a groundcover such asgrass or mulch. Stopping ants from reaching the canopy by girdling tree trunkswith grease prevents the transport of propagules up the trunks of the trees. Theactivities of nest-building ants may be reduced by adopting as yet undeterminedpractices to encourage ground-dwelling ants which outcompete tent-buildingspecies. Wider plant spacing and regular pruning open the canopy resulting inIower relative humidity which discourages disease development. While pruning,ant nests and infected plant material can be removed reducing dispersal withinthe canopy by raindrops, dew, rainsplash, ants and beetles. However, care mustbe taken not to inoculate healthy branches by using contaminated pruningimplements. Frequent harvesting of ripe pods allows removal from the canopy ofinfected pods which are sources of secondary inoculum.The discovery in the l96os of systemic chemicals which can eradicatepathogens in infected plants may have encouraged pathologists to rely too heavilyon chemical control. However, the recognition of resistance to many of thesechemicals soon after their introduction ensured that practices were developed forusing systemic fungicides strategically in conjunction with other disease controlpractices.Since the mid l97Os, the term'disease management'has tended to replace theterm 'disease control', reflecting a change in attitude. Plant pathologists haverealised that control, which implies the complete or near complete elimination ofa pathogen, is usually biologically unrealistic and economically unnecessary. Theterm management, however, suggests the rational use of measures to achieveboth biological and economic goals. Disease management programs aim tomanipulate the pathogen population so that it does not reach levels at whicheconomic losses occur as well as to improve the efficiency of crop production.Disease management also focuses attention on the dynamics of the crop-pathogen interaction, an approach that is more likely to be successful thanconcentrating on the pathogen, the approach encouraged by the term diseasecontrol.22.6 Crop health managementThere are many factors that influence the productivity of a crop, includinginsects, diseases, weeds and nutrition. When a problem occurs in any one ofthese areas, it is dealt with by entomologists, plant pathologists, weed scientistsor plant nutritionists, respectively. In recent years, specialist scientists haveincreasingly worked together, integrating their disciplines in an effort to achieve alevel of pest and disease control that is acceptable in economic terms to farmersand at the same time causes minimal disturbance to the environments of non-target organisms. Plant production is therefore a complex process requiringeither an individual with a very diverse range of skills or, more often, a teamapproach. Integrated or holistic approaches to crop production have resulted inthe publication of health management manuals for crops such as wheat, peanutsand potatoes (e.g. those published by the American Phytopathological Society).These manuals cover every aspect of crop production from site selection topostharvest care, including the economics of production.


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22. Disease management: general.concepts 355There are several factors that motivate growers to adopt an integratedapproach to crop production, especially pest, weed and disease control. One isthe reduction in the number of chemicals available to control pests andpathogens. Pesticides are being deregistered as concerns about their safety riseand many have become redundant following the development of resistance in the

organisms they were meant to control. The farming community will receivefavourably any strategies for controlling pests and pathogens that also reducechemical usage because of its concern about the health of farm workers who arein close proximity to chemical pesticides. Another factor that obliges growers tolook for alternative ways of producing healthy crops is the encroachment ofsuburbia into farming areas. More intensive population of farming areas restrictsthe use of chemical pesticides because of the risk of spray drift affecting humansand animals and the danger of sprays contaminating water supplies.On the other hand, the more complex management systems involved inintegrated pest and pathogen control programs compared with the simplicity ofusing chemical sprays may retard the adoption of integrated control. Chemicalsprovide fast, simple, cheap and, usually effective, control of diseases. In the past,spraying for disease was often carried out by the calendar (e.g. on a weekly ortwo-weekly basis) or by the growth stage of the crop (e.g.sprays applied at bud-burst and again two weeks later). Spraying for disease was often combined withspraying for insects pests, further reducing the cost and effort involved. Crophealth management systems require farmers to take a more active part inevaluating available control options, assessing the risk of disease or pestoccurrence and monitoring epidemic development. By doing this, the level ofthreat should be met with an appropriate level of control. In seasons favourablefor epidemic development, more sprays would be applied while in seasons inwhich the threat is low, fewer sprays would be applied. Integrated control relieson good data gathering and the involvement of farmers at all stages of thedecision making process. Many farmers do not have the time, experience orexpertise to carry out these practices so some employ trained consultants toprovide necessary advisory and monitoring services.Legislation could be used to force growers to adopt integrated plant healthprograms, but would be difficult to enforce. Whilst claims of 'organic produce' canbe verified by testing for chemical residues, produce resulting from integratedcontrol regimes are not necessarily the result of reduced pesticide applicationsoverall, although the chemicals used are usually considered 'softer' or lessharmful to the environment. Any reference to reduced or safer pesticide usage onthe packaging of produce grown under integrated control systems may invoke ahostile response in potential consumers who often perceive any indication ofchemical use as negative.Successful adoption of new technologies or strategies by farmers is the resultof a complex decision-making process tempered by economic and sociologicalpressures. Whilst it might be a sound disease-management decision to leave afield fallow to break a pathogen's life cycle, it might not be economical to do so,and farmers will receive conflicting advice from their accountant and theiragricultural adviser.Conflicting and complementary management strategiesOne of the most challenging tasks in designing a workable integrated cropproduction system is combining management strategies for insect pests anddiseases with agronomic practices. The requirements for one control method mayconflict with those for another or with another farm management practice. Many


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35 6 John Brotun, Helen Ogle and Mtchele DaLe

of the procedures mentioned in this section are discussed more fully in Chapters23-27.The use of minimal tillage has obvious advantages for controlling erosion andmaintaining good soil structure (see Chapter 25). However, it may hinder thecontrol of soil-borne pathogens where tillage is used to expose propagules todegradation by solarisation or desiccation. The use of herbicides to control weedsin minimal tillage regimes may also encourage the formation of resistant sclerotiaby pathogens such as Macrophominain weed hosts.Some herbicides applied to crops to control weeds change the sugarconcentrations of host tissues and consequently change their susceptibility toinfection by pathogens. Fungi such as ALternaria soLaniprefer 'low sugar' tissueswhile others such as the rust fungi prefer'high sugar' tissues. Thus, herbicidessuch as 2,4-D which decreased the sugar content of host tissue lead to anincrease in the severity of diseases such as those caused by ALternaria solanr anda decrease in the severity of some rust diseases. In some cases, germination ofthe spores of pathogens is inhibited by the use of pre-emergent herbicides,perhaps due to the induction of phytoalexins in the host plant. Herbicidetreatments also increase leakage of host metabolites. Because the growth ofmany pathogens is stimulated by host metabolites, herbicides applied to the soilcan lead to an increase of seedling diseases.Micro-organisms vary in their sensitivity to pesticides. A pesticide applied forone purpose may lead to a previously insignificant disease becoming important.For example, treating soil with several fumigants to control soil-borne pathogensand weeds stimulates the germination of sclerotia of Sclerotinia spp. leading to anincrease in the incidence of sclerotinia rot in lettuce. Application of the herbicideatrazine increases the incidence of fusarium rot in navy beans by stimulatingspore germination, germtube growth and chlamydospore formation. Germinationof coffee rust urediniospores is stimulated by low concentrations of copper so theapplication of copper-based fungicides may increase the incidence of rust.Machinery used for cultivation to control weeds and for applying chemicalsprays to control pests and diseases compacts soil. Compaction restricts rootgrowth, interferes with water drainage, reduces gas exchange within the soil,allows toxins to accumulate, reduces biomass and yield, predisposes plants toroot rots and contributes to root rot severity.Since fungal diseases are generally more severe in densely-planted crops,increasing row spacing generally reduces disease severity because themicroclimate within the crop is less moist and less favourable for fungalestablishment (see Chapter 25). However, some viral infections such asgroundnut rosette disease become more severe in widely spaced crops becauseinsect vectors are attracted to bare ground and are more likely to land in suchfields.Plants fertilised with high levels of nitrogen grow larger and faster but aremore susceptible to diseases caused by bacteria and biotrophic fungi, such asrusts and powdery mildews (see Chapter 25). Ammonium-containing fertiliserscan have an indirect effect on disease by lowering the pH in the rhizosphere,which can increase susceptibility to diseasessuch as fusarium wilts.Fumigating soil before planting has been used in many cropping systems toeliminate weeds and soil-borne pests and pathogens (seeChapter 23). However,the chemicals used are generally biocidal in action and lethal to soil organismssuch as mycorrhizal fungi which assist many plants in nutrient uptake.Mycorrhizas have also been implicated in the suppression of various soil-borne


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diseases, both directly by occupying potential infection sites on the host, andindirectly by contributing to the general vigour of the host.Predatory mites are used to control pest mites in many Australian crops.However, fungicides such as benomyl and insecticides such as ox5rthioquinoxmay have adverse effects on the predators and should not be used in conjunctionwith them.In some cases, the use of insecticides has led to an increase in the severity ofcertain foliar diseases. However, this was caused by the emulsifring agents in theinsecticide dissolving the wil(y cuticles on the leaf surfaces making it easier forfungi to penetrate the leaf surface, rather than any direct action of the activeingredient.22.7 ConclusionDisease management can be considered a 'tool kit' approach to crop protection.Plant pathologists have been developing 'tools' for crop protection for many years.Planting crop varieties with resistance to specific diseases (Chapter 26) is oftenthe first line of defence. Chemical tools (Chapter 24) may be the major componentof a disease control program. Research is under way to provide biological toolsusing organisms that reduce disease incidence or severity by mechanisms suchas induced resistance, hyperparasitism, hypovirulence or competition (Chapter27). There are many cultural tools for controlling disease, including quarantinemeasures, mulching, tillage and solarisation (Chapters 23 and 25). These areparticularly important in traditional farming systems because they areinexpensive in terms of capital investment, but are labour intensive. Diseasemanagement systems will be judged successful if they produce significantincreases in productivity or economic returns. To do so, they need to be based ona sound knowledge of the specific disease in question, the organism involved, theeconomic advantage of control and the various management options available tothe grower.22.8 Further readingAllen, S.J., Brown, J.F. and Kochman,J.K. (1982).The control of alternaria blight ofsunflowers in eastern Australia. Proceedings J the tenth international sunflotuerconference, urfers Paradise,Queensland, p. 142-145.Carlile,W.R. (1995).ControloJcrop iseases.CambridgeUniversityPress,Cambridge.Clifford,B.C. and Lester,E. (eds), 1988).ControloJpLant iseases: ostsandbeneJtts.Blackwell Scientific Publications,Odord.Kable,P.F. (1991).Crop diseasemanagementwith fungicides-an overviewof its origins,progress, current status and future developmentsusing modelling and climatedata. Plant ProtectionQuarterlg6, 19-28.Konam, J.K., Dennis, J., Saul, J., Flood,J. and Guest, D. (rn press). ntegratedmanagementof Phytophthoradiseases f cocoa n Papua NewGuinea. ProceedtngsoJ the 12th internattonal cocoa research conJerence,Salvador, Brazil, 17-23November, 996.Sill, W.H. (1982).Plantprotection: ntntegratedinterdisctpltnary pproach. he IowaStateUniversity Press,Ames, owa.Thurston, H.D. (1992). SustainablepracticesJor plant disease managenLentn traditionalJarmingsgstems.WesMewPress,Boulder,Colorado.


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