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Plant pathogens &ecological management

Horticulture 150Spring 2011

PLANT PATHOLOGY Plant pathology is the study of the

organisms and environmental conditionsthat cause disease in plants

PLANT DISEASE

Plant disease is defined as:

“ A microscopic agent or abiotic conditionthat interferes with the manufacture,translocation, and/or utilization of food,mineral nutrients, and water in such a waythat the plant changes in appearance oryields less than a comparable normalhealthy plant of the same variety.”

OutlineVariety of symptoms, causal organisms, anddiagnosis of plant pathogensDiseases in less disturbed ecosystemscompared to agroecosystemsEcological approaches to disease managementFoliar fungal disease epidemiology and culturaland biological control – Botrytis cinerea of grapesand strawberries

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Plant pathogenic bacteriaSmall, microscopic. Single procaryotic cells: no nucleus,one chromosome. Limited over-all size, ‘unlimited’ reproductionby fission. Absorptive nutrition. Most bacteria are saprophytic.Pathogens cause blights (rapid, toxin killing of plant tissue), rots(mushy breakdown), wilts (plugging of vasculature) and galls (growth regulator-mediated cancers). *Very sensitive to environ-ment, except when dormant. Spread: wind, water, vectors

Main genera include:AgrobacteriumCorynebacteriumErwiniaPseudomonasXanthamonasStreptomyces

Bacterial diseases

Bacterial ooze

Fire blight (Erwinia amylovora)

Bacterial leaf spot (X. campestrus) on lettuce.

Plant pathogenic fungi•Plant-like but without chlorophyll or vascular system. •Nuclei, multiple chromosomes, mitochondria. •Chitin in cell walls rather than cellulose.•Overall size unlimited, but poor connection/communication between cells. •Easily fragment into multiple bodies. •Most form differentiated structures (mushroom, spores).•Most saprophytic. •Many symptoms (rot, blight, leaf spots, wilts). •Fairly sensitive to light and dry when active. •Can make very resistant structures when dormant.• Spread: wind, water, seed, vectors.

8000 species cause disease in plants.

Filamentous protists - once thought to be fungi - which mustabsorb their food from the surrounding water or soil, ormay invade the body of another organism to feed.

Plant parasitic water molds - Oomycota

Late blight fungus (Phythopthora infestans),causal organism of the Irishpotato famine, is classic example.

Saprolegnia is another

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Fungal diseases – soil born

Phythium damping off on soybean

Healthy vs. Fusarium wilted plant.

Verticillum wilt in cucumber

Monterey Pine Pitch Canker

Armillaria mellea

Nectria galligena canker

Fungal diseases - trunks &branches

Fungal diseases – foliar & fruit

Rhizoctonia leaf spot on tobacco

Leaf rust on barley.

Powdery mildew on apple.

Peach leaf curl on peach.Gray mold on grapes.

Plant pathogenic virusesRod shaped or polyhedral entities too small to be seen with a light microscope, multiply only in living cells, and can cause disease. Viruses attack plants, microorganisms, animals etc. They are nucleaic acids (double or single stranded) with protein coats, either only RNA or only DNA, and usually with only one kind of protein. •The nucleic acid codes for a few proteins, takes over cell. •Upsets normal metabolism: excess, shortage of molecules used•by the cell.•Symptoms mimic genetic abnormalities: mosaics, distortions,•yellows, death.•Spread by mechanical, seeds or vectors (esp Homopterans)

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Viral diseases

Pea with red clover mosaic virus

Peach with various prunus viruses.

Peanut with Tomato Spotted Wilt virus

Squash mosaic virus

Nematodes are multicellular animals in the group Ecdysozoa,or animals that can shed their cuticle. Phytoparasitic nematodesrange in size from 250 um to 12 mm in length, averaging 1 mm,to about 15-35 um in width.

http://www.apsnet.org/education/IntroPlantPath/PathogenGroups/intronematodes/

Nematodes

Parasitic higher plants

2500 plant species live parasitically offhigher plantsMistletoes, dodder, witchweed,broomrapes

Toxicities – nutrient & chemical

Paraquat on beans.

Fluoride toxicityon ornamental.

Sulfur dioxide injury on alfalfa.

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Plant disease diagnosis – in the field

Visual assessment of signs and symptomsDiscussion with grower or consultant abouttheir impression of disease, weather, & recentactivities.Knowledge of host range – the big red bookTake photographs – to share with colleagues,develop own photo database.Take samples – range of samples, to catchgrowing edge of lesion, mature infection etc.Proper sample handling – keep cool andinspect as soon as possible.

Plant disease diagnosis – back at thelab

Back at the lab - look at infected tissueunder dissecting and compoundmicroscopes.Fungal or bacterial:– Using proper sterile technique, plate tissue

samples on selective media– Place surface sterilized tissue in humid chambersViral – isolate, purify, look for occlusionbodies in plant tissue, transmission studieswith indicator plants, use serological tests,EM, PCRNematodes:– Use Baermann funnel or sieves to catch

nematodes from soil or tissue samples.

Plant Disease Triangle Botrytis Bunch Rot/Gray Mold

Host

Environment PathogenTemperatureRelative HumidityVapor PressureWind SpeedHours of Surface WetnessLight Intensity

VirulenceGerminationInfectionSporulationGrowthDispersal

VarietyMaturityFoliage & flower statureBerry cuticle & waxCanopy densityNutritionIrrigation

Factor SpecificAttribute

AgriculturalSystems

NaturalSystems

Plantcommunity

Speciescomposition

One to fewspecies

Many species

Plantpopulation

Genotypiccomposition

One to fewgenotypes(cultivars)

Many genotypes

Age,developmentstage

Uniform Variable

Plant spacing Uniform Variable

Physicalenvironment

Uniform Variable

Factorsdeterminingcommunity &population traits

Humans, abiotic& bioticenvironment

Abiotic & bioticenvironment

From Alexander 1989

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Need to study “natural”, less disturbedecosystems

Disease in natural, undisturbed communities(Harper, Antonovics, Alexander, Augspurger,Burdon, Burdon & Chilvers).Pathogens regulating plant populations andcommunity structure and vice versa –maintenance of genetic diversity.Density dependence and disease – most studiesfound positive correlation (62%) but there weresome negative (27%) due to indirect effects.

Genetically variable plant populations havereduced disease incidence and spread – lower #susceptible plants in mixture, increased distancebetween susceptible plants, interference ofresistant plants with spread, induced resistance.Spatial variation in physical environment willaffect disease spread – more disease in shadyenvironments, successes of canopymanagement for disease control. This in turncan influence host resistance within a plantpopulation to the pathogen, if over enough time.

Need to study “natural”, less disturbedecosystems

Use of ecological approaches in planthealth management

Plant spatial patterns at multiple scales – cropdensity, genetic diversity, non crop interactions,alternate hosts– Species diversity through intercropping – physical,

chemical barriers to pathogen or vectordissemination, infection, growth, and/or reproduction(Power, Mundt).

– Host genetic diversity using multilines formanagement of cereal rusts – Mundt et al.

– Plant host density – affects on dispersal, infection,overwintering, microclimate etc.

Plant temporal patterns – importance of croprotation, fallow to break up specific diseasecycles, allelopathic plants (Brassica)Modify plant microclimate – less moisture, morelight, wind/air movement, evaporative potentialBiological control – intrinsic, classical andaugmented (Baker & Cook)Use of clean seed, propagating material – infield avoidanceExclusion of pathogens at a regional or countryscale through quarantine and regulations.

Use of ecological approaches inplant health management

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Ecologists have proposed thatpathogens are important in:

Regulation of plant population sizeMaintenance of species diversity in plantcommunitiesEvolution and maintenance of sexualreproduction in plantsMaintenance of genetic variation in plantpopulations

Organic amendments and disease (Hoitnik)Complex microbial communities in soil and onplant surfaces – in time and space - successionHost nutrient status and susceptibilityInduced Resistance (IR) and Systemic AcquiredResistance (SAR) - induced with pathogens,insects, other organisms applied to soil andplants.Suppressive soils – combination of soil microbialcommunity structure, other biological, andchemical characteristics.

Use of ecological approaches in planthealth management

Employ horizontal and vertical resistance in host– gene for gene hypothesisHost water status and susceptibilityK vs. r strategist pathogensIsland biogeography theory – Distance anddispersal of pathogen and “founding effects.”Competition and succession - pruning wounds ingrapes or newly opened strawberry flowers andease of introducing biocontrol agents.

Use of ecological approaches in planthealth management Botrytis cinerea

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Beach Road 2002, evaluating Broome et al. infection model on strawberries.

Weather station to monitormicroclimate and predict infection risk.

Leaf wetness sensor

Atmometer - measuresevaporative potential.

ModelCalendarsprays

Biochemical and physical aspects of hostresistanceVariety stature and flower and fruit positionwithin the canopy, most exposed (upright)flowers and fruit may have less disease,e.g. San Juan variety.Choose varietieswell suited toyour region

Variety choice may influencedisease through genetic resistance

but also host microclimate

Cultural practices that influencehost microclimate and inoculum

level & reduce diseasePlanting density within row, # rows per bedBed direction relevant to prevailing wind,bed height and slope of land.Canopy management, leaf and debrisremoval.

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Density of plants per bedIn Florida they evaluated 23, 30, 38, and 46cm spacings (offset) with 2 rows per bedplantings of Camarosa (bed width was 71cm).The widest spacing had a 40-44% lowerincidence of disease than the narrowestspacing with this variety.However yield was lower in with lowerdensities even if there was less disease.

From Legard et al. 2000

Sanitation – removal of infectedfruit and leaves

Remove dead or infected plant material, afterrains remove infected fruit, and after harvestremove or incorporate all plant residue(Legard et al. 2000 and Daugaard 2000).Leaf sanitation (removal of senescent andnecrotic leaves) reduced disease from 12.6%to 8.2% in 1996-97 and from 17.6% to 11.8%in Florida.Fruit sanitation (removal of unmarketable fruitfrom alleys between beds) did not have asignificant effect on disease incidence oryield.

Additional cultural controls forstrawberry gray mold

Site selection:– select fields that are isolated from conventional

growing areas.– Try to avoid areas with heavy fogs or dew events.

Use plastic mulches that prevent berry-soilcontact.Use mulches that provide optimum aircirculation.Reduce spore movement, bug vacs?

Proper crop management

Manage fertility and organic matter tosupport complex soil community and tightnutrient cycles.Do not over fertilize.Do not over irrigate.

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Use of tunnels for coveredproduction

Researchers found that the mean incidenceof Botrytis fruit rot was 88 to 94% lower intunnels than in the field (Xiao et al. 2002).Tunnels completely exclude rain and impactthe length and frequency of dew events aswell as increasing temperature and reducingrelative humidity. These researchersobserved 30 days with an average of 4.5hours of free moisture in the tunnels but 77days with an average of 12.9 hours wetnessjust outside the tunnels.Tunnels will also reduce/eliminate splashdispersal of pathogen, and plastic excludesnear UV radiation which will reduce fungalactivity.

Tunnels in Southern California

From K. Larson, UC

EM Photos of Trichodermaviride attacking Botrytiscinerea.

Biological control of Botrytis cinerea

As bees exit the hive on their way to the field, they walk acrossthe 'footbath' of powder, picking up as many as 100,000Trichoderma spores per bee. As they forage for nectar andpollen, the bees leave spores of T22 behind in the flowers.

Novel delivery system for biological controlagents in strawberries, Cornell University.

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SummaryFungal, bacterial, viral, Mollicutes, filamentous protists,nematodes, and parasitic plants can cause disease.Wide ranging symptoms and disease cyclesValue in studying “natural” systems, and other lessdisturbed ecosystems, and compare to agroecosystemsEcological approaches to disease management arevarious but often limited by economic andmarket/structural restrictions as well as research data.Foliar fungal disease epidemiology and cultural andbiological control – Botrytis cinerea of grapes andstrawberries.

Biological control - Definition“the reduction of the amount of inoculumor disease-producing activity of apathogen accomplished by or through oneor more organisms other than man”.Disease producing activity involves -pathogen growth, infectivity,aggressiveness, virulence etc.

Organisms involved in biologicalcontrol

Avirulent or hypovirulent individuals orpopulations within the pathogenic species itselfThe host plant manipulated genetically, bycultural practices, or with microorganisms towardgreater or more effective resistance to thepathogenAntagonists of the pathogen, microorganismsthat interfere with the survival or disease-producing activities of the pathogen.

Biological controlRole of the pathogenHost resistanceAntagonists - introduced and residentAbiotic environment - temperature, waterpotential, radiation, pH, surface charges,partial pressures of gases, ions andelements, and energy-containing carboncompounds.Population and community level interactionsof above

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Mechanisms of antagonism

Antibiosis - inhibition or destruction of oneorganism by a metabolic product ofanotherCompetition for resources (food, mineralslike iron, space etc.)Parasitism or predation

Differences between classic biologicalcontrol of insects and biological control

of plant pathogens

Insects are impacted by fewer environmentalfactors, i.e. temperature, and limitedbiologically by one or a few predator orparasite species.Pathogen life histories are oriented towardextremely rapid exploitation of limitedresources under the constraints of a multitudeof biotic and abiotic factors which vary greatlyover short time and space intervals.

Predation and parasitism in soil

Mycophagous springtails (Collembola)have been shown to reduce the inoculumdensity of Rhizoctonia solani (Curl et al1986)Sporidesmium sclerotiorum is amycoparasite of Sclerotinia minor (Adamset al 1986)Trichoderma spp. numerous studies

Pathogen inoculum reductionthrough biological activity

Earthworm burial of apple leaves andreduction in apple scab inoculum, Raw etal 1962.Mulches with urban green waste andreduction in brown rot of peaches, impactson mummies on the ground.

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Bio-enhanced mulchesSuppression of plant disease by compost,Hoitink, HAJ et al. 1997.Compost induced systemic acquiredresistance in cucumber to Pythium root rotand Anthracnose, Hoitink et al. 1996.Urban and agricultural wastes for use asmulches on avocado and citrus and fordelivery of microbial biocontrol agents(Trichoderma harzianum, Gliocladium virens,and Pseudomonas fluorescens), Casale et al.1995.

Organic matter (OM) additions to soil &impacts on disease - depends on OM

quality, decomposition stage, orcompost maturity

Additions of compost, plant residues andorganic materials have been shown tosuppress root pathogens and diseaseincidenceHowever, fresh organic matter has beenshown to enhance saprophytic growth anddisease intensity by various pathogens, i.e.damping-off pathogens such as Pythium.

Host and soil nitrogen status - useof excessive nitrogen increases

diseaseIncreased host susceptibility - withincreased use of synthetic nitrogen.– Brown rot and hull rot in almonds– Brown rot in peaches– Botrytis bunch rot in grapesCover crops and nitrogen budgets, organicsources of nitrogen affect plant availablenitrogen for the host and othermicroorganisms in soil.

Spring time leaf infection.

Grape flower in full bloom.Floral debris as inoculumsource.

The fungus overwinters in oldclusters on the vine, in the soil.

Botrytis Bunch Rot (Botrytis cinerea)

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Cluster var. Zinfandel with Botrytis:Note the nesting or movement of the fungus from one berry to the next.

Direct infection of the berrywith enzymatic degradation andfurther rotting of the berry.

Sporulation

Orange tortrix.

Some lepidoptera feeding on berries can allow entry of bunch rot disease organisms.

Omnivorous leafroller nest in young berries.

Photos by Jack Kelly Clark.

Noble Rot - Late Harvest Wine Far Niente’s Dolce - from SemillonDolce Web Cam: http://www.farniente.com/

Gray Mold of Strawberry (Botrytis cinerea)

The fungus overwinters in dead plant tissue in the soil.

Floral tissue becomes infected throughout season.

Spores.

Fruit can be directly infected, or a floral infection grows into the ripening fruit.

Wind, splashing water, & human activity

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Temperatures between 65° to 75°F (18° to 23°C) and free moisture on the foliage from rain, dew, fog, or irrigation create ideal conditions for infection.

Methods permitted for plant disease management underthe national organic program standards

Grape canopy management, systemredesign, and the importance of

microclimate

Trellis systemShoot positioningwiresPruningShoot removalLeaf removalHedging

Two wire vertical trellis with shootpositioning wires and leaf removal 2weeks post bloom, Oakville, Napa.

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The microclimate around the clusteris critical, air movement or wind speed was found to be the most important.

A Chilean poet knew about the importance of wind and grapes…...

Leaf Removal and Evaporative Potential (grams of water/hour) in the Grapevine Canopy

From Thomas et al. 1988 From English et al. 1990

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Evaporative PotentialEffects on Mycelium &Conidium Development

From Thomas et al. 1988. From Marois et al. 1986

Effects of berrycontact on epicuticularwax

Grape berry cuticle and epicuticular wax can protect berriesfrom infection. Cuticle thickness is influenced by microclimate, in particular sunlight and air movement in the canopy.