epidemiology-170505192252.pdf

plant disease epidemiology

1. What is Plant Pathology or Phytopathology?
PlantPathologyorPhytopathology
isthestudyofsufferingofplant
diseases.
Pathology comes from 2 Greek words
Pathos
Suffering
Logos
Study

2. Why do we study Plant Pathology?
1.To study the causesof diseases
2.To study the interaction(s) between the plant
and the pathogen
3.To study the mechanism(s) of disease
development.
4.To develop management strategy of the
diseases

3. Disease=disturbancefromplantpathogenor
environmentalfactorthatinterfereswithplant
physiology
•Causes changes in plant appearance or yield
loss
•Disease results from:
•Direct damage to cells
•Toxins, growth regulators, or other byproducts
that affect metabolism
•Use of nutrients and water or interference
with their uptake

4. Whenapathogenspreadstoandaffectsmany
individualswithinapopulationoverarelatively
largeareaandwithinarelativelyshorttime,the
phenomenoniscalledanepidemic.
Epidemic and Epidemiology
Anepidemichasbeendefinedasanyincreaseof
diseaseinapopulation.
Asimilardefinitionofanepidemicisthedynamics
ofchangeinplantdiseaseintimeandspace.

5. Plant Disease Epidemiology
Study of epidemics
Thestudyofepidemicsandofthefactorsthat
influencethemiscalledepidemiology.
Epidemiologyisconcernedsimultaneouslywith
populationsofpathogensandhostplantsasthey
occurinanevolvingenvironment.

6. Edpidemiologyorepiphytologyisthestudyofthe
outbreakofdisease,itscourse,intensity,cause,effects
andthevariousfactorsgoverningit.
Thescienceofpopulationsofpathogensin
populationsofhostplants,andthediseases
resultingtherefromundertheinfluenceofthe
environmentandhumaninterferences

7. Basedontheoccurrenceandgeographicaldistribution
theyareclassifiedasfollows:
1.EndemicorEnphytoticWhenadiseaseismoreorless
constantlyoccurringyearafteryearinamoderatetosevere
forminacountryorlocalitythenitiscalledasendemicdisease.
eg:wartdiseaseofpotato(Synchytriumendobioticum)is
endemicinDarjeeling,citruscanker(Xanthomonasaxonopodis
pvcitri)inAsiaandsorghumrust(Pucciniapurpurea)
Certaindiseaseareendemicinoneareaandbecomeepidemic
inanotherarea.Eg:CitruscankerisendemicinAsiabut
epidemicintheintroducedplace,Florida(U.S.A).Thedowny
mildewofcornisaendemicdiseaseinIndiabutbecame
epidemicinthePhilippine

8. 2.EpidemicorEpiphytotic
Itisasuddenoutbreakofadiseaseperiodicallyoverawidespread
areainadevastatinglysevereformcausingseverelossesor
completedestruction.
Thisisconstantlypresentinalocalitybutitassumessevereform
onlyonoccasions.
Thisisbecauseoftheoccurrenceoffavorableenvironment
responsiblefortherapiddevelopmentofdisease.
eg:wheatstemrust(Pucciniagraministritici),lateblightof
potato(Phytophthorainfestans)Irishfamine1845-1746,andrice
blast(Pyriculariaoryzae)Bengalfamine1943.

9. 3.Pandemic
Whenanepidemicdiseasespreadsovercontinentsor
subcontinentsandinvolvesmassmortalityitisconsideredas
pandemic.TheoutbreakofblackstemrustofwheatinIndia
during1947isbestexampleforapandemicdisease.
4.Sporadic
Diseaseswhichoccuratirregularintervalsoverlimitedareasor
locationsarecalledsporadic.Theyoccurrelativelyinfew
instances.Eg:Fusariumwiltofcotton(Fusariumoxysporumfsp.
vasiinfectum)andloosesmutofwheat(Ustilagonuda)

10. Anepidemicmaycausewidespreadandmass
destructionofcropinashorttimeormaypersistfor
longperiodsdependinguponthefollowingfactors
responsibleforthedisease:
Elements of an epidemics

11. Vanderplank’sEquivalence Theorem
“Effects of host, pathogen and environment can
be translated into terms of the rate parameter
of an epidemic”
Changes in any component has an equivalent
effect on disease
-More-less susceptible host All affect
-More-less favorable environment amount of
-More-less aggressive pathogen disease

12. 12
Typical Disease cycle
Knowinghowparticularpathogensgothrough
theirdiseasecycleisimportantindeveloping
managementstrategies.

13. Therefore,diseasemanagementprinciplesand
practicesareoftencenteredaroundtheconcept
oftheDiseaseTriangle.sothatmanagement
tacticsoftenseektomanipulateoneormoreof
thecomponentsofthediseasetriangle.

14. Elements of a disease cycle
1.Host
2. Pathogen
3. Environment
Interactions of the 3 main
components are described by the
disease triangle.
The Disease Triangle
Disease development is also affected
by
4. Time
5. Humans
Interactions of the 5
components are
described by the disease
pyramid.

15. The Disease Triangle

16. Eg:OutbreakofPhytophthorawiltofbetelvineoccurs
duringrainyseasoninBangladesh.Thisdiseaseonce
againbecomedestructiveduringrainyseason.This
typeofepidemicisknownasseasonalepidemicor
annualepidemic.
Eg:Epidemicscausedasaresultofintroductionof
newpathogensinthelocality.Thewellknown
epidemicsoflateblightofpotatoinEuropeandblast
diseaseofriceinSouthEastAsia.

17. Factors that affect the development of
epidemic

18. Hostfactors
A.LevelofgeneticresistanceorSusceptibilityof
thehost
Susceptibility-inherent character, lack of ability to resist the
effect of a pathogen or other damaging factors
Resistance-the ability of an organism to exclude or
overcome completely or in some degree.
a. true resistance-gene control, eg: vertical and
horizontal resistance
b. apparent resistance-not genetically control
# disease escape-to overcome the disease
developing factors
# disease tolerance-disease occur but not
economic loss
Race : a genetically and often geographically distinct mating
group with in a species of a pathogen.

19. B. Degree of genetic uniformity of the host plant
Eg: Vegetative propagated crop, Self-pollinated
crop, Monoculture, especially Clones
C. Types of crop-
-Annual crops & foliar or fruit diseases develop much more
rapidly (in weeks)
-Perennial woody diseases take longer time to develop
(in years
D. Age of crop plant –
Plantchangeintheirsusceptibilitytodiseasewithage-Some
plantsaresusceptibleonlyduringgrowthperiod&become
resistantduringmatureperiod
.
Hostfactors
E.Introductionofnewhosts-

20. Age of crop plant

21. Pathogenic Factors
A.Level of virulence and aggressiveness –
Virulence-degree of pathogenecityof a given pathogen
Aggressiveness –production of a large number of inoculums
with in a very short period of time.
B. Quantity of inoculums near host-

22. Acentralconcepttoepidemiologyisthatdifferentpathogen
populationshavedifferentdiseasecycles.
I.Monocyclic=singlecycle(simpleinterest)
Pathogensthatcompleteoneorevenpartofonedisease
cycle/yeararecalledmonocyclic
Inmonocyclicpathogenstheprimaryinoculumistheonly
inoculumavailablefortheentireseason.Thereisnosecondary
inoculumandnosecondaryinfection.
Theamountofinoculumproducedattheendoftheseason,
however,isgreaterthanatthestartoftheseasonsotheamount
ofinoculummayincreasesteadilyfromyeartoyear.
C. Types of reproduction of the pathogen-

23. Thisrepresentationofplantdiseaseovertimeisreferredto
asa“DiseaseProgressCurve”
Graphically,diseasecausedbymonocyclicpathogenslooks
likeasaturationcurve.

24. Rate of increase of disease over time can be
represented by a simple interest function.

25. Itisclearfromtheabovemodelofamonocyclic
epidemicthatQ,R,andthaveequalweightin
theireffectonx.Areductionintheinitial
inoculumortherateofinfectionwillresultina
reductioninthelevelofdiseasebythesame
proportionatanytime,t,throughouttheepidemic.
Iftcanbereduced(forexample,byshorteningthe
season),diseasewillbereducedproportionately.
The monocyclic
model(Van der
plank in 1963)

26. 1.Reduce the initial inoculum(Qin the
monocyclic model and x
oin the polycyclic
model). (Actually x
ois the initial incidence of
disease, which is proportional to the initial
inoculum.)
2.Reduce the rate of infection (Rin the
monocyclic model and rin the polycyclic
model)
3.Reduce the duration of the epidemic (the time,
t, at the end of the epidemic)
Examining these models, we can see that in both there are
three ways in which we can reduce xat any point in the
epidemic:

27. ExamplesofMonocyclicDiseases
Blacklegofpotato(Erwiniacaratovora)
Verticilliumwilt
CerealCystNematode

28. Polycyclic=multiplecycles/year(compound
interest)
Mostpathogensgothroughmorethanone(2-
30)diseasecycleinagrowingseasonandare
referredtoaspolycyclic.
Onlyasmallnumberofsexualsporesorother
hardystructuressurviveasprimaryinoculum
thatcauseinitialinfections.

29. Onceinfectiontakesplace,largenumbersof
asexualsporesareproducedassecondary
inoculumateachinfectionsite.
Thesesporescanproducenew(secondary)
infectionsthatproducemoreasexualspores
andsoon.
Witheachcycletheamountofinoculumis
multipliedmanyfold.

30. Graphicallythistypeofpopulationgrowthis
representedasasigmoidcurve

31. Rateofincreaseofdiseaseovertimecanbe
representedbyacompoundinterestfunction.

32. The polycyclic model
•If ris very high, the apparent
effect of reducing x
ois to delay
the epidemic.
•If ris very high, x
omust be reduced to very low
levels to have a significant effect on the epidemic.
•Reducing rhas a relatively greater effect on the
epidemic than reducing x
o.
•Reducing x
omakes good strategic sense only if ris
low or if ris also being reduced.

33. Manyofthesepathogensaredisseminatedprimarily
byairOrair-bornevectorsandareresponsiblefor
mostoftheexplosiveepidemicsinmostcrops
Examples of PolcyclicDiseases
•Downy mildews
•Powdery mildews
•Late blight of potato
•Leaf spots
•Blights
•Grain rusts
•Aphid borne viruses
•Root-knot nematodes

34. III.Polyetic(multi-year)cycles
Somepathogenstakeseveralyearsbeforeinoculumthey
producecanbedisseminatedandinitiatenewinfections.
Maynotcausemanynewinfectionsoveragivenareain
ayear,amountofinoculumdoesnotincreasegreatly
withinayear.
However,becausetheysurviveinperennialhoststhey
havealmostasmuchinoculumastheyhadattheendof
thepreviousyear.
Inoculummayincreasesteadily(exponentially)fromyear
toyearandcancausesevereoutbreakswhenconsidered
overseveralyears.

35. Examples of PolyeticDiseases:
Some diseases of trees
Dutch elm disease
Citrus tristeza
Fungal vascular wilts
Mycoplasmalyellows
Viral infections

36. Pathogenic Factors
D. Ecology of the pathogen-
-Presence of inoculums in the host
-vascular fungi reproduce inside of the host and vector
transmitted cause epidemic
-Soil borne pathogen produce inoculums on the infected field.
E. Mode of spread of the pathogen-
-Wind, Water, Soil, Seed, insect, Human, Machinery used in
agriculture

37. Environmental Factors
Temperature
-Host effect-high temperature break the host resistance
specially horizontal resistance
-Pathogen effect-high temperature increase the
growth of pathogen but low temp. reduce the amount
of inoculumalso reduce the movement of vectors.

38. Moisture
•Activates resting stages
•Affects germination of spores and penetration
into host eg.Soil borne pathogen produce
inoculums on the infected field
•Water on leaves
•Humidity
•Splashing water-distributes inoculum
•Leaf wetness= best indicator but difficult to
measure
Environmental Factors

39. Effectoffoliagedensityon
developmentofPhytophthora
infestansduringaperiodofpartly
favorableweather(May–June)
andofveryfavorableweather
(November–December).
Moisture

40. Human Factors
A. Site selection and preparation
-Poor drainage, low land, infected field
B. Selection of propagating materials
-Using pathogen free or treated planting materials reduce
disease
C. Cultural practices
-Monoculture, higher level of nitrogenous fertilizer, dense
planting, overhead irrigation, poor sanitation, creation injury in
plant.
D. Disease control
-cultural , biological chemical control reduce the epidemic

41. DiseasedevelopmentisalsoaffectedbyTime
Timefactors
1.Seasonoftheyear
2.Duration&frequencyoffavorabletemp.&
rains
3.Appearanceofvectors,etc.

42. ImplicationsforDiseaseManagementStrategies
MonocyclicDiseases
•Reducetheamountofprimaryinoculum,oraffect
theefficiencyofinvasionbytheprimaryinoculum.
PolycyclicDiseases
•Reducingtheamountofprimaryinoculumhasless
impact.
•Reducingtherateofincreaseofthepathogenmore
beneficial.

43. Purposeofdiseasemanagementistoprevent
diseasefromexceedingsomelevelwhereprofitor
yieldissignificantlydiminished.

44. Principlesofepidemiologyindicatesthat
controlmeasurescandothisinonlytwo
ways.
1.Theymayreduce(ordelay)diseaseatthe
beginningoftheseason(x)or
2.Theymaydecreasetherateofdisease
development(r)duringthegrowingperiod.

45. Waystoreducedisease(inoculum)atbeginning
(x
0)
•Fumigation,Certifiedseed
•Sanitation,Seedtreatments
•Quarantine,Hostplantresistance
Waystodecreasetherateofdiseasedevelopment
(infectionrate)(r)
•Changetheenvironment
•Fertilizerapplication
•Hostplantresistance
Waystochanget
•Harvestearlybeforediseasebecomessevere.
•Plantearly(cerealcystnematode)

46. PATTERNSOFEPIDEMICS
Interactionsamongtheelementsofepidemics,as
influencedovertimebyfactorsoftheenvironment
andbyhumaninterference,areexpressedinpatterns
andrates.
disease–progresscurve
Diseasegradientcurve

47. Diseaseprogresscurve
(A)Saturationtypeofcurve
Threemonocyclicdiseasesofdifferentepidemic
rates.
(B)Sigmoidalcurve
Polycyclicdisease,suchaslateblightofpotato.
(C)Bimodalcurve
polycyclicdisease,suchasapplescab,inwhichthe
blossomsandthefruitareinfectedatdifferent,
separatetimes

49. Diseasegradientcurve
disease–gradientcurve:Thepercentageofdiseaseand
thescalefordistancevarywiththetypeofpathogenor
itsmethodofdispersal
beingsmallforsoilbornepathogensorvectorsand
largerforairbornepathogens.
Theprogressofan
epidemicmeasuredin
termsofchangesinthe
numberoflesions/the
amountofdiseased
tissue,andthenumberof
diseasedplantsasit
spreadsoverdistance,is
calleddiseasegradient
curve(spatialpattern)

50. Fungi Bacteria Viruses Nematodes
Survival Crop residue,
Soil,
Alternate
hosts
Crop residues,
Soil,
Alternate
hosts,
Insect vectors
Alternate
hosts,
Insect vectors
Crop residues,
Soil
DispersalWind
Rain
Insects
Wind
Rain
Insects
Insects Tillage
Equipment
Water run-off
InfectionDirectly
Wounds
Insect feeding
Wounds
Insect feeding
Insect feeding Directly
50
Comparison of Disease Cycles