Oomycota w.s.r. to Phytopthora sp.

This PPT intends to give a bird's eye view of the fungi, Oomycota w.s.r. to Phytopthora sp.

1. MYCOLOGY
Oomycota: General Characteristics,
Ecology with special reference to
Phytopthora
By
Dr. N. Sannigrahi, Associate Professor,
Dep't. Of Botany, Nistarini College,
Purulia
West Bengal, India

2. LATE BLIGHT OF POTATO

3. OOMYCOTA -MEMBERS
OomycotapopularlycalledOomycetesareclassoffungi
havingfilamentousgrowthhabit,absorptivemodeofnutrition
andreproductionbyspores.Theyarecalledwatermoldsand
beingtreatedaslowergroupoffungi.Oomycotameansegg
fungireferringtothelargeroundoogonia.Evolutionary
studiesconfirmedthecloseresemblanceofthisgrouptoalgae.
Thegroupconsistsofmostdevastatingpathogenscause
variousdiseaseslikeseedlingblights,dampingoff,rootrots,
foliarblight,downymildews,lateblightofpotato,rootand
stemrotsofsoybeanetc.
ThemostcommonmembersarePhytopthora,Pythium,
Albugo,Sarolegnia,Peronosporaetc.
Itdiffersfromeumycotaduetoabsenceofsepta,cellwall
composition,productionofwalllessbiflagellateswimming
sporesanddifferenceinmetabolicpathways.

4. MEMBER OF OOMYCOTA

5. TAXONOMY
Intraditionalclassification,itisplacedinPhycomycetesand
OomycetidaeisasubclassofPhycomycetes.
Alexopolus(1962)consideredastruefungiandkeptinaclass
Oomycetes.
Ainsworth(1971)keptasaclassunderthesubdivision
MastigomycotinaofdivisionEumycota.
WebsterandWeber(2007)keptOomycotaasadivisionof
StraminipliahavingordersSaprolegnials,Pythialesand
Peronosporalesinthisdivision.
Inthemodernclassification,Oomycotahasbeenplacedinthe
3
rd
phylaofthekingdomChromista.
Oomycotahasbeendividedunder13orderswith110genera
and1000species.

6. ECOLOGY
Thewatermoldssuggestsitsabundanceinmostlydamp
placesbutnumberofspeciesareterrestrialinnature.
Foundinfreshwater,sea,soilandabovetheplantorgans,
Obligateaerobes;liveeithersaprophyte,obligateorfacultative
parasitesonplants;somespeciescausediseaseonanimals
particularlyoninsectsandfishes.SaprolegniaonSalmon,
AphanomycesastracionCrayfish,Lagenidiumgiganteumon
parasiticinsects,
Mostofthespeciesarepathogenictoplantsandverymuch
aggressiveonseriousdamageofeconomiccrops.

7. INFECTED POTATO

8. GENERAL CHARACTERS
1.Filamentous,branchedmyceliumwithoutsepta;mycelium
multinucleate,diploid;cellwallcontainscelluloseandglucans
withoutchitin;eucarpicorholocarpic.
2.Reproductionoccursbyasexualandsexualmeans
3.Asexualreproductionbyzoosporesonsporangia.
4.Sporangiahavingdiverseshapeandposition-eitherterminal
orintercalary,bulbousornot.
5.Zoosporesareflagellate-anterioristinselandposterioras
whiplash,swiminwater,soilwaterornaturalbodies,
6.Afterswimming,zoosporessettleonsurface,retractflagella
andsecretemucilaginousmatrixtosettlethem,
7.Homomorphicordimorphiczoosporesareformed;
zoosporesmaybeprimaryorsecondary,

9. GENERAL CHARACTERS
8.Sexualreproductionheterogamousviaantheridiaandoogoniaas
reproductiveorgans,
9.Femaleoosporesisproducedinoogoniaanditmaybe1-many,
10.Malegameteisproducedinantheridium-eitherparagynous
(sideoftheoogonium)oramphigynous(atthebaseofoogonium),
11.Mostlyhomothallicbutheterothallicwhenbothmatingtypesare
presentduetostimulationofhormone,
12.Meiosisoccursduringthegameteproduction
13.Plasmogamytakesplacebygametangialcontactanddiploid
zygoteisproducedasapartofkaryogamy,
14.Diploiazygotedevelopsthickwalledresistantsporesand
germinatetogiveriseto2nvegetativehyphaeandreproduce
asexuallybyzoospores.
15.Diplonticlifecycleisthekeyfeatureofthisgroup,haploid
phaseisrestrictedonhaploidgametesonly.

10. LIFE CYCLE PATTERN

11. DISEASE AND ECOLOGY
Lateblightisaseriousfungaldiseaseofpotatoes.Itisworldwide
initsdistribution.Itoccursinpotatogrowingareasoftheworld.
WinteristhemainpotatogrowingseasoninIndia.Itisfollowed
byhotsummermonthsintheplains.Thedroughtandhigh
temperaturekillthefungusinthesoil.Thelateblightepidemics
arethusrareintheplainsinIndia.Itisdestructivetothecrop
grownintherainyseason.Thediseaseoccursannuallyinthe
coolerHimalayanregionsextendingfromAssamtoKashmiratan
altitudeof6,000ft.ormoreasthecropisgrownintherainy
season.
Moreover,thetemperatureduringthedayisneverabove22°-23°C
whichisfavorablefortheappearanceofdisease.Thecropsgrown
intheplainshavebeenusuallyfreefromtheepidemicsoflate
blightbecausethechiefpredisposingfactors(temperatureand
moisture)thatrenderpotatoplantssusceptibletodiseaseare
absentduringtheperiodoftheirgrowth.

12. DISEASE OCCURRENCE IN INDIA
Thetemperatureishighforthedevelopmentofthedisease.Nowit
hasestablisheditselfintheIndo-Gangeticplainandoccurs
annuallyinthestatesofPunjab,UttarPradesh,Bihar,andW.
Bengal.Thediseaseisalsodestructivetotomatoes.
Thedamagecausedbythediseaseisfrequentlyveryhigh.Severe
damagetothefoliageshortensthegrowingseason(Fig.22.5).
Consequentlythetubersremainsmallandreducedinweight.
Theyareproducedinsmallernumbers.Thisresultsinthereduced
yield.Inseverecasesofinfectionthereiscompletelossofthe
crop,Infectionalsoresultsinthedecayoftubersinthefieldand
storage.

13. PRIMARY INFECTION & SYMPTOMS
Thediseasefirstappearsonthetopsoftheplantsgenerallyafterthe
blossomingperiodbutmostlyinthemonthofJanuary.Itmayappearas
wellatanytimeduringthegrowthperiodoftheplant.Theconditioning
factoristhefavorableenvironment.
Thediseasemakesitsappearanceassmall,dead,brownishtopurplish
blackareasorlesions.Theseappearonthetipsandmarginsofthe
leaflets,rachis,petioleandstem.Underfavorableconditions(low
temperatureandhighhumidity)thelesionsrapidlyincreaseinsize
involvingthewholesurfaceoftheleaf.
Thediseasegenerallyfirstattackstheleaves,andpetiolesnearthe
groundandthelesionsappearonthelowersurfaceoftheleafletson
individualplantsandthenspreadsupwards.
Finally,arapidandgeneralblightingoffoliageoccurs.The
blightedleavescurlandshrivelindryweather.Undermoist
conditionstheydecayandemitacharacteristicoffensiveodour.

14. INFECTION & SYMPTOMS
Examinationofthelesionsonthelowersurfaceoftheleafona
dewmorningrevealsadelicategrowthofthefungusparasitein
theformofwhitishpowderybloom.Itconsistsofsporangiophores
andsporangiaofthepathogenpushingoutthroughthestomata.
Thesporangiaservetospreadthediseaseinthegrowingseason.
Potatotubersareofteninfectedinthefieldafterthetopshave
beenblighted.Theygetseparateinfectionswhileinthehill.There
isbrownishdiscolorationoftheskinofthosepartsofthetubers
whichlienearestthesurfaceofthesoil.
Thesedryrotspotsremainfirmandextendtoabouthalfaninch
belowthesurface.Duringstorage,thebacteriaassisttosetinthe
wetrotphase.Incoolanddryconditionstheprogressofthe
diseaseisslowerandthewetrotphaseisgenerallychecked.
Undermoistconditionshyalinemycelialhyphaeand
sporangiophorespushoutthroughthelenticelsandappearonthe
surfaceofinfectedtubers.

15. SECONDARY INFECTION
Theinfectedtubersaregenerallyconsideredasthemainsourceof
primaryinfectioninIndia.Thesurvivalofthefungusinthesoilinthe
Indianclimaticconditionsinanyformappearsremote.Accordingtothe
widelyheldview,thefungalparasiteoverwintersasadormant
myceliumintheinfectedtubers.
Itbecomesactivatedatthetimeofgerminationofthediseasedseed
tubersamongtheplantingstockorwastetubersindumpheapsor
infectedtubersremaininginthegroundafterapreviouscrop.The
activatedmyceliuminvadesthehealthysprouts.
Thesecondviewisthatthethick-walledrestingoosporeswhichare
foundinabundanceintheinfectedtubersaretheimportant
overwinteringstructures.Theyplayasignificantroleasthesourceof
primaryinfection.
Attheplantingtime,therestingoosporegerminates.Thegermtubeafter
emergenceusuallyendsinaterminalsporangium.Thecontentsofthe
latterdividetoformzoospores.Thereleasedzoosporesinvadethe
healthysproutsandbringaboutinfection.

16. DISEASE & REPRODUCTION OF FUNGUS
Accordingtosome,thesexualphaseseemstoplaynoroleinthe
lifehistoryofthepathogen.Theinfectedsproutsemergeabove
groundandproduceshootswhichcontainthemycelium.
Itgrowsandramifiesintheintercellularspacesabsorbing
nutritionbyputtinghaustoriaintothehostcells.Undersuitable
conditionsoftemperatureandhumidity,themyceliumpushesout
hyaline,branched,indeterminatesporangiophoresthroughthe
stomataofthehostleaves.
Thethin-walled,ovoidorlemon-shapedsporangia,eachwithan
apiculatetip,arebornesinglyatthetipsofsporangiophoresor
theirbranches.Asthesporangiumreachesmaturity,thesupporting
hyphalbranchimmediatelybelowitswellsslightlyandcontinues
togrowturningtheattachedsporangiumtotheside.The
elongationofthebranchproceedsandanewsporangiumis
formed.Theprocessisrepeated.Afertilebranchor
sporangiophoreisthuscharacterizedby9or10suchswellings
occurringatintervals.

17. PREDISPOSING FACRORS OF DISEASE
Eachnodularswellingmarksthepointwherethesporangiumwas
borne.Thematuresporangiaarereadilydetachedandspreadby
splashingrainoraircurrentstonewpotatoplants.
Grosier(1934)whostudiedthebiologyofPhytophthorainfestans
reportedthatthesporangiaareformedwithinatemperaturerange
of3°to26°Cwithanoptimumof18°to22°C.Theminimum
relativehumidityrequiredis91percentwithanoptimumof100
percent.
Onreachingasuitablehost(potato),thesporangiagerminateon
theleaves.Germinationisinfluencedbymoistureandtemperature
conditions.

18. DIRECT GERMINATION
Incoolmoistweatherthesporangiafunctionaszoosporangia(F
1-
3).Theoptimumtemperaturefortheformationofzoosporeis12ºC
(54°F).Intheindirectgerminationtheprotoplasmiccontentsof
sporangiumdividedtoformanumberof(usually8)biflagellate
zoospores.
Theyareliberatedinagroupthroughterminalporeformedby
ruptureoftheapicalpapilla.Thereleasedzoospores,afterabrief
periodofactivityinrainwaterordewcometorest.Eachretracts
itsflagellaandsecretesawallaroundit.
Theclothedzoospores(cyst)thengerminatesbypushingouta
germtubeorinfectionthread.Thezoosporesgerminaterapidlyat
12°to15°C.Coolandmoistnightsarethusfavorableforthe
formationandgerminationofzoospores.Thegermtubesshow
rapidgrowthat21°C.Afterinfectiontheygrowbestataslightly
highertemperature.

19. INDIRECT GERMINATION
Underdryandwarmerconditionsnozoosporesareformed.The
sporangiumfunctionsasaconidium.Itdirectlyputsoutagermtubeor
infectionthread.Theoptimumtemperatureforthisdirectgerminationof
sporangiaisabout24°or’25°C.
Theindirectmethodofgerminationofsporangiabytheformationof
zoosporesinaterrestriallateblightfungusisaninstanceofretentionof
anancestralprimitivecharacterwhichwasnormallyusedbyitsaquatic
ancestor.
Theinfectionthreadproducedonthesurfaceofthehostleafin
eitherofthetwoabove-mentionedmethodsentersthehosttissue
(leavesorstem).Itmakesitsentryoccasionallythroughthestoma
butmoreoftenitpenetratesdirectlythroughthecuticlebya
penetrationhyphaarisingfromanappresorium.
Thelowersurfaceoftheleafismoresusceptiblethantheupper.
Theinfectedleavesproduceanothercropofsporangia.Theseare
carriedbywindtothehealthyplantswhicharethusinfected.This
constitutessecondaryinfection.Theprocessisrepeated

20. INFECTION OF TUBERS
Thetubersgetseparateinfections.Itiscausedbyzoospores
producedinfoliagelesions(blightedtops)orpresentinthe
contaminatedsoil.Sporangiaandzoosporescomeincontactwith
thetubersintwoways.
Firstly,bycontactfreshlyliftedhealthyandwoundedtuberswith
diseasedhaulmsandcontaminatedsoil.
Secondly,duringcropgrowth,thezoosporesandsporangia
washeddownthestemsintothesoilbyraincomeincontactwith
thetubers.
Tuberinfectionisdependentonthegerminationofsporangia,
releaseandmotilityofzoospores.Thereleasedzoosporeshaveto
movethroughsoiltotheinfectionsites.Thelongerthezoospores
continuetoswimandgreatertheirnumber,thegreaterarethe
chancesofinfection.Thegermtubesgainentrancethroughthe
eyes,woundsandlenticels.

21. DISEASE CYCLE

22. CONTROL MEASURES
Thediseaseofthelateblightofpotatocanbecontrolledbyadoptingthe
followingmeasuresasstatedbelow:
1.Selectionofthehealthy,diseasefreetubersanddiseasefree
certificationofthetubersbeforetheplantation.
2.Storageoftheseedsbelow28℃orbelowtoreducethechanceof
infection.
3.Treatmentofthetuberswiththedifferenttypeoforganicchemicals
beforestoragetoreducetheinfection.
4.UseoffungicideslikeBordeauxmixture,Perenox,Blitox-50,
Fytolan,DiathaneM-45,DiathaneZ-78,DiathaneM-22andothersto
reducetheinfectiontoeradicatethepathogen.
Culturalpracticesinvolvedtheproperdisposalofinfectedtubersandthe
killingofthevinesbytheapplicationofweedicidescanminimizethe
infection.
Growingthediseaseresistantinfectionaspreventionisbetterthancure
toobtaindesiredoutput.

23. THANKYOUTOVISITME
REFERENCES
1.IntroductiontoMycologyandPhytopathology-Mishra&Dash,
AtextbookofFungi-Vasistha
3.AtextbookofBotany-Hait,Bhattacharyya&Ghosh
4.studiesinBotany
5.CollegeBotany
6.Googleforimages
7.DifferentWebPagestoexplorethetopic.
DISCLAIMER:
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