Ecological studies of Phytophthora cactorum (Leb. and Cohn) shroet, with reference to collar rot disease of apple

In culture studies temperature optima progres81ve~ deoreased from 25 to 7°0 for ~oelial growth, sporangial produotion, oospore produotion, zoospore release and zoospore motility.Oospore foroation was slightly inhibited and sporangial production greatly stimulated by light and aeration.In free water susponsions, zooaporeo remained motile for upto 71 H at 7°c.In saturated particulate systems, the motile period was always less and deoreased with the partioles1ze or the system. :;Z~oapores were dirrerential~ attraoted to the tissues of varied plant speoies, .but the degree of attraotion wan not neoessarily related to subsequent colonisation.Zoospores infeoted tissue fragments oi several common plants, inoluding grasses, incubated on nonsterile soil surfaoes for upto 7 days beiore inooulation.The fungus sporulated freely on and in the i~~ediate vioinity ot the plant fragments.Zoospore cysts recained viable for upto 13 days on nonaterile s01l; persistence was inoreased in 60ils amended with relatively fresh erass lear fragments, but reduoed in 90110 amended 14 days before infestation.lfonsterile 0011e stimulated spornn(;1nl produotion and induced the germination of oospores within 1 days.On soil a emall proportion only of oospores germinated at any one t1me dur1ng a year, but none germinated in the winter m~nths.~e periodicities of oospore germination and or isolation from Boil by baiting were similar, indicating the probable signifioant role of ooaporea in perennation.trhe spring-time cormnenoemant or aotivi ty or 1. oaotorum in soil ooincided with that of apple blossom development -a period 4 when host resistanoe to infeotion was low.In this pariod, inooulation dates a few days apart resulted in great differenoes in lesion development.It was oonsidered that in oertain springs the late resumption of aotivity by the pathogen might result in somo disease eooape.Resistance or apple varieties was also modified by the rootstock type.This effect of the rootstook was anaooiuted with ita influence on tree vigour and not its inherent roaistanoe.Differenoes in the resistanoes of rootstooks wero demonstrated by zoospore inooulations of the root system~, but in no instance did progressive diaease situatifln8, resulting in tree deaths, develop~~.This poasibility provides further ovidenoe or severe restriotion of the Dubterranean aotivity of 1onotorum. ACYJTO•.