Isolation of oospores of sunflower downy mildew, Plasmopara halstedii, and microscopical studies on oospore germination.
A method was elaborated to isolate oospores of Plasmopara halstedii from tissue of its host, Helianthus annuus. Isolated oospores were studied microscopically and germination was documented with respect to the time course and the mode of germination. Formation of primary sporangia was similarly observed in oospores, harvested from 4- to 6-week-old systemically infected sunflower plants, grown under constant conditions at 16°C, as well as from field plants, harvested late in the season. Pretreatment of oospores with cold temperatures, previously assumed to stimulate the rate and to accelerate the speed of oospore germination, did not result in such effects. Germination usually occurred within 10-30 days of incubation at a highly variable rate of about 1 to 17% (average 6.7%) in deionized water.