Isolation of Oospores of Sunflower Downy Mildew, Plasmopara halstedii, and Microscopical Studies on Oospore Germination

A method was elaborated to isolate oospores of Plas! mopara 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 oosporesharvested from 3! to 5!week!old systemically infected sun~ower plants\ grown under constant conditions at 05>C\ as well as from _eld plants\ harvested late in the season[ Pretreatment of oospores with cold temperaturespreviously assumed to stimulate the rate and to accelerate the speed of oospore germination\ did not result in such e}ects[ Germination usually occurred within 09 29 days of incubation at a highly variable rate of about 0 to 06) "average 5[6)# in deionized water