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A Procedure for Isolation and Maintenance of Peronospora destructor on Onion. A. A. Abd-Elrazik, Former visiting associate professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853, Present address of senior author: Department of Plant Pathology, Assiut University, Assiut, Egypt; J. W. Lorbeer, professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 70:780-782. Accepted for publication 29 January 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-780.

Procedures described by previous workers were not satisfactory for the isolation and subsequent maintenance of Peronospora destructor on onion plants in the greenhouse. High levels of infection and subsequent sporulation were obtained when leaves of onion plants grown from bulbs in a greenhouse were wiped once with a dry pad of cotton immediately prior to inoculation. Sporangia collected from naturally infected onion plants in commercial onion fields in New York (fields sprayed weekly with chlorothalonil and/or mancozeb) were washed three or four times, suspended in tap water, and used as inoculum. Inoculated plants were incubated in a moist chamber at 14 C for 24 hr in the dark, placed in a growth chamber (21,500 lux = 2,000 ft-c) at 14 C for 4 days, and then placed on a greenhouse bench with supplemental fluorescent light (16 hr) at 18 C for 8 days. Sporulation was induced overnight by placing the infected plants in a moist chamber at 14 C in the dark commencing at 1600–1700 hours. Once isolated, the pathogen was maintained by transfering 1- to 2-day-old unwashed sporangia to healthy plants every 3 wk and using the cotton wipe technique and the inoculation and incubation procedures described. All inoculated plants became infected and abundant sporulation was obtained.