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Semiselective Media for the Isolation of ElsinoŽ fawcettii from Citrus Scab Pustules. J. O. Whiteside, Professor, Department of Plant Pathology, University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred 33850. Plant Dis. 70:204-206. Accepted for publication 31 August 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-204.

Isolating Elsinoe fawcettii from citrus scab pustules or detecting viable propagules of this pathogen in them can be difficult or even impossible by conventional methods, particularly if the pustules are old. This slow-growing pathogen is confined to the pustuleís stromatic portion, which is colonized by many fast-growing organisms including Cladosporium spp. and yeasts. E. fawcettii was isolated successfully by scraping small fragments from the pustules onto semiselective media in isolation plates. Potato-dextrose agar (PDA) was the preferred basal medium, because E. fawcettii formed a unique, compact, raised, and easily identifiable colony on it. Dodine was the only material found that suppressed yeasts and Cladosporium spp. sufficiently to permit use of PDA. The most useful medium developed contained 39 g of PDA, 100 mg of streptomycin sulfate, 100 mg of tetracycline hydrochloride, and 400 mg a.i. of dodine per liter of water. Chlorothalonil showed some potential for the selective isolation of E. fawcettii, but because of inactivity against yeasts, it had to be used in water agar to avoid abundant yeast growth.