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Growth and Zoospore Germination of Phytophthora spp. on P₁₀VP Agar with Hymexazol. F. C. S. Tay, Professional officer, School of Environmental and Life Sciences, Murdoch University, Western Australia 6150; K. Nandapalan(2), and E. M. Davison(3). (2)Graduate assistant, School of Environmental and Life Sciences, Murdoch University, Western Australia 6150, Present address: Department of Virology, Royal Victoria Infirmary, Newcastle upon Tyne, U.K. NE1 4LP; (3)Mycologist, Department of Conservation and Environment, 1 Mount St., Perth, Western Australia, 6000. Phytopathology 73:234-240. Accepted for publication 11 August 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-234.

Mycelial growth of 49 isolates of eight Phytophthora spp., including 25 isolates of P. cinnamomi, on P₁₀VP + hymexazol agar in a half-strength potato-dextrose agar (PDA) base (P₁₀VPH) was compared with growth on half-strength PDA (½PDA). Hyphal extension rates of isolates from groups I- III and V were suppressed on the P₁₀VPH, whereas isolates from group VI showed enhanced growth. The effect of the P₁₀VPH medium on mycelial growth varied among isolates of the same species. The germination of zoospores and the germ tube growth of P. nicotianae and P. palmivora were suppressed on P₁₀VPH; motile zoospores were more sensitive than encysted ones. The age of the medium (1 or 4 days) had no effect on the percentage germination or germ tube length, and the sterility of the zoospore suspension had much less effect than the type of medium. Zoospore germination of 20 Western Australian isolates of P. cinnamomi was also suppressed on P₁₀VPH. The mean germination of motile zoospores on P₁₀VPH was 72% of that on ½PDA, whereas the mean germination of encysted zoospores was 85% of the control value. The mean germ tube length after 5 hr on P₁₀VPH was 81% of that on ½PDA. Therefore, when P₁₀VPH agar is used for the quantitative detection of P. cinnamomi zoospores in soil, viable propagules are underestimated due to the antibiotics in the agar.