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Comparative Antifungal Activity of Four Phosphonate Compounds Against Isolates of Nine Phytophthora Species. D. G. Ouimette, Graduate research assistant, Department of Plant Pathology, University of California, Riverside 92521-0122; M. D. Coffey, professor, Department of Plant Pathology, University of California, Riverside 92521-0122. Phytopathology 79:761-767. Accepted for publication 20 February 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-761.

The systemic fungicide potassium phosphonate, three alkyl-substituted phosphonate compounds (monoethyl, dimethyl, and diethyl phosphonate), and potassium hypophosphonate were compared for their activity in vitro against 34 isolates of nine species of Phytophthora. Potassium phosphonate proved to be the most inhibitory compound against most isolates. In contrast, potassium hypophosphonate was the least inhibitory, in some cases over 100 times less than potassium phosphonate. Among the alkyl phosphonates, monoethyl phosphonate was generally more inhibitory than the two dialkyl phosphonates. Compared to diethyl phosphonate, dimethyl phosphonate was either equally or more inhibitory toward the majority of isolates tested. In vivo, the four phosphonate compounds were equally effective in controlling stem rots of Persea indica L. and pepper (Capsicum annuum L.) caused by P. citricola and P. capsici, respectively. Potassium hypophosphonate provided no control of either disease. In contrast, no phosphonate compound was effective in controlling stem rot of pepper caused by a laboratory-produced, phosphonate-resistant mutant of P. capsici. The rapid hydrolysis in plant tissues of the organic alkyl-substituted phosphonates to the more antifungal inorganic phosphonate anion probably explains the similar level of disease control achieved by all four phosphonate compounds.