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Effects of Phosphorous Acid and Fosetyl-Al on the Life Cycle of Phytophthora cinnamomi and P. citricola. M. D. Coffey, Professor, Department of Plant Pathology, University of California, Riverside 92521; M. C. Joseph, staff research associate, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 75:1042-1046. Accepted for publication 6 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1042.

Mycelial growth of both Phytophthora cinnamomi and P. citricola was inhibited by low concentrations of phosphorous acid (H3PO3). Concentrations required for 50% inhibition (EC50) ranged from 1.3 to 1.7 μg/ml for P. citricola, and from 4.1 to 6.2 μg/ml for P. cinnamomi. Sporangium formation by P. citricola and P. cinnamomi also was sensitive; EC50 values for H3PO3 inhibition were 1.4 and 1.8 μg/ml, respectively. Aluminum, calcium, and sodium tris-O-ethyl phosphonates (fosetyl-Al, fosetyl-Ca, fosetyl-Na) were as inhibitory to sporangium development as H3PO3. With P. citricola and P. cinnamomi the EC50 for inhibition of zoospore release by H3PO3 was 6 μg/ml. Cyst germination was not affected. H3PO3 inhibited oospore formation by P. citricola; 1 μg/ml caused 89- 97% inhibition among different isolates. Oospore production by P. cinnamomi was less sensitive; 1 μg/ml caused 60- 78% inhibition. The EC50 for H3PO3 inhibition of chlamydospore production by isolates of P. cinnamomi was 15- 44 μg/ml. Fosetyl-Al and H3PO3, used at equivalent rates of phosphite, were very similar in their effects on chlamydospore formation by P. cinnamomi and P. parasitica.