Effect of Metalaxyl on Sporulation and Growth of Metalaxyl-Resistant and Metalaxyl-Sensitive Isolates of Phytophthora parasitica In Vitro. D. M. Ferrin, Assistant Professor, Department of Plant Pathology, University of California, Riverside 92521. M. L. Wadsworth, Laboratory Assistant, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 76:492-495. Accepted for publication 9 January 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0492.

The in vitro production of sporangia by two metalaxyl-resistant isolates of Phytophthora parasitica in the absence of the fungicide was within the range of that by five sensitive isolates. Sporangium production by the resistant isolates was not affected by metalaxyl at 1, 10, or 100 μg/ml, whereas that by three sensitive isolates was inhibited by as little as 1 μg/ml of the fungicide. The in vitro production of chlamydospores by one of the resistant isolates (P-015F) was greater than that by three sensitive isolates in the absence of the fungicide, but chlamydospore production by the second resistant isolate (P-014F) was less than that by the sensitive isolates. Chlamydospore production by P-014F was not affected by metalaxyl at 1, 10, or 100 μg/ml, whereas that by P-015F was inhibited at these concentrations. Chlamydospore production was inhibited by as little as 1 μg/ml for the three sensitive isolates, but the degree of inhibition was generally greater than for P-015F. Linear growth of P-015F was stimulated by metalaxyl at concentrations up to 100 μg/ml but was inhibited at 1000 μg/ml. In contrast, mycelial dry weight in metalaxyl-amended liquid media was not affected at 1 or 10 ?g/ml but was inhibited at 100 and 1,000 μg/ml.