In Vitro Insensitivity to Metalaxyl of Isolates of Phytophthora citricola and P. parasitica from Ornamental Hosts in Southern California. D. M. Ferrin, Assistant Professor, Department of Plant Pathology, University of California, Riverside 92521. J. N. Kabashima, Farm Advisor, University of California Cooperative Extension, Anaheim 92805. Plant Dis. 75:1041-1044. Accepted for publication 3 April 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1041.

In vitro sensitivity to metalaxyl was evaluated for 13 isolates of Phytophthora citricola and 26 isolates of P. parasitica recovered from ornamental hosts or soil from 12 locations in southern California. One isolate of P. citricola from azalea and two isolates of P. parasitica from periwinkle were insensitive to metalaxyl. Mean EC₅₀ values for inhibition of linear growth were 219.9 µg a.i./ml for the metalaxyl-insensitive isolate of P. citricola and 717.4 and 742.4 µg a.i./ml for the two metalaxyl-insensitive isolates of P. parasitica. In contrast, mean EC₅₀ values ranged from 0.087 to 4.970 µg a.i./ml and from 0.255 to 3.080 µg a.i./ml for metalaxyl-sensitive isolates of P. citricola and P. parasitica, respectively. Based on mean EC₅₀ and slope values of the linearized dose-response curves, the metalaxyl-sensitive isolates of P. citricola could be divided into three distinct groups, whereas the metalaxyl-sensitive isolates of P. parasitica formed a single heterogeneous group.

Keyword(s): fungicide resistance, Subdue.