INVIVO EXPRESSION OF RESISTANCE TO METALAXYL BY A NURSERY ISOLATE OF PHYTOPHTHORA-PARASITICA FROM CATHARANTHUS-ROSEUS

Populations of a metalaxyl-insensitive isolate of Phytophthora parasitica from a nursery in southern California were determined every 2 wk for 10 wk from the root zones of container-grown Catharanthus roseus either treated or not treated with metalaxyl or fosetyl-Al. When compared with no treatment, metalaxyl applied every 4 wk as soil drenches at 18.8 or 37.4 mg a.i./L and fosetyl-Al applied every 4 wk as foliar sprays at 4.8 g a.i./L had no effect on populations of the isolate. However, increase of the pathogen population was delayed and the population was significantly less (P < 0.05) than that of the nontreated control at 6 and 10 wk when fosetyl-Al soil drenches at 1.92 g a.i./L were applied every 4 wk. Based on determinations of fresh shoot and root weights, metalaxyl failed to control disease and fosetyl-Al provided acceptable control.