EFFECT OF METALAXYL ON FORMATION AND GERMINATION OF OOSPORES OF PHYTOPHTHORA-INFESTANS

Oospores of Phytophthora infestans were produced in potato leaf discs floating on metalaxyl solution (100 mu g mL(-1) a.i.) and inoculated w ith all combinations of two metalaxyl-sensitive and two-resistant pare ntal isolates. Numbers of oospores produced varied between different m atings, depending on parents, in the absence of the fungicide and when metalaxyl was added 0, 7, 14 and 21 days after inoculation. Oospores were not produced when metalaxyl was added at the time of inoculation (0 days) when either one or both parents were sensitive to metalaxyl. In two of three such matings further oospore formation was arrested wh en metalaxyl was added either 7 or 14 days after inoculation. Oospores extracted from leaf discs 14, 21 and 28 days after inoculation were a ssessed for germination on water agar after 21 days. Germination of oo spores from water control treatments varied between 6 and 30% dependin g on the cross. Germination was significantly reduced in oospores of m etalaxyl-sensitive parents extracted 28 days after inoculation of leaf discs treated with metalaxyl 0, 7 and 14-days after inoculation compa red with the 21-day treatment, Minimal differences in germination were observed for oospores from the mating of resistant parents irrespecti ve of metalaxyl treatment, although germination was generally low, not exceeding 8.5%.