In vitro Selection of Strains of Phytophthora cactorum Resistant to Metalaxyl

Abstract Isolates of Phytophthora cactorum resistant to the systemic fungicide metalaxyl were obtained by exposing them to sequentially increased concentrations of metalaxyl. A linear relationship was observed between the concentrations of metalaxyl and percentage inhibition of mycelial growth of P. cactorum. The stability of metalaxyl‐resistant isolates 150R and 250R was confirmed after six serial transfers on corn meal agar without fungicide. The in vitro metalaxyl‐resistant isolate (Ph10) was less aggressive on apple rootstocks compared with the Ph07 isolated from metalaxyl‐treated trees and the Ph03 isolated from untreated trees. Metalaxyl‐resistant and sensitive isolates remained sensitive to the chemically unrelated fungicide fosetyl‐Al at high concentration (600 μg/ml), to mancozeb, and to a mixture of metalaxyl + mancozeb. Significant differences in resistance to metalaxyl existed among P. cactorum field isolates.