Bioassay for validation of metalaxyl persistence to control Phytophthora palmivora infection in durian

Metalaxyl is the most widely used fungicide to control Phytophthora diseases. A detached leaf bioassay was developed to quantify metalaxyl efficacy using metalaxyl-sensitive (CbP01) and metalaxyl-resistant (CbP03) isolates of Phytophthora palmivora as biological indicators. The linearity of pathogen growth on metalaxyl-treated leaves ranged from 1.6–200 ppm. This assay was applied to determine metalaxyl efficacy on durian leaf after foliar spraying and soil drenching at concentrations of 2 and 4 g/l. Metalaxyl applied as foliar spray inhibited the growth of CbP01 up to 80% for 30 days after application and up to 62–65% inhibition 60 days after application. Soil drenching strongly inhibited the growth of both CbP01 and CbP03 isolates with inhibition greater than 90% for at least 60 days after application. Although soil drenching was more effective than foliar spraying in controlling P. palmivora, soil drenching with 4 g/l metalaxyl resulted in seedling death within 15 days after application. This simple bioassay to evaluate the efficacy and persistence of metalaxyl in planta is beneficial for designing and optimizing metalaxyl application with maximum efficiency and minimum negative impacts on human health and the environment.