Variation in Growth Rate and Colony Morphology of Phytophthora parasitica Induced by Metalaxyl.

Growing isolates P991 and P731 of Phytophthora parasitica on medium containing 25μg/ml metalaxyl for 6 weeks, caused 66-74% of their single-zoospore cultures to increase the range of growth rates on metalaxyl-free medium. Metalaxyl treatment also caused each isolate to produce six colony types, including the parental type, on medium without metalaxyl. Growth rate and colony morphology of six single-zoospore cultures each representing a different colony type did not change after five successive subcultures. However, all the five colony-type variants produced single-zoospore cultures with greater variation in growth rate than did the parental type.