Abstract
Eight field isolates of P. cubensis, the fungal causal agent of downy mildew in cucurbits, were tested for sensitivity to metalaxyl and mancozeb. While mancozeb efficiently controlled downy mildew in cucumbers inoculated with 3 metalaxyl-sensitive isolates, it failed to do so in cucumbers inoculated with 5 other metalaxyl-resistant isolates. The practical and theoretical implications of this finding are discussed.