Metalaxyl Stimulation of Growth of Isolates of Phytophthora infestans

Metalaxyl is a systemic organic fungicide used to control Phytophthora infestans, causal agent of potato and tomato late blight, and other plant-pathogenic oomycetes. Metalaxy-resistant isolates of P. infestans are common and have been found worldwide. Four independent metalaxyl-resistant isolates were cultured on media containing or lacking metalaxyl. Metalaxyl stimulated vegetative growth of three of the four isolates. One isolate was stimulated by metalaxyl only when nutrients were limiting. The data indicate that under certain conditions metalaxyl is beneficial to some metalaxyl-resistant isolates of P. infestans.