Fungicides induce phenotypic changes in the late blight fungus

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Late blight is a devastating disease of tomato and potato caused by the fungus Phytophthora infestans, the pathogen responsible for the Irish potato famine, one of the most awful tragedies recorded in human history. Late blight is largely controlled by chemical fungicides, and the cost of chemical control worldwide could well exceed US$1 billion annually.Crop losses in the field and during storage are probably several-fold higher. Metalaxyl, introduced in the 1970s, has been the most effective control. However, resistant strains of P. infestans have begun to appear and, concomitant with the use of Metalaxyl, there has been a resurgence of late blight in the past two decades accompanied by the appearance of exotic pathogen strains and a shift in mating types.Carol Groves and Jean Ristaino have shown that compared with a metalaxyl-sensitive isolate, a wide range of chemically diverse fungicides can induce normally heterothallic isolates of P. infestans to form oospores asexually in vitro after short periods of exposure to commercially formulated fungicides. Furthermore, alterations in the phenotypic expression of mating type were also observed in some of these isolates.Copious amounts of fungicides are applied annually to potato and tomato production areas to control late blight. The data presented 1xCommercial fungicide formulations induce in vitro oospore formation and phenotypic change in mating type in Phytophthora infestans. Groves, C.T. and Ristaino, J.B. Phytopathology. 2000; 90: 1201–1208Crossref | PubMedSee all References1 indicate that a wide range of chemically diverse fungicides can induce normally heterothallic metalaxyl-resistant isolates of P. infestans to form oospores. This should serve as a wake-up call that these chemicals have major non-target effects on the reproductive biology of the pathogen.