Commercial fungicide formulations induce in vitro oospore formation and phenotypic change in mating type in Phytophthora infestans

A wide range of commercially formulated fungicides cause in vitro effects o n mating behavior in specific isolates of Phytophthora infestans, the causa l agent of late blight of potato and tomato. Four isolates of P. infestans representing each of the four common US genotypes, US-1, US-6, US-7, and US -8 and varying in their sensitivity to metalaxyl, were exposed to a variety of fungicides used to control late blight in petri dish assays at concentr ations ranging from 1 to 100 mug a.i./ml. Exposure of each of these normall y heterothallic single mating type isolates of I! infestans to 9 of the 11 commercial fungicide formulations tested resulted in the formation of oospo res after 2 to 4 weeks. The highest numbers of oospores were formed on medi a amended with Ridomil 2E (metalaxyl) and Ridomil Gold EC (mefenoxam) at 0. 1 to 10 mug a.i./ml, averaging as many as 471 and 450 oospores per petri di sh, respectively. Several other fungicides including Maneb, Manzate (Mancoz eb), Curzate (cymoxanil + mancozeb), and Acrobat MZ (dimethomorph + mancoze b) also induced oospore formation, producing from 0 to 200 oospores per pla te at fungicide concentrations from 0.1 to 10 mug a.i./ml. The metalaxyl re sistant isolates formed oospores in response to the fungicides more often t han the metalaxyl sensitive isolates. No oospores were formed on media amen ded with Brave (chlorothalonil) or Tattoo C (chlorothalonil + propamocarb H Cl) and these compounds completely suppressed growth of the isolates at 0.1 and 1 mug a.i./ml. Three metalaxyl resistant A2 isolates mated with both A l and A2 isolates after exposure to the fungicides Ridomil 2E and Ridomil G old EC. Alterations in mating type expression were also observed in a metal axyl sensitive Al isolate after exposure to Benlate (benomyl). Copious amou nts of chemicals are applied annually to potato and tomato production areas to control late blight. Our results indicate that a wide range of chemical ly diverse fungicides can induce normally heterothallic metalaxyl resistant isolates of P. infestans to form oospores in vitro after short exposures t o the fungicides.