VIEW ARTICLE

Disease Control and Pest Management

Insensitivity of Thick-Walled Oospores of Pythium ultimum to Fungicides, Methyl Bromide, and Heat. T. E. Stasz, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; S. P. Martin, University of California, Santa Barbara 95616. Phytopathology 78:1409-1412. Accepted for publication 17 May 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1409.

Thick-walled oospores, thin-walled, quiescent oospores, and quiescent sporangia of Pythium ultimum were treated for 24 hr with up to 1,000 mg of a.i./L of the fungicides captan, etridiazol, fenaminosulf, maneb, and thiram. Propagules also were treated with heat at 50 and 70 C for 30 min, and with methyl bromide gas at a concentration of 60 mg/L for up to 6 hr. Thick-walled oospores were killed only by heat at 70 C; their viability was reduced by heat at 50 C and by high levels of etridiazol, but was not affected by captan, fenaminosulf, maneb, methyl bromide, or thiram. In contrast, thin-walled oospores and sporangia, when treated while quiescent, were killed by heat at 50 or 70 C, by captan, thiram, or etridiazol at 100–500 mg a.i./L, by maneb at 1,000 mg of a.i./L, and by methyl bromide at 60 mg/L for 6 hr. Surprisingly, quiescent thin-walled oospores and sporangia were not greatly affected by fenaminosulf at up to 1,000 mg of a.i./L. However, when treated during germination, oospores and sporangia were killed by low levels of all toxicants tested, including fenaminosulf. Insensitivity of thick-walled oospores to fungicides and heat may result in the reappearance of germinable propagules in treated soils due to conversion of these survival structures to the thin-walled condition.