Variation in the sensitivity to metalaxyl of Pythium spp. isolated from carrot and other sources

SUMMARY Over a 3‐yr period 261 isolates of 17 species of Pythium were tested for sensitivity to metalaxyl at concentrations of 5, 50 or 100 μ/ml. A wide range of responses was observed, from isolates where growth ceased at 5 μg/ml to those where growth at 100 μg/ml was similar to that of the untreated controls. In further tests isolates of 11 different species had ED50's < 1 μg/ml. A lower sensitivity was detected in isolates of six Pythium spp. where values in the range 1–10 μg/ml were obtained. This lower sensitivity was not related to previous known use of metalaxyl. Three isolates of Pythium dissotocum from sites where the fungicide had been used repeatedly had ED50's > 100 μg/ml and were considered resistant. The resistance was stable over a 2‐yr period and isolates were cross‐resistant to furalaxyl, benalaxyl, ofurace, cyprofuram and oxadixyl. Increasing concentrations of metalaxyl reduced or prevented the production of zoospores by four species of Pythium, although when zoospores were produced, this was followed by the normal processes of encystment and germination. Culturing P. dissotocum on different sub‐lethal concentrations of metalaxyl for 18 wk did not induce a high level of resistance to the fungicide.