Sensitivity of Phytophthora Infestans to cymoxanil

The efficacy of cymoxanil, Pulsan (oxadixyl + mancozeb + cymoxanil, 1:7:0.4) and Sandocur-M (oxadixyl + mancozeb + cymoxanil, 1:7:2) in controlling late blight was examined in potatoes (cv. ‘Alpha’). Nine oxadixyl-sensitive and seven oxadixyl-resistant isolates of Phytophthora infestons from six countries were tested. ED₉₀ values of cymoxanil ranged between 64 and 467 mg a.i. /l. No relationship was observed between sensitivity to cymoxanil and sensitivity to oxadixyl. Isolate S6A (1305) from California was the most sensitive to cymoxanil (ED₉₀ = 64 mg/l), whereas isolate R11 from Israel was the least sensitive (ED₉₀ = 467 mg/l). Most isolates from Israel were less sensitive (ED₉₀ = 151–467 mg/l) than isolates from Switzerland, the Netherlands, Ireland, Mexico or California (ED₉₀ = 64–152 mg/l). Three-way mixtures of cymoxanil with mancozeb and oxadixyl were more inhibitory than cymoxanil alone, regardless of the isolate. Sandocur-M was, for most isolates, more effective than Pulsan. The correlation coefficient between sensitivity to cymoxanil and sensitivity to Sandocur-M was 0.67, while that between cymoxanil and Pulsan was 0.76.