Competition between oxadixyl-sensitive and -resistant field isolates of Phytophthora infestans on fungicide-treated potato crops

Potato crops (cv. Alpha) were grown in 19 walk-in plastic tunnels (33 m²) in fall 1986 and spring 1987. Beginning 4-5 weeks after planting, crops were treated, at 2-week intervals, with oxadixyl or oxadixyl-containing fungicidal formulations. One day before the first spray, crops were inoculated with a 9:1 mixture of oxadixyl-sensitive (S) and -resistant (R) sporangia of Phytophthora infestans. Late blight development and the proportion of R in the fungal population were estimated for 8 weeks after inoculation. Results showed that in the absence of fungicide treatment the proportion of the R isolate increased to 85% and then, towards the end of the epidemic, declined to 28%. In crops treated with SAN371F (oxadixyl), SAN518F (oxadixyl + mancozeb), Pulsan and Sandocur-M (oxadixyl + mancozeb + cymoxanil), the proportion of R reached a final level of 100, 97, 34 and 22%, respectively. In sprayed crops, blight development was positively correlated (R² = 0·86) with the mean proportion of R in the fungal population. Tuber yield was inversely (R² = -0·98) related to the area under the disease progress curve.