Efficacy over time of cymoxanil mixtures in controlling late blight in potatoes incited by a phenylamide-resistant isolate of Phytophthora infestans

Field-grown potatoes (cv. Alpha) were sprayed with fungicides at the commercially recommended rates 0-15 days before being inoculated with a phenylamide-resistant isolate of Phytophthora infestans (Mont.) DeBary. Numbers of lesions, or disease severity, in fungicide-treated versus fungicide-free plants were used to calculate the percentage control efficacy of fungicides over time. Efficacy over time fitted a declining logistic model in 26 out of 31 cases. Non-transformed data showed a significantly longer duration of > 90% efficacy of the cymoxanil mixtures Mancur (mancozeb + cymoxanil, 4 + 1), Pulsan (mancozeb + oxadixyl + cymoxanil, 7 + 1 + 0 · 4) and Sandocur-M (mancozeb + oxadixyl + cymoxanil, 7+1+2) (9-11 days) compared with SAN-518 (mancozeb + oxadixyl, 7+1), Ridomil-MZ (mancozeb + metalaxyl, 1 + 7 · 5), mancozeb or Curzate (4-6 days). Predicted T₅₀ values (number of days with >50% efficacy) were significantly larger for Pulsan and Sandocur-M (16 days) than for Ridomil-MZ (11 days), SAN-518 (10 days), mancozeb (9 · 5 days) and Curzate (7 · 5 days). Numeric integration of the area under the predicted control efficacy curves gave significantly larger values for Pulsan and Sandocur-M compared with the other fungicides except Mancur. Linear regression of logit disease control values over time resulted in the smallest slope for Sandocur-M. Analysis of variance showed no significant interaction between fungicide and cumulative rain. We predict that Pulsan and Sandocur-M may effectively control phenylamide-resistant Phytophthora infestans in the field at 14-day spray intervals, and Mancur at 10-day intervals.