Chemical control of late blight epidemics incited by metalaxyl-resistantPhytophthora infestans in potato crops in Israel

Four field trials were conducted to evaluate the efficacy of various fungicide formulations in controlling late blight in potato crops artificially inoculated with A2 metalaxyl-resistant isolates ofPhytophthora infestans. Two trials (Winter 1986, Spring 1987) were performed at the research farm of Bar-IIan University (central coastal plain) and two trials (Winter 1987, Spring 1988) at the Besor Experiment Farm (western Negev), with seven formulations tested in each trial. At Bar-IIan, Mancur (mancozeb + cymoxanil, 4:1) applied once a week was most effective in controlling the blight. Sandocur-M and Pulsan (mancozeb + oxadixyl + cymoxanil, 7:1:2 and 7:1:0.4, respectively) applied at biweekly intervals were the second best, while SAN 518F (mancozeb + oxadixyl, 7:1) and Ridomil-MZ (mancozeb + metalaxyl, 7.5:1) applied at biweekly intervals, and mancozeb or cymoxanil applied once a week, were the least effective. At the Besor farm Ridomil-MZ at biweekly intervals was the least effective, but the efficacy of the other formulations varied between the two trials. Mancur, Sandocur-M, Previcur-N and mancozeb were highly effective in Winter 1987 but less so in Spring 1988. In the latter season Mancur, Sandocur-M and Previcur-N were significantly more effective than mancozeb or Ridomil-MZ. The correlation coefficient between yield and final disease level was -0.5322.