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.