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Metalaxyl Controls Downy Mildew and Supplements Horizontal Resistance to Bremia lactucae in Lettuce Grown on Organic Soil in New York. J. E. Yuen, Graduate Research Assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. J. W. Lorbeer, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. 67:615-618. Accepted for publication 3 November 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-615.

In field trials during 1979 to 1981, metalaxyl (Ridomil 2EC and 5G) controlled downy mildew of lettuce caused by Bremia lactucae on lettuce grown on organic soil in New York state. In 1979, four weekly sprays of metalaxyl (Ridomil 2EC) at three rates from 0.14 to 0.56 kg a.i./ha or a single spray of metalaxyl (Ridomil 2EC) at 1.12 kg a.i./ha were more effective in controlling downy mildew than four weekly sprays of mancozeb at 1.79 kg a.i./ha. In 1980, metalaxyl (Ridomil 5G) from 0.5 to 1.5 kg a.i./ha placed in the seed furrow at planting reduced the incidence of downy mildew in the lettuce cultivars Ithaca, Mesa 659, and Minetto. This effect was uniform despite the various levels of horizontal resistance in the three lettuce cultivars. No interaction between cultivars of lettuce and metalaxyl was detected. In 1981, there was an interaction between metalaxyl (Ridomil 5G) applied at planting at 1.5 kg a.i./ha and metalaxyl (Ridomil 2EC) applied midway through the growing season at 1.0 kg a.i./ha, although all fungicide treatments controlled downy mildew.

Keyword(s): Lactuca sativa.