Metalaxyl Controls Downy Mildew and Supplements Horizontal Resistance toBremia lactucaein Lettuce Grown on Organic Soil in New York

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Abstract
In field trials during 1979 to 1981, metalaxyl (Ridomil 2EC and 5G) controlled downy mildew of lettuce caused by B. lactucae on lettuce grown on organic soil in New York state. In 1979, 4 weekly sprays of metalaxyl (Ridomil 2EC) at 3 rates, 0.14-0.56 kg a.i.[active ingredient]/ha or a single spray of metalaxyl (Ridomil 2EC) at 1.12 kg a.i./ha were more effective in controlling downy mildew than 4 weekly sprays of mancozeb at 1.79 kg a.i./ha. In 1980, metalaxyl (Ridomil 5G) at 0.5-1.5 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 3 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.

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