Fungicidal control of downy mildew ( Bremia lactucae ) on lettuce

Field experiments to evaluate fungicides for the control of downy mildew (Bremia lactucae Regel) on lettuce were undertaken because of reports of poor disease control with acylalanine fungicides, and the possible withdrawal from the market of ethylene bisdithiocarbamate fungicides.Of 6 fungicides evaluated, dimethomorph at 0.375 and 0.75 g a.i./L and phosphonic acid at 2.4 g a.i./L were the most effective. Dimethomorph and phosphonic acid controlled downy mildew where acylalanine and, in most cases, acylalanine plus mancozeb formulations were ineffective. The use of lettuce cultivars with resistance to downy mildew and alternating spray programs of either dimethomorph and phosphonic acid should prolong the effectiveness of cultivars and fungicides.