Evaluation of fungicides for control of downy mildew of lettuce

Downy mildew (Bremia lactucae) is an economically important disease of lettuce in New Zealand. At present, few chemicals are registered in this country for control of this disease. The aim of this experiment was to evaluate the efficacy of 0 chemicals for control of downy mildew of lettuce. The experiment was laid out in a randomised block design with four replicates. An Oxford precision sprayer, calibrated to 500 litres/ha, was used to apply the chemicals. Disease assessments were carried out weekly. On 25 October 2006, 20 plants in the center two rows of each plot were harvested and assessed for downy mildew. Non-sprayed plants had 24.5% basal area infected. All ten fungicides tested reduced downy mildew. Fosetyl-aluminium had significantly (P<0.05) lower levels of downy mildew (2.0% basal area affected) than the other fungicides. Metalaxyl plus mancozeb and azoxystrobin gave good control of downy mildew (with 2.8% and 3.9% basal area affected respectively). Copper hydroxide and mancozeb gave poorest control of the disease (7.4% and 6.8% basal area affected respectively). Based on the results of this experiment, fosetyl-aluminium, metalaxyl plus mancozeb, and azoxystrobin show promise for control of lettuce downy mildew in New Zealand.