Downy Mildew:

       Downy mildew is a widespread, serious disease of beans, crucifers, cucurbits, maize, potato, tomato, rice but most common with grapevines. Previously considered a fungus, the downy mildew pathogen called Plasmopara viticola, is now classified an algae. Initial leaf symptoms are light green to yellow spots, called “oil spots” because they may appear greasy. Under humid conditions, white, downy spore masses can be seen on the lower leaf surface. These spores are wind dispersed. The lesions eventually turn brown as the infected tissue dies. Severely infected leaves drop prematurely, which can reduce winter hardiness of the vine. Infected flower clusters dry up or become covered with white spores under humid conditions. Infected berries turn a mottled dull-green or reddish purple and readily fall from the cluster. Although berries become resistant to infection within three weeks after bloom, the rachis remains susceptible for several weeks longer.

     Downy mildew needs warmth and water. As for most algae, in most stages of its life cycle, take away water and it will die. As a reminder of its dependence on free-water and because water runs downhill, think of downy as ‘down-hill mildew’! In contrast, we may call the other mildew ‘powdery-dry’ mildew because it grows in the absence of free water. Periods of high risk from downy can be determined by monitoring the vineyard microclimate for factors such as temperature, rainfall, relative humidity (RH) and leaf wetness. This allows optimum timing of control actions when they are needed, and gives confidence to withhold sprays when not required. Affected leaves turn yellow and are rapidly shed from the plant. A fine white fungal growth may be visible on the lower leaf surface, but affected leaves decay rapidly. Flowers are also commonly shed, and the plant is often reduced to bare branches with a small tuft of yellow leaves and flower buds at the tip. Severely affected plants will eventually die

      Spreads in warm moist weather Downy Mildew begins with primary infection when spores are splashed from the soil to the leaves. Downy mildew spreads in secondary infection when spores are splashed from leaf to leaf to bunch.

     A useful rule of thumb is the three t’s of good spray application: type, timing and technique. Like three links in a chain, each of the t’s must be of good quality for good management of disease. We shall discuss the first two here. The third, technique, includes spray coverage and dose. Fungicide Types: the two main types of downy mildew fungicide are classified according to their timing in relation to infection events. They are best applied when they are most effective, that is either pre- or post-infection. Downy mildew can be avoided by rotating annual flowers and vegetables and by avoiding overwatering, overcrowding, and poorly drained soil. Other avoidance measures are growing resistant varieties, sowing disease-free seed, removing diseased parts and crop refuse, eliminating weeds, and maintaining balanced soil fertility. The application of copper, maneb, or zineb is effective against many downy mildews, but the amount of residue on vegetables must be considered.

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