Recent research findings indicated that disease occurrence can be predicted accurately by measuring the number of hours that free moisture is present on the leaf surface. When 10 to 12 continuous leaf-wetness hours occur, the disease will develop when it is present. Fungicide application should be made when these conditions occur. Downy mildew is a relative new disease of onion in the Lower Rio Grande Valley, having made sporadic appearances during the last five years. When it does occur, it can cause severe economic damage to the crop.
Symptoms consist of white to violet spots on leaves, later turning dark or almost black.
Typical symptom of this disease as it occurs in Texas is a series of contiguous verticle spots up and down the leaves. A fuzzy growth is observed on the surface of the leaves, particularly during periods of high humidity. Fields should be monitored closely, particularly during prolonged cold wet weather, when the disease is more likely to occur. Fungicide applications should begin on plants two to three months old as soon as these conditions prevail. Downy mildew is seldom found in Texas as the season progresses into warmer weather.