Downy Mildew

Downy Mildew is a fungal disease that is caused by a number of different fungi such as Bremia, Peronospora, Basidiorphora and others. It can be identified by white, blue, gray, or violet patches of mildew that will form mostly on the undersides of leaves. Eventually the tops of the leaves will turn to a pale green or yellow and then will wilt and often die. Downy mildew can affect the rest of the plant such as the fruit, flowers or stems and this disease can often cause seedlings to collapse and die. Downy mildew can be devastating to commercial grape crops and can affect a number of other vine crops such as cucumbers, pumpkins, squash and watermelon. Onions, certain bush fruits, some trees, and shrubs can also be affected by downy mildew. Downy mildew like many fungal diseases overwinters in dead plant materials in the soil. Spores are carried to new leaves by the splashing of the rain, as the disease requires a film of water for germination and infection.

The first lesions will appear approximately 5-17 days after the initial infection. Downy mildew appears to thrive in cool wet conditions, with high humidity and frequent rain or dew, spores generally germinate between 50 -75 degrees. Prevention of downy mildew is much like other fungal diseases, it starts with good sanitation practices, such as cleaning up dead plant debris and disposing of diseased plants properly by burning or in the garbage. Be sure to purchase disease free seeds and plants, and make sure that your soil is well drained and contains adequate nitrogen and other nutrients, an under nourished plant will succumb to disease much fast than a healthy plant. When planting be sure that rows are planted in the direction the wind most commonly comes from, this will increase air circulation, and help keep the plants much drier. Crop rotation is helpful in larger operations, particularly when disease has been present in recent years.

Be sure to plant crops that are not susceptible to downy mildew in the place where downy mildew has been spotted. Remove all diseased plant stock and burn or dispose of in such a way that will prevent the disease from spreading. Water only when necessary ,early in the morning and as close to soil level as possible, to encourage leaves to dry as quickly as possible. Fungicides can help with downy mildew, but be sure to do your research as not all fungicides are as effective as others on this difficult to control disease. It is important to use the fungicide before downy mildew becomes evident if at all possible, if not be sure to remove all infected plant parts before applying it. Fungicides will need to be applied every seven to ten days to be most effective in disrupting the disease cycle. The disease is much easier to prevent that it is to treat so be sure to start your spray program early and be consistent for best results. If you need effective disease control for downy mildew then take a look at Marrone Bio Innovations, they sell a green fungicide that increases the plants ability to fight disease, without harming the environment.