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Powdery mildew is a fungal disease that affects a wide range of garden plants. It is easily identified, as its symptoms are quite distinctive. Infected plants will display white powder-like spots on the upper surfaces of the leaves and stems. The lower leaves are the most affected, but the mildew can appear on any part of the plant that shows above the ground. As the disease progresses, the spots get larger and thicker, in many cases forming a solid mat over the entire infected surface. Severe infections can cause stunting, distorted foliage, leaf drop, and may eventually be fatal on some plants if left uncontrolled.
Typically powdery mildew only occurs on the upper surfaces, a similar but unrelated group of fungi that grow on the lower sides of leaves are downy mildews.
The fungi that cause powdery mildews generally feed only on the epidermal cells of the plant, which they penetrate using organs called haustoria.
In early spring, the overwintering bodies release spores which are spread by wind or water splashing onto the leaves and other plant parts. The fungi then spread over the plants' surfaces, while also penetrating the epidermal cells with haustoria, which are short, specialized hyphae. The fungi that cause powdery mildews generally feed only on the epidermal cells of the plant.
During the growing season the fungus continually produces conidiospores, which reinfect the plant or are spread to other hosts.
Late in the season, specialized fruiting bodies called cleistothecia are produced, which either overwinter on the soil surface or release aeciospores, which infect the next years' buds. Cleistothecia are visible to the eye as small yellow to black dots.
The various species of powdery mildew affect a wide variety of plants in hundreds of genera.
- Exclusion: Removing infected materials can provide some control, but total exclusion is nearly impossible due to the wind-borne spores.
- Cultivation: Mulches around infected plants should be removed and replaced over winter.
- Cultural Controls: Overhead irrigation to wet the leaf surfaces helps reduce infections. Avoid nitrogenous fertilizers which cause overly lush foliar growth, creating an attractive target for the mildew.
- Physical Removal: Blasting with water to remove the fungus is effective, but must be repeated every 7-10 days
- Chemical Controls (organic): It has been reported that a dilute solution of 1 part skim milk to 9 parts water sprayed every 3 to four days on affected plants is an effective means of organically controlling many powdery mildew infestations. This method has been most studied in squash and grape plants. In some tests, the milk mixture has outperformed popular synthetic fungicides.
- Other organic means of control include mixtures of baking soda, soap, and horticultural oil.