Powdery mildew is a fungal disease that appears as a dusty gray or white coating on plants. Vegetables such as cucumber, pumpkins, squash and melons; houseplants such as African violet and begonia; and outdoor plants such as phlox, lilac and rose are all susceptible to powdery mildew. Not only is the disease unattractive, but it can distort and stunt a plant's leaves, buds, tips and fruit. If enough leaves or tissue are affected, the plant may die.
How Powdery Mildew Forms
Wind carries powdery mildew spores. The disease can appear at any time, but it's more likely to grow rapidly in shady areas, if there is slow or nonexistent air circulation or when the humidity is high. Powdery mildew can reproduce both sexually and asexually.
If you're planting a susceptible specimen, choose an area with as much sun as the plant can handle. Don't crowd the plant; make sure there is enough room for air to circulate. Use a slow-release fertilizer; avoid liquid fertilizers that may splash spores back onto the leaves. Water the plants at the root level, not from above. Plant varieties resistant to powdery mildew.
There are several organic treatments that fight powdery mildew. Milk contains salts and amino acids beneficial to plants and that combat the disease. To use, mix 1 cup of skim milk with 9 cups of water and spray on infected areas every two to three days.
Make a spray of 1 tbsp. of baking soda in 1 gallon of water and spray on infected areas every three to five days.
Organic Neem oil is a broad-spectrum fungicide. Mix 2 tbsp. and 1 1/2 tsp. dish soap per gallon of water. Spray all plant surfaces, including the undersides of leaves, until wet.
Cleanup is key. Gather all fallen leaves and flowers that are affected and place in the trash. Do not compost them.