Powdery mildew looks like a dusty white or gray coating on stems or leaf surfaces. It is sometimes thought to be dust, which allows it to spread due to a lack of treatment. As the infestation becomes more serious, it turns to circular white spots of powder. Powdery mildew usually occurs later in the growing season. Prevention is the best cure for powdery mildew.
Because powdery mildew is caused by moisture, watering technique can help reduce the chances of its occurrence. If you use sprinklers or a sprinkler hose head, water in the morning so that the sun can quickly dry the leaves and stems of your plants. If you water at night, using a soaker hose that will help to keep moisture away from the leaves and stems. Soaker hoses also work well for daytime watering.
Space Plants Appropriately
Leave enough space between your plants. Crowding can help to increase humidity around your plants and can also help to reduce air circulation. Plant your vegetables at the distances recommended on the seed package or on the spike that was included with your nursery plants.
Remove spent or problem plants as soon as you can. Leaving those plants in the garden can reduce air flow. Allowing dying plants to remain can attract mildew and other potential garden pests and diseases. Make sure you remove any dead plants before planting in the spring to reduce the potential spread of disease.
A number of liquid sprays are said to have a positive effect in preventing powdery mildew. Liquid seaweed, applied directly to the leaves, is one. Sulfur spray can help prevent an infestation and can also help stop an infestation from spreading. Baking soda, mixed at between 0.1 and 0.5 oz. per quart of water can also help. A mixture of 1 part milk to 9 parts water, sprayed on the leaves once a week, can also help prevent or slow a powdery mildew infestation.