Patches of white or grayish powder on garden plants, lawns, shade trees, or shrubs are probably powdery mildew. Powdery mildew is a fungal disease found throughout North America. It can cause severe problems in irrigated areas. Powdery mildew can attack a wide variety of turf plants, garden vegetables, and landscape trees and shrubs. The degree of damage varies with the plant and environmental conditions. In some cases, yield is lost or plants might die. In other species of plant, yield is not affected and damage is negligible.
In gardens and orchards powdery mildew is commonly found on peas, cucumbers, squash, strawberries, raspberries, apples, and cherries. On garden plants, look for patches of grayish-white powder on either the upper or lower surfaces of leaves or on various parts of the plants. As soon as diseased garden plants are discovered, prune out and destroy the infected parts. Disinfect your pruning tool in a bleach solution of one part household bleach to four parts water after each cut. If the infestations are severe, remove and destroy entire plants that are infected.
In addition to pruning, fungicidal soap, which is manufactured by several companies, can help to control the disease. Fungicidal soaps are safe for both humans and the environment and are made from highly refined fatty acids, a form of soap. Another option is to spray once a week with a solution of one teaspoon baking soda dissolved in one quart of water. Antitranspirants, such as Wilt-pruf or VaporGard, have also been shown to prevent powdery mildew when applied every two or three weeks. Sulfur-based fungicides applied every 7-10 days will also help prevent spread of powdery mildew to healthy plants. Be sure to spray the undersides of leaves as well as the upper surfaces when using any of these sprays.
To manage powdery mildew in fruit trees, follow the same methods just discussed. In areas where powdery mildew is a chronic and severe problem in fruit trees, a preventative spraying program should begin in spring, with weekly applications of a fungicide such as lime sulphur to prevent establishment of the disease.
Shade trees and ornamentals are usually not severely damaged by powdery mildew. Many times all that is needed for trees and ornamentals is pruning and cleanup of fallen leaves and debris in the fall, unless the infestation is serious.
Powdery mildew in lawns is seldom serious enough to warrant chemical control. In shady areas that might be more prone to infection, shade tolerant grasses such as creeping red fescue can be planted. Raise the mowing height for grasses in shady areas that are prone to powdery mildew infection, and avoid over- or under-application of nitrogen. Organic fertilizers or slow-release formulations of lawn fertilizers are good choices. Watering lawns in the morning gives the plants the rest of the day to dry off, discouraging establishment of diseases, including powdery mildew. The longer plants remain wet, the more susceptible they are to infection by diseases. Do not work around plants when they are wet, as the disease spores can easily be transported to other plants on your clothes, hands, and tools under these conditions. Thin plants to allow air circulation and penetration of sunlight.
References:
Agrios, G.N. 1988. Plant Pathology. Third edition. Academic Press, San Diego, CA. 803 pp.