The Plant Doctor
Powdery Mildew Disease
Environmental Horticulture Farm Advisor
UC Cooperative Extension, Santa Cruz County
Powdery mildew is a common disease on many types of plants, and they are usually very well adapted to the climate of the Monterey Bay Area. Powdery mildew fungi tend to infect either host plants in the same family or only one species of plant.
You can recognize this disease by the white powdery appearance caused by the fungal filaments and spore growth on leaf surfaces and shoots and sometimes on flowers and fruits. Powdery mildews may infect new or old foliage. This disease can be serious on woody species such as rose, crape myrtle, and sycamore where it attacks new growth including buds, shoots, flowers, and leaves.
New growth may be dwarfed, distorted, and covered with a white, powdery growth such as found on coast live oak. Infected leaves generally die and drop from the plant earlier than healthy leaves.
Wind carries powdery mildew spores to new host plants. Although relative humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of free water. Interestingly, most other fungi and bacterial pathogens require a certain amount of free water to infect their host plants. Moderate temperatures of 60°to 80°F and shady conditions generally are the most favorable for powdery mildew development. Powdery mildews are often sensitive to extreme heat and sunlight, and leaf temperatures above 95°F may kill the fungus.
Avoiding the most susceptible cultivars, placing plants in full sun, and following good cultural practices will adequately control powdery mildew in many situations. Some ornamentals do require protection with fungicide sprays if mildew conditions are very favorable. Horticultural oils, soaps, sulfur, and other conventional and biological fungicides can be effective.
Adapted from UC Pest Notes Publication 7493