Posted by: Kim_Hamilton on 02/02/2010 01:07 PM
Updated by: Kim_Hamilton on 02/02/2010 07:30 PM
Expires: 01/01/2015 12:00 AM
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Powdery Mildew~by Ken Churches
Powdery mildew is a common disease on many types of plants. Powdery mildews generally do not require moist conditions to establish and grow, and normally do well under warm conditions; thus they are more prevalent than many other diseases under our dry summer conditions. The disease can be recognized easily on most plants by the white powdery mycelial and spore growth that forms on shoots, both sides of leaves, and sometimes flowers and fruit. An exception is the powdery mildew that affects tomatoes, eggplants, peppers, artichokes, and some ornamentals, which produces yellow patches on leaves, but often no powdery growth.....
The disease can be serious on woody species such as grapes, fruit trees, roses, crape myrtle, and sycamore where it attacks new growth including buds, shoots, and flowers as well as leaves. New growth is dwarfed, distorted, and covered with a white, powdery growth. On apples and grapes, young fruit develop web-like russetted scars and sometimes a rough corky skin.
Grapes may also crack or split as they continue to grow and expand after a severe mildew infection.
Powdery mildew spores are carried by wind to new hosts. Although humidity requirements for germination vary, all powdery mildew species can germinate and infect in the absence of water. In fact, spores of some powdery mildew fungi are killed and germination and mycelial growth are inhibited by water on plant surfaces. Moderate temperatures and shady conditions are generally the most favorable for powdery mildew development. Spores and mycelium are sensitive to extreme heat and direct sunlight. Fruit and ornamentals require protection with fungicide sprays where conditions are most favorable for mildew. Fungicide applications are most often needed on susceptible varieties of apples, grapes, cucurbits, roses, and crape myrtle.
In some situations, especially in the production of apples, grapes, susceptible cucurbits, and roses, fungicides may be needed. Sulfur and synthetic fungicides are available. Garden Fungicide, a formulation of sulfur that is combined with surfactants similar to those in dishwashing detergent, is available from Safers. This product is especially useful for garden use. To prevent plant injury to any plant, do not apply sulfur when temperature is near or over 90°F. Proper timing of fungicide applications is critical to successful control; this is especially true with sulfur treatments that primarily prevent rather than eradicate infections. Fungicides must be applied to highly susceptible plants at the earliest signs of the disease; once mildew growth is extensive, it is generally too late for effective control with fungicides. Timing for sulfur treatment in apples and grapes is described below. Fungicides can also be used on other tree fruits and vegetables but should rarely be needed for powdery mildew control.
Grapes. Powdery mildew is a perennial problem in some grapes. Begin applying sulfur when all buds have pushed. Thereafter, repeat at 10-day intervals until the sugar content of grapes is 12 to 15 percent, which is when they begin to soften and approach ripeness and are no longer susceptible to infection.
This article adapted from information at the UCIPM Center. Please contact the Farm Advisor’s office at 754-6477 or http://cecalaveras.ucdavis.edu with your agricultural questions. Information for this article collected at UC DANR.
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