Eb1054 Powdery Mildews

EB1054 POWDERY MILDEWS Powdery mildews are diseases that affect a large number of plants. The severity of the disease depends on several factors including the variety, age and condition of the plant, when the plant is infected, and weather conditions during the growing season. A wide range of plants are attacked by the powdery mildews—ornamental flowers and shrubs, grasses, fruit trees, and vegetables. Only flowering plants are affected; conifers and ferns are not hosts of the powdery mildews. Among the more important plants that are seriously affected in Washington are apples, roses, grapes, bluegrass, wheat, potatoes, peas, and zinnias. Many others including native plants are susceptible to these diseases. Mildew-infected plants typically have a grayish white, powdery coating over affected leaves and shoots. Later in the season this coating may be dotted with small brown or black specks. Mildew-infected leaves are often distorted, curled or twisted, and may be smaller than normal. Severely infected leaves may be yellow with small patches of green (the so-called "green island effect") and may fall prematurely. Infected fruit of some plants such as apple, peach, and gooseberry may be severely russeted. Powdery mildews are caused by a group of similar fungi that live largely on the outer surface of the host plant and obtain their nutrients from the plant by means of small, rootlike organs called haustoria, that penetrate the outer or epidermal layer of the plant. The fungus can feed upon any green tissue, and powdery mildew may be found on leaves, green stems, flower buds, and green fruit. Figures 1—3. Powdery mildew on flowering crab, rose, and gooseberry, respectively. Powdery mildew is named for the grayish white powder consisting of fungal strands (mycelia) and masses of summer spores that accumulate on the plant surface. These summer spores are often produced in chains and serve to spread the fungus during the growing season. After the mildew has been established for a time, small black fruiting bodies (cleistothecia) may be produced throughout the mycelium. This is the overwintering stage of the fungus. Each cleistothecium is a hollow sphere in which one or more sacs containing ascospores are formed. Under mild winter conditions cleistothecia are not critical to the survival of some powdery mildews. In apple powdery mildew, and perhaps in others such as rose or grape, the fungus survives as mycelium in dormant buds. These infected buds, however, are more sensitive to cold than are healthy buds and only survive mild winters. Cold winters kill the infected buds thereby reducing the amount of early infection. There are several genera of fungi that cause powdery mildew, the most common being Erysiphe, Sphaerotheca, Phyllactinia, Uncinula, Microsphaera, and Podosphaera. These are separated on the basis of the type of appendages on, and the number of spore sacs in, each cleistothecium. Some species of powdery mildews can infect only a few closely related hosts while others can attack many genera of plants. For example, Uncinula necator has been reported in Washington only on grape and Virginia creeper, but Erysiphe cichoracearum has been reported on 58 genera of plants ranging from snapdragon, cucumber, and lettuce to potato and zinnia. Powdery mildews are favored by humid conditions such as occur when days are warm and nights are cool. The disease also is common in crowded plantings where air circulation is poor as well as in damp, shaded areas. The disease is most severe on young, succulent tissue such as that resulting from heavy nitrogen fertilization and watering. Since environment plays a large role in the development of powdery mildew, control can sometimes be obtained by making conditions less favorable for the disease. Space plantings to provide good air circulation and avoid poorly drained soils and damp and shaded areas. Collect and remove lawn clippings. Do not apply too much water and fertilizer. Rake and burn infected leaves and shoots to destroy overwintering stages of the fungus. Powdery mildew conidia cannot survive in a film of water because they absorb so much water that they rupture. Because of this and the ease with which spores are washed from the plant surface, spraying a plant with water can noticeably reduce the amount of mildew. This control technique can sometimes be used with individual ornamental plants such as roses. The most practical and long-lasting control measure is to plant resistant varieties if they are available. These varieties are often noted in seed and nursery catalogs or in publications like the Pacific Northwest Plant Disease Management Handbook. When the disease becomes severe and some immediate action is desired, chemical control is effective. Since most of the mildew fungus is external to the plant, this is one of the few diseases that can be easily controlled after infection has occurred. Several fungicides are available for powdery mildew control. (Be aware that another group of fungi called the downy mildews do not respond to the same fungicides used for the powdery mildews.) These should be used with care and according to the manufacturer's recommendations. By Otis C. Maloy, Ph.D., Extension plant pathologist (retired), WSU, Pullman. Warning. Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock. Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended. Reprinted September 1998. Subject Code 356. A. EB1054.

Load more