Powdery Mildew of Apple
Powdery Mildew of Apple
Powdery mildew, caused by the fungus Podosphaera leucotricha, attacks buds, blossoms, leaves, new shoots, and fruit of wild and cultivated apples and crabapples. It interferes with the proper functioning of leaves, reduces shoot growth, reduces fruit set, and produces a netlike russet on the fruit of some cultivars. It is often a serious problem in apple nurseries.
Disease cycle The fungus overwinters as mycelium (fungus threads) inside infected buds. As these buds open in spring, all of their parts become covered with a powdery coating of spores. The spores, easily windblown, infect new leaves, fruit, and shoots. Fruit infection takes place during and shortly after the blossom period. Leaf and shoot infection may continue as long as shoot growth continues. Buds can become infected as they begin to form until they are matured for overwintering. Infections occur at temperatures of 65 to 80°F when relative humidity is high, such as at night. No moisture is required for spore germination to occur. Mildew-susceptible varieties include Jonathan, Rome Beauty, Cortland, Monroe, and Idared.
Disease management
On leaves of new shoot growth symptoms of powdery mildew are feltlike, white patches on the margins and lower surfaces. Infected leaves curl upward and soon become covered with a powdery coating of spores. Photo by K. Peter.
Symptoms The first sign of powdery mildew in spring is a 3- to 4-day delay in the opening of infected buds. Leaves and blossoms of these buds soon become covered with a white to light gray powder, the spores of the powdery mildew fungus. Flowers do not develop normally, are likely to be greenish white, and produce no fruit. On leaves of new shoot growth symptoms of powdery mildew are feltlike, white patches on the margins and lower surfaces. Infected leaves curl upward and soon become covered with a powdery coating of spores. New infections of succulent leaves and growing shoots reduce the size of the entire shoot. By midsummer, leaves and shoots may turn brown.
Mildew sprays should begin at the tight cluster bud stage, where the disease is severe. Fungicides need to be continued until new shoots stop growing or about the fourth cover spray. Sterol inhibitor fungicides (FRAC Group 3 fungicides) applied to control apple scab are very effective in controlling powdery mildew. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.
Contact Information Kari A. Peter Assistant Professor [email protected] 717-677-6116
Penn State College of Agricultural Sciences research and extension programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture. Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied. This publication is available in alternative media on request. Penn State is an equal opportunity, affirmative action employer, and is committed to providing employment opportunities to all qualified applicants
without regard to race, color, religion, age, sex, sexual orientation, gender identity, national origin, disability or protected veteran status. © The Pennsylvania State University 2017
Page 2
Powdery Mildew of Apple
Powdery mildew, caused by the fungus Podosphaera leucotricha, attacks buds, blossoms, leaves, new shoots, and fruit of wild and cultivated apples and crabapples. It interferes with the proper functioning of leaves, reduces shoot growth, reduces fruit set, and produces a netlike russet on the fruit of some cultivars. It is often a serious problem in apple nurseries.
Disease cycle The fungus overwinters as mycelium (fungus threads) inside infected buds. As these buds open in spring, all of their parts become covered with a powdery coating of spores. The spores, easily windblown, infect new leaves, fruit, and shoots. Fruit infection takes place during and shortly after the blossom period. Leaf and shoot infection may continue as long as shoot growth continues. Buds can become infected as they begin to form until they are matured for overwintering. Infections occur at temperatures of 65 to 80°F when relative humidity is high, such as at night. No moisture is required for spore germination to occur. Mildew-susceptible varieties include Jonathan, Rome Beauty, Cortland, Monroe, and Idared.
Disease management
On leaves of new shoot growth symptoms of powdery mildew are feltlike, white patches on the margins and lower surfaces. Infected leaves curl upward and soon become covered with a powdery coating of spores. Photo by K. Peter.
Symptoms The first sign of powdery mildew in spring is a 3- to 4-day delay in the opening of infected buds. Leaves and blossoms of these buds soon become covered with a white to light gray powder, the spores of the powdery mildew fungus. Flowers do not develop normally, are likely to be greenish white, and produce no fruit. On leaves of new shoot growth symptoms of powdery mildew are feltlike, white patches on the margins and lower surfaces. Infected leaves curl upward and soon become covered with a powdery coating of spores. New infections of succulent leaves and growing shoots reduce the size of the entire shoot. By midsummer, leaves and shoots may turn brown.
Mildew sprays should begin at the tight cluster bud stage, where the disease is severe. Fungicides need to be continued until new shoots stop growing or about the fourth cover spray. Sterol inhibitor fungicides (FRAC Group 3 fungicides) applied to control apple scab are very effective in controlling powdery mildew. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.
Contact Information Kari A. Peter Assistant Professor [email protected] 717-677-6116
Penn State College of Agricultural Sciences research and extension programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture. Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied. This publication is available in alternative media on request. Penn State is an equal opportunity, affirmative action employer, and is committed to providing employment opportunities to all qualified applicants
without regard to race, color, religion, age, sex, sexual orientation, gender identity, national origin, disability or protected veteran status. © The Pennsylvania State University 2017
Page 2
Powdery Mildew of Apple
