Powdery mildew, Podosphaera leucotricha, can stunt and russet apples.
The best method of powdery mildew control is prevention. Avoiding the most susceptible varieties and following good cultural practices will adequately control powdery mildew in many situations. However, where conditions are favorable, susceptible fruit trees and berries may require protection with fungicide sprays. Fungicide applications are most often needed on susceptible varieties of apple and on almost all grape and strawberry varieties.
Where possible, choose resistant varieties that meet your growing requirements and personal preferences. Be aware that control actions will probably be necessary when planting more susceptible varieties.
The most resistant varieties are Red Delicious and Stayman Winesap. Moderately susceptible varieties include Braeburn, Golden Delicious, Granny Smith, Jonagold, and McIntosh. The most susceptible varieties include Gravenstein, Jonathan, Rome Beauty, and Yellow Newtown.
Blackberry is not affected by powdery mildew. Resistant raspberry varieties include Chief, Marcy, Malling Orion; the variety Logan is immune. Highly susceptible raspberry varieties include Glen Clova, Latham, Ottawa, and Viking.
The most susceptible varieties are Bing, Black Tartarian, and Rainier.
Most varieties are susceptible.
Most varieties are susceptible.
Freestone varieties such as Crest, Flame Crest, Flavor Crest, and O’Henry are less susceptible than varieties such as Elegant Lady, Fairtime, Fay Elberta, and Summerset.
Some highly susceptible varieties of plum that may need protection are Black Beaut, Gaviota, Kelsey, and Wickson.
Day-neutral (everbearing) varieties such as Fern, Seascape, Sequoia, and Yolo are more susceptible than short-day varieties (those that fruit in May and June only) such as Chandler.
Fungus species: Podosphaera leucotricha
Controls: tolerant varieties; prune out infections in apple trees during dormant season; fungicides if necessary
Fungus species: Podosphaera clandestina
Controls: fungicides if necessary
Fungus species: Podosphaera tridactyla
Controls: tolerant varieties; fungicides if necessary
Fungus species: Sphaerotheca macularis
Controls: resistant varieties; removing infected tissue; fungicides if necessary
Fungus species: Sphaerotheca pannosa
Controls: fungicides if necessary; remove or treat roses
Fungus species: Uncinula necator
Controls: tolerant varieties; water sprays; prune during dormancy; fungicides
Shade and moderate temperatures favor most powdery mildews. Plant in sunny areas as much as possible, provide good air circulation, and avoid applying excess fertilizer. A good alternative is to use a slow-release fertilizer. Long duration overhead sprinkling may actually reduce active powdery mildew infections because spores are washed off the plant. However, spores can be disseminated in water to new noninfected leaves if watered only briefly.
As new shoots begin to develop on perennial plants, watch closely for the appearance of powdery mildew. Where infection is limited, prune out and bury or discard diseased tissue as soon as it appears. Infected tissue can be recognized by the young emerging leaves being deformed or showing a puckered condition. Soon after emergence infected leaves begin to exhibit white mycelial growth on the leaf surface. This combination of symptoms is characteristic of early season mildew onset. If powdery mildew has been present during the season on woody species, prune out infected tissue during the dormant season.
Prune grapevines during dormancy and position shoots during the growing season to allow exposure of fruit to sunlight and good air flow through the canopy. Pruning and training are also helpful in controlling Botrytis bunch rot.
Because one common powdery mildew fungus, Sphaerotheca pannosa, often spreads disease from roses to stone fruits, try to avoid planting apricot or plum trees near highly susceptible rose bushes. If roses are nearby and can’t be removed, control powdery mildew infections on them.
On apple trees, look carefully for infected shoots and buds in the dormant season and remove them. Infected buds are flattened or shriveled compared to normal buds. The buds and infected shoots have a thin layer of fuzzy white fungus on their surfaces that usually is easy to see. Where practical, remove and dispose of overwintering leaves on strawberry plants that are infected. If raspberry canes develop powdery mildew, remove the canes down to the roots during the dormant season. Infected canes of berries and grapevines have distinctive weblike russetting. Remove infected prunings from the garden area and destroy them.
Where powdery mildew has been a problem in the past, fungicides may be needed. Fungicides function as protectants, eradicants, or both. A protectant fungicide can only prevent a new infection from occurring, but an eradicant will kill an existing infection. Apply protectant fungicides to highly susceptible plants before the disease appears. Eradicants should be used at the earliest appearance of the disease. Once mildew growth is extensive, control with fungicides becomes more difficult.