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Powdery Mildew

Podosphaera leucotricha

Powdery mildew, caused by Podosphaera leucotricha, is an important disease on some cultivars in the Southeast. Losses occur from the reduced grade of infected fruit and from stunting of leaves, which reduces yield and can affect return bloom the next season.

Symptoms

Leaves on infected primary terminals are stunted, distorted, and covered with a silver gray mat of mycelium (Fig 1). Secondary lesions are most common on the lower leaf surface of younger leaves and appear as powdery patches (Fig 2) that cause leaves to become puckered and distorted. When the temperatures increase during the summer and lesions age, affected leaf tissues usually become light red to purple and sporulation is sparse. Severely infected leaves may become necrotic and abscise. Late in the summer, sexual fruiting structures may be produced in what appear to be dark patches on infected shoots (Fig 3). Fruit infections are characterized by a netlike russet (Fig 4).

Fig 1 - Infected leaves

Fig 1. Infected leaves

Fig 2 - Powdery patch

Fig 2. Powdery patch

Fig 3 - Infected shoot

Fig 3. Infected shoot

Fig 4 - Infected fruit

Fig 4. Infected fruit

Disease Cycle

The fungus overwinters as mycelium in dormant apple buds. Terminals with infected buds are usually silver and infected buds may break later than those not infected. By the tight cluster to pink growth stage, the fungus colonizes newly emerging leaves and flower clusters and produces spores on the surfaces of the leaves and flower parts. These spores are blown to new leaves and fruit and initiate infections. Mildew infections are favored by warm, humid weather. Unlike scab and most other apple diseases, the powdery mildew fungus does not require free water (rain or dew) for infection. Secondary infections can continue throughout the summer until the terminal bud sets.

Control

The need for fungicides to control powdery mildew depends to a large extent on the susceptibility of the cultivar grown. Many of the new cultivars are more susceptible (www.caf.wvu.edu/kearneysville/tables/powmilsus.html) than cultivars such as Delicious that have been grown widely in the Southeast. When mildewcides are needed, the first applications should be made at the tight cluster to pink bud stage. These early sprays are very important in reducing the inoculum available for secondary spread. Sprays prior to this period provide little control. On susceptible cultivars, mildewcides may need to be applied at 10-14 day intervals until the terminal bud forms. If 20% or more of the terminal leaves are affected with powdery mildew, then a more aggressive control program is needed. Removal of white- or silver-appearing terminal shoots while dormant pruning will help reduce the primary inoculum and potential for infections during the next growing season.

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Format updated March 29, 2011