Powdery mildew ( Podosphaera leucotricha )

Powdery Mildew is one of the most common diseases in Kashmir apple orchards, which is caused by the fungus, Podosphaera leucotricha. The disease occurs during periods of high humidity (above 70%) and warm temperatures. Infected trees develop a white, powdery appearance on the underside of the leaves. This powdery growth is mycelium and spores (conidia) of the fungus. As the infection develops, the disease spreads to twigs, flowers, and fruit. Infected leaves curl upward, and infected fruit develops a net-like russeting on the surface. Late in the season tiny black fruiting bodies (cleistothecia) of the fungus may appear on infected leaves and twigs. Severe infection causes stunted trees with reduced vigor, yield, and fruit quality.

• Disease appears when the buds develop into new leaves and shoots.

• Small patches of white or grey powdery masses on under surface of leaves occur.

• Leaves grow longer and narrower than normal leaves and the margin is curled.

• Twigs covered with powdery mass.

• Affected fruits remain small and deformed and tend to develop roughened surface.

The disease appears on buds, leaves and young shoots, flowers and rarely on fruits. Young leaves tend to increase in length, but not in width and becomes longitudinally folded. Severely infected leaves become inrolled, hard and brittle and give scorched appearance. The symptoms are first noticed on leaves as small, irregular, white or greyish felt like powdery patches on or under the surface. Soon the entire leaf becomes covered with white mycelium and powdery masses of spores. Young shoots also start showing the typical disease symptoms. Infected flower buds wither fail to open or open later than healthy buds which rarely set fruit.

Conidia are the fungus's short-term survival spore. However, they can withstand hot, dry periods for many weeks. Once the disease begins, it is a potential threat throughout the season. This fungus may also produce fruiting bodies, called cleistothecia, which contain ascospores. These spores are protected from the winter climate. Thus, if produced, the fungus has two mechanisms for overwintering, as dormant mycelium in buds and as ascospores.

Powdery mildew overwinters as fungal strands (mycelium) in buds infected the previous year. The infected terminals may be silvery-gray in color, stunted, and misshapen. When the buds break dormancy, the new leaves and flowers are infected by the fungus. The powdery fungal growth produced on infected tissue consists of thousands of tiny spores, called conidia, which are responsible for secondary spread and infection. Conidia are disseminated throughout the orchard in wind currents and water splashes. High humidity (greater than 70%) and relatively warm temperatures are required for the conidia to germinate. Although spore germination depends on high humidity, they will not germinate in free water. Thus, while the leaf surface is wet, the fungus is not active. When the water evaporates from the plant surface, the humidity in the plant canopy increases and the fungus becomes active. A new batch of conidia are produced 5 days after infection. In favorable conditions, the disease spreads rapidly.

Management of powdery mildew on apple begins before the orchard is planted by selecting cultivars with some degree of tolerance to the disease. Highly susceptible cultivars include Jonathan, Golden Delicious and Granny Smith. Avoid these cultivars in areas with a history of severe powdery mildew problems. No cultivars are completely resistant to infection. Check with tree producers for information regarding the tolerance of individual cultivars.

Additional management practices include good sanitation practices and protective fungicide spray programs. Sanitation programs should include removing fallen leaves and pruning shoots suspected of infection during dormancy or in early spring. This will help to reduce the primary infection and limit the amount of fungus present in the environment. Routine pruning and thinning are also helpful in allowing good air circulation around the trees, reducing humidity and chances for severe disease development.

In orchards where powdery mildew is known to be a problem, a preventative fungicide spray program can be helpful in controlling the disease. Fungicides are most effective when applied at 7- to 10-day intervals from prepink stage through petal fall. In some areas continued sprays may be necessary until terminal growth stops. A number of registered fungicides control powdery mildew on apple. However, pesticide registrations are constantly changing, making it difficult to compile pesticide lists that will stand up over time. Check with your local county extension service or chemical representative for product availability. Always be sure to read the current label before using any pesticide