Diseases of Fruit Plantation Medicinal and Aromatic Crops

Powdery mildew of apple

Causal organism: Podosphaera leucotricha (Ell. and Eva.) Salmon
Symptoms:

• Symptoms of powdery mildew infection may appear on the primary leaves, buds, shoots, blossoms, and fruits.
• On leaves the symptoms most often are seen as whitish, felt-like patches of fungal mycelium and spores.
• These most commonly appear first on the lower surface of the leaves.
• Lesions may also appear on the upper surface as chlorotic spots, or cover the entire leaf with powdery white spores and mycelium.
• Curling and crinkling of leaves can occur as a result of infections along the leaf margin.
• Leaves infected severely by the disease may defoliate buds which are infected with powdery mildew can become more susceptible to winter injury.
• Fruit infections on certain cultivars may result in a netlike russeting.
Disease cycle

• The fungus overwinters in the dormant buds that had been infected in the preceding growing season.
• Buds which have been infected are more susceptible to winter damage.
• Below–12° F temperatures the survival rate of the overwintering buds is generally less than 5%.
• The over wintering fungus within the bud produces spores that initiate the primary infection of the disease cycle in the spring.
• Unlike apple scab fungus, the spores of powdery mildew do not require a film of water for germination.
• The resulting infection of young leaves and blossoms provide inoculum for secondary cycle of the disease.
 Dissemination

• In spring, the spores are produced by powdery mildew fungus which has over wintered in the buds and are carried by winds to vulnerable (young) tissues.
• PSI: Dormant mycelia
• SSI: Barrel shaped conidia
Management

• Key time for management varies from tight cluster stage until terminal growth stops, particularly the period after petal fall when vegetative growth is rapid. Start looking for signs of infection when leaves emerge from the bud.
• Management Options: Depending upon the susceptibility of cultivar and impact of cold weather temperatures on overwintering inoculum survival, chemical control may be applied.
• Cultural Controls: Plant cultivars which are less susceptible to the disease. Reduce humidity in the tree canopy by pruning and also to increase air, light, and spray penetration.
• Chemical: Bavistin( 0.5gm per lit ) spray has been found effective.