Powdery Mildew is a serious fungus affecting major apple growing regions of the world, especially serious in nursery production. Powdery mildew is caused by the fungus Podosphaera leucotricha, which affects leaves, buds, shoots and fruits. The fungus produces a white powdery on terminal growth and developing fruit. It reduces growth of the branches and fruit set, and produces netlike russet on the fruit of some varieties. The disease causes economic damage by reducing tree vigor, flower bud production, and fruit quality.
The symptoms of powdery mildew can be seen on young shoots, leaves, blossoms and fruit. Dormant shoots which were heavily infected in last growing season are covered with white powder like substance, and terminal buds are not developed. In late spring the leaves get covered with a light grey powder, which are spores of the powdery mildew fungus. Infected leaves curl upward and soon become covered with a powdery mildew. Infected terminal shoots appear stunted and young shoots may be killed in the spring. The white lesions can also be seen on blossoms, resulting in poor fruit-set. Later on the symptoms are seen on the fruit, which produces a network of russet on the fruit.
Powdery mildew ovewinters as mycelium in fruit buds, which were infected in last growing season. The fungal strands get killed in low winter temperatures, reducing the source of the infection. The buds which are infected breaks dormancy later than the healthy buds. As soon infected buds break dormancy fungus starts growing on shoots and leaves. Spores from these infected leaves give rise to secondary powdery mildew infections on the leaves of shoots as long as shoot growth continues. There are multiple generations per year, as long as they are actively growing. Powdery mildew is favored by moderate temperatures and high relative humidity.
Powdery mildew can be managed by pruning off infected shoots in dormancy or in spring. The spores of the fungus can be destroyed by destroying fallen leaves in autumn season. Mulching the tree debris can reduce the water stress which helps pants less prone to the infection.
Most of the growth of powdery mildews is found on the plant surface, they are easily targeted with fungicides. Early spring applications of fungicide are necessary to prevent secondary spread of powdery mildew. Neglecting control early in the year will result in poor control during the season. The fungicides found effective against powdery mildew are—