With a wet spring and now humid summer weather there is a fungus among us……Apple Scab. This fungus survives the dormant season in the infected leaves and twigs. Black fruiting structures develop in leaves. Release of spores occurs during wet periods in the spring and they infect young leaves, shoots, and fruit.
One to three weeks after infection velvet-like, dark-green fruiting structures and spores form. The fungus can infect new growth if conditions are wet for about six hours and the air temperature is mild. Repeated episodes of spore release and infection occur throughout the growing season. Moist spring and early summer weather accompanied by cool temperatures favors this disease.
The disease primarily attacks apple, crabapple, several species of mountain ash, cotoneaster and pear trees causing premature drop of the foliage during summer months. Leaves, flowers, fruit and green twigs are susceptible to infection. The first symptoms are water soaked lesions that turn olive green to dark gray and develop a velvety appearance. Eventually infected leaves turn yellow and drop from the tree prematurely, leading to completely defoliated trees by mid to late august.
It is wise to remove and dispose of infected leaves near the tree during and after the growing season. Prune rapidly growing water sprouts emerging from branches, since many of these water sprouts apple scab lesions on them. In addition, prune to increase air circulation and sunlight penetration into the canopy, which promotes drying of foliage. To minimize disease problems, the best time to prune is in the late winter or very early spring before new growth starts. Otherwise, prune when the bark and weather conditions are dry. Consider growing varieties or species of apple, crabapple, and mountain ash that are resistant to apple scab.