Apple scab, caused by the fungus Venturia inaequalis, occurs on apples and crabapples (Malus spp.). In late fall and early spring, fungal reproductive structures, appearing as round black pimples, develop primarily on infected leaves from the previous year. Spores are released from these structures in the spring and blown by wind or splashed by rain to new growth on nearby trees. There they start new infections. The fungus may infect and colonize the leaves, fruit, petioles, and blossoms of the tree, with symptoms commonly observed on the leaves and fruit. Later in the spring, these primary infections produce secondary spores which infect other leaves and fruit. These secondary infections may continue throughout the growing season during wet periods.

Fig. 1 Photo by U of MN Plant Disease Clinic

Fig. 2 Photo by U of MN Plant Disease Clinic

Symptoms on infected leaves initially appear as velvety brown to olive colored spots. Later, these spots turn black (Fig. 1). Heavily infected leaves, with many spots, may turn yellow and drop from the tree. Fruit may also be infected, initially displaying lesions similar to those appearing on leaves (Fig. 2). Later these lesions become brown and corky. Severe infections cause fruit to mature unevenly and crack.

Apple scab does not seriously harm apple trees. However, heavy defoliation causes apple trees to be less attractive, reduces growth and yield, and increases susceptibility to winter injury. During dry seasons, apple scab may be controlled through cultural practices. These practices include sanitation procedures, proper pruning, and watering during dry periods. Rake fallen leaves in autumn to reduce the number of spores available for infection the following spring. Prune suckers and branches in the crown to improve air circulation so leaves dry quickly. Perform annual pruning in late winter through early spring (February to early April, depending on environmental conditions).

	Spores infect young leaves & flower buds Fungi survive winter on fallen leaves.	Spores released from initial infections are blown or splashed, causing new infection throughout the season.
Spores are released in spring from reproductive structures on fallen leaves.		Additional leaves are infected. Apples are infected

During wet seasons, fungicide applications may be necessary for controlling apple scab. The need for fungicides depends upon cultivar susceptibility and weather conditions, since rainy weather and moisture are required on plant surfaces for infection by the fungus. Fungicides listed below are currently labeled for controlling apple scab. Read fungicide labels carefully and apply as directed. For more information on when to apply fungicides, see "Home Fruit Spray Guide" (University of Minnesota Extension Service Publication #FO-0675-B).

Fungicides currently labeled for controlling apple scab (January, 1999)
Ornamental crabapples	Edible apples
Captan Chlorothalonil (Daconil 2787) Thiophanate-methyl (Cleary's 3336)	Captan Thiophanate-methyl (Cleary's 3336) All-purpose fruit sprays (must contain one of the above fungicides to be effective against apple scab)

Representative trade names may be included along with generic names. This information is supplied with the understanding that no discrimination is intended and no endorsement implied.

P220A
Revised 1/99