LEXINGTON, Ky. — Apple scab is the most consistently serious disease of homegrown apple and flowering crabapple in Kentucky. The most noticeable losses on apple result from reduced fruit quality and from premature drop of infected fruit. Scab also causes a general weakening of the host when leaves are shed prematurely. Summer defoliation of flowering crabapple due to scab invariably results in fewer flowers the next spring. Resistant cultivars and fungicides are available; however, sanitation is a critical step in prevention and management.
Apple Scab Facts
- Leaf symptoms begin as olive-green to brown spots (lesions) with indefinite, feathery margins (Figure 1) on upper and/or lower surfaces. As disease progresses, lesions become more distinct, develop a greenish-black, velvety growth, and then thicken, and bulge upward (Figure 2).
- Infected fruit develop symptoms similar to those on leaves. Older lesions turn dark brown to black, develop a corky (“scabby”) appearance, and frequently become cracked as fruit enlarge (Figure 3). If infections occur on young fruit, uneven growth near “scabs” may cause fruit to be deformed.
- Heavily infected leaves and fruit may drop prematurely.
- Hosts include apple, crabapple, hawthorn, and mountain ash.
- Primary infection occurs during periods of continuous leaf wetness from bud break until 2 to 4 weeks after petal fall.
- Subsequent infections result from a second spore type (conidia) that are produced in lesions throughout the remainder of the season.
- Caused by the fungus Venturia inaequalis.
- The apple scab fungus overwinters in fallen leaves.