Apple Scab

Problem type: Disease

Name of problem: Apple scab

Plant name(s): Apple, ornamental flowering crabapple, mountain ash,

Symptoms / Characteristics:

In the spring, small olive-green lesions develop on the leaves. These spots darken as the season progresses and the edges become more distinct. Infected leaves may be deformed, turn brown and drop prematurely. Fruit lesions are dark and may be raised or domed. As the disease advances, these lesions become brown and corky. Initially, the lesions are only found on the blossom end of the fruit but later develop on the entire surface. Infected fruit is deformed and cracked. Premature fruit drop may occur. Apple scab also infects leaf stems and blossoms but symptoms are most prominent on the leaves and fruit.

Apple scab is a fungal disease caused by Venturia inaequalis. The fungus overwinters in fallen leaves and debris. Wet and cool spring conditions facilitate the ejection of spores from the infected debris. The spores are then carried by wind to young shoots and buds where infection takes place.

Control / Preventions:
Remove and destroy dead leaves and other debris to prevent overwintering of the apple scab fungus and reduce subsequent infection. Pruning or thinning the canopy promotes good air circulation and reduces disease incidence. Ensure adequate spacing between trees at planting. Registered protectant and systemic fungicides are available but timing is critical. Protectant fungicides are used to prevent infection from occurring and may be applied if long periods of wet weather are forecast that may contribute to fungal infection. A typical application time is shortly after bud break. Planting scab resistant apple varieties is another method of prevention. Contact your local nursery for a list of available resistant varieties suitable for your area.