Symptoms

Apple scab infects leaves, flowers, and fruit. The most obvious symptoms occur on leaves and fruit in the spring and summer. Leaf spots are small, velvety brown to olive green spots that enlarge and darken to become more or less circular. The spots have feathery, indistinct margins. When infections are numerous, young leaves become curled and distorted. Severely infected leaves and fruit fall prematurely. Scab lesions on fruit become corky, cracked, and rough. Symptoms on mountain-ash and other hosts are similar to those on crabapple, with emphasis on foliar lesions and premature leaf fall.

Disease Cycle

Apple scab overwinters as fungal spores in the diseased leaves on the ground. The first fungal spores mature and are capable of causing infections in spring at about the time of budbreak (leaf expansion). Apple scab is most severe during spring and early summer when the humidity is high and the temperature is moderate. Fungal spores are ejected into the air following rainfall and continue to be produced over much of the growing season. The largest number of spores are produced near the end of flowering on apples and crabapples. Whether infection occurs or not depends on the rainfall and temperature. Fewer hours of wet leaf surfaces are required for infection at high temperatures than at low. The severity of disease increases with the duration of wetting of the leaves.

Once the fungus has become established on the host, it produces secondary spores which help to re-infect new leaves throughout the summer. These spores are disseminated by splashing rain or irrigation and wind to new leaf or fruit surfaces, and give rise to new lesions. Several secondary cycles may occur during the growing season if wet weather prevails during the summer.

Management

Cultural Management

Since the apple scab fungus overwinters on fallen leaves and infected twigs, collecting and removing these leaves and twigs will reduce the source of infection. Sanitation practices, such as leaf litter removal and pruning, should be done in the fall or winter before bud break occurs.

Resistant Cultivars

The best way to prevent apple scab is to plant resistant apples and crabapples. Many species and cultivars of apple and crabapple are resistant to the scab fungus. A partial list of crabapple cultivars is available on the Arboretum’s Search Trees and Plants database. Check with local nurseries regarding the availability of these and additional disease resistant cultivars.

Chemical Management

Apple scab can be effectively managed with fungicides by treating new emerging leaves in spring. This can reduce the primary infection. It is important that sprays are applied according to plant development, with the first spray at bud swell and additional sprays applied according to label directions.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).