Apple scab is a fungal leaf and fruit disease. The fungus, Venturia inaequalis, over winters in infected fallen leaves and fruit. The severity of the disease varies with the weather and the susceptibility of the hosts. Although this disease is not fatal to a plant, continued infection and defoliation weakens the plant allowing other diseases and pests to take hold.

The disease attacks apples (Malus), crabapples (Malus), pear (Pyrus), quince (Chaenomeles), mountain ash (Sorbus), and other species. The fungus has two types of spores. The initial or primary infection forms on fallen fruit and leaves from the previous year and is blown by the wind onto the new leaves in the spring.

The second type of spore germinates from the primary spore and is spread by splashing rain or irrigation water. A warm, wet, humid spring will encourage the infection to spread to adjacent leaves and plants. If the summer is also particularly wet, further infections will continue to occur.

Symptoms appear first on the young leaves in spring as brown to olive green spots on either side of the leaf surface. Heavy attacks can affect entire leaves turning them yellow and then brownish in color. On older leaves, the spots appear velvety black and slightly raised. As the disease develops, the leaves usually turn yellow and drop prematurely. Blossoms have similar symptoms to the leaves. The fruit is deformed by dark olive colored spots that are sunken, causing the fruit to be corky.

The key to prevention of this disease is plant selection. Avoid species that are very susceptible to this disease, such as mountain ash and quince.

When selecting a crabapple, be sure to check its resistance to apple scab. There are many other cultivars being developed, and currently on the market that are resistant to apple scab.

See the plant list below.

Control of this disease requires diligence. Begin with sanitation. Clean the area around previously infected plant by raking up fallen leaves and fruit to eliminate some of the inoculum spores. This must be done before new leaves open in the spring.

The second step may be chemical sprays for plants having a history of repeated or heavy infections. Fungicidal sprays will effectively control apple scab if applied regularly. Remember, the initial spores are carried by the wind from other infected plants. Application must first occur when the flower buds are showing a half inch of green tissue.

The Midwest usually has very wet springs, so application will need to be repeated every 14 days, or whenever dew or rain threatens to wet the leaves for over six hours. If the summer is also very wet, additional spraying may be required. Usually three or four sprayings at 10-14 day intervals starting in the early spring when the buds swell will prove sufficient on ornamental crabapples. For fruiting trees, sprays should continue until close to harvest. Follow directions on the fungicide label to determine when to stop applications prior to harvest of the fruit.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

Common crabapple cultivars and species:
Severely Susceptible Fairly Resistant
‘Bob White’
‘Red Jewel’
Malus floribunda
M. sargentii
M. x zumi calocarpa


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