University of Illinois Extension


Barbara Larson
Unit Educator, Horticulture
Boone & Winnebago Counties

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Apple Scab & Black Knot

Members of the apple and cherry families provide many of our common small flowering trees but each family is susceptible to a disfiguring fungal disease. During spring these two diseases may infect disease prone plants.

Cool, wet weather in spring creates ideal conditions for the fungal disease apple scab. Apple scab affects susceptible apples and ornamental crabapples. Leaves on severely infected trees develop gray blotches, turn yellow and drop in midsummer. Apple scab will also affect flowers, fruits, and in rare cases twigs.

Apple scab does not permanently damage a tree. However, severe defoliation two or three successive years can weaken a tree to the point that it becomes prone to winter or drought injury.

Reduce the chance of apple scab infection with the following practices. Rake and destroy leaves in the fall to reduce the disease potential the following spring. Maintain trees by watering during summer droughts and fertilizing every five years. Remove trees that defoliate yearly and replace with resistant varieties. In new plantings always use scab resistant crabapples. Spotting may occur on resistant varieties but the leaves should not yellow and drop. A number of crabapple varieties have shown excellent resistance in trials including ‘Adirondack,’ ‘Louisa,’ ‘Prairifire,’ ‘Purple Prince,’ ‘Harvest Gold,’ ‘Red Jewel,’ and ‘Sugar Tyme.’

Correctly applied fungicides may help reduce apple scab damage. Fungicides act as protectants and must be present before the disease begins. Make the first application when new green growth first appears. Repeat every seven to ten days until frequent and prolonged rainy periods are finished in early July. Thorough coverage of buds, young leaves, and fruit is essential. Spray on calm days or nights when temperature is between 40° and 80° F. Benomyl and Captan are two of the fungicides registered for controlling apple scab. Carefully read and follow label directions.

Ugly growths on plum and cherry trees are a disease called black knot. Black knot is a common disease of plums, cherries, and other members of the Prunus genus. The disease can be controlled with pruning and fungicide sprays. Black knot causes elongated, rough, black swellings on twigs, branches and trunks. The growths begin velvety olive green then the following year turn hard, brittle, and coal black. Stems girdled by black knot will die outward from the growth. In spring, fungal spores are released from existing knots and blown in the wind to other trees. Trees will gradually weaken and die if effective control measures are not taken.

Do not purchase trees with visible knots or abnormal swellings on the branches. Resistant plants are available. Most infections occur between budbreak and two weeks after bloom if the weather is wet with 55-77°F temperatures. A copper fungicide should be applied every two weeks from the time the leaf buds open until three weeks after flower petals fall. Fungicides prevent new infections but do not cure disease already present. There are many formulations of copper fungicides. The fungicide you use must be labeled for black knot fungus on your type of plant.

Besides spraying you must prune out all knots to remove old infections. Prune all infected wood in late winter or as soon as new knots appear. Cut four to eight inches behind any black knot growths. Knots on the trunk or large limbs may be carefully cut out with a knife or chisel, removing about an inch of healthy bark beyond the swelling. Destroy infected material by burning, burying, or sending to municipal composting facilities.

Alertness and care will reduce the incidence of apple scab and black knot in our landscape


February - March 2001: Hybrids & Heirlooms | Put the Right Plant In the Right Place | Windbreaks Can Help Save On Energy Costs | Apple Scab & Black Knot

Past Issues

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