Apple Scab

April 2, 2015 – Kenosha News

Photo by Brian Hudelson,  UW-Madison Plant Pathology

Photo by Brian Hudelson,
UW-Madison Plant Pathology

Question: We moved into a condo about a year ago. Last summer we noticed the crabapple tree near our unit lost most of its leaves in late summer.  Is there something we can spray on the tree to prevent leaf loss? A group of trees in the commons area are covered with lumpy black growths. What are the growths and can the trees be saved? A.E.

Answer: Apple scab is a very common disease of susceptible ornamental crabapples and apples. Cool, wet weather in spring creates ideal conditions for development of the fungal disease. Leaves on severely infected trees develop gray blotches, turn yellow and drop in midsummer.  Apple scab will also affect fruit.

Cleaning up fallen infected leaves and fruits in fall reduces the amount of pathogenic fungi which decreases disease potential the following spring.

Trees that defoliate yearly from scab infection should be removed and replaced with disease resistant cultivars.  In new plantings always use scab resistant crabapple and apple varieties.  Spotting may occur on resistant cultivars, especially in wet years, but the leaves won’t yellow and drop.

Fungicide treatments are not needed for resistant or semi-resistant trees. Correctly applied fungicides may help reduce apple scab damage in disease susceptible trees.  Fungicides are protectants and must be present before disease begins.  Make the first application when new leaves first appear in spring.  Repeat every seven to fourteen days until frequent rains end in early July.  Thorough coverage of buds, young leaves, and fruit is essential. A number of fungicides including mancozeb, chlorothalonil, myclobutanil, propiconazole, or thiophanate methyl, are available for scab control. Carefully read and follow label directions.

The ugly black growths are black knot.  The rough black galls may appear on twigs, branches, and trunks.  Stems girdled by black knot will die outward from the growth.

Black knot is a fungal disease of plums, cherries, and other members of the Prunus genus.  The disease can be managed with pruning. Remove all infected wood in late winter or as soon as new growths appear.  Prune six to eight inches behind 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.

Severely infected trees, like in your photos, should be removed.  Plants resistant to black knot are available.


Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from UW. Plant and gardening questions can be sent to  or call 262-857-1942.