University of Illinois Extension
Bruce Spangeberg

These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.

Stateline Yard & Garden

Apple Scab Management Starts Early

March 23, 2000

With the return of spring this week, it won't be long before trees and shrubs are again leafing out. As leaves develop in spring, certain diseases may infect some trees. One example is apple scab, a fungus disease of flowering crabapples and apples in the orchard. There is still some time, but consider management options now.

Apple scab has been a widespread disease in the landscape and orchard the past few years. While the problem may not become evident until leaves of crabapples and apples turn yellow and drop, control needs to start early in the season as buds open and leaves start to unfold. If apple scab has not appeared on your crabapple or apple tree the past few years, chances are it is a resistant cultivar.

Apple scab first appears as olive green to black spots or lesions on leaves. As the season advances, leaves turn yellow or brown and drop from the trees. Once the leaves are infected and dropping, there's nothing to do to stop the disease that season. Infection actually occurs early in the season during cool, wet weather as leaves develop.

There are some options to consider for managing apple scab. Trees that have shown the problem previously are scab-susceptible cultivars; thus the only way to prevent scab is to apply protective fungicide sprays to newly developing foliage. Applications are needed at regular intervals to provide adequate protection for an ornamental crabapple; product labels should give the time interval between sprays.

Begin fungicide sprays as growth first appears and continue until prolonged wet periods are uncommon (usually about July 1). Captan, chlorothalonil, copper, maneb, mancozeb, sulfur, thiophanate-methyl are among the fungicides suggested. Read and follow all label directions.

In the backyard orchard, home fruit tree sprays containing fungicides need to be applied at about the same times as for crabapples to provide protection from scab. Captan is suggested. Dormant oil sprays are for certain insects only, and do not control scab.

Resistant varieties are the best way to manage scab, especially for ornamental crabapples. Some commonly grown crabs, such as Almey, Hopa, Eleyi and Radiant are very susceptible to scab. There are many resistant cultivars available, so be sure to ask for them when selecting crabapples. Consider replacing extremely susceptible trees with resistant cultivars. Some apples also have resistance.

 

Click here for the full article index