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

Once Again Time for
Apple Scab Control

April 9, 1998

Apple scab has been a serious disease problem on crabapples and apples the past few seasons as cool, wet spring weather is ideal for this disease to develop. While it is difficult to predict what will happen in 1998, apple scab management focuses on prevention, rather than "curing" an infected tree in midseason. During the past few seasons, leaves on susceptible trees dropped by mid-summer.

Scab affects both ornamental crabapples and apples grown in orchards. Cultivars will vary in susceptibility. If your crabapple or apple did not get this disease the past few seasons, either you have a resistant cultivar or were spraying the tree with a fungicide to prevent scab.

Apple scab appears as olive green to black spots or lesions on leaves. Heavy infection, as happened the past few seasons, makes leaves turn yellow or brown and drop from the trees. Once the leaves are infected and dropping, there's nothing to spray on the tree.

Controlling apple scab involves several steps. If a susceptible crab already exists in the landscape, 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 as growth first appears and continue until prolonged wet periods are uncommon (usually about July 1). Captan, chlorothalonil, copper, maneb, mancozeb, sulfur,and thiophanate-methylare among the fungicides listed in the 1998 control guide from the University of Illinois.

For backyard apple trees, home orchard sprays containing fungicides need to be applied at about the same times as crabapples to provide protection from scab. Captanis suggested. Dormant oil sprays 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.


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