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