Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001669
11/12/99

Scab


Symptoms: Scab is similar to a large leaf spot (Vis. 1), but
the affected area looks dull, is usually dark olive green
and has indefinite margins. The fruit symptoms are
usually sunken, corky, dark olive areas.

Cause: Scab is a fungal disease that attacks many species,
including crab apple, apple, quince and pear. The disease
in some years can completely cover leaves of susceptible
cultivars. Such severe infections, referred to as "sheet
scab," will defoliate an entire tree. The loss of
infected leaves weakens the tree. Scab can defoliate a
tree or shrub several times in one season.

The scab fungus forms two types of spores. The first type
is blown about in the wind and causes the first infections
in the spring. These infections form a second type of
spore that is spread by rain splash and infects only the
same plant or other plants of the same species. The more
rainy the season is, the more the disease spreads
throughout adjacent plants.

If you can prevent the first type of spore from infecting
the plant in early to mid-spring, the plant will escape
further infections that season. The first type of spore
is formed on fallen leaves of the previous year.

Control: Rake and burn fallen leaves before spring. If
your plant has a history of serious scab infections,
apply a fungicide when swollen flower buds show a half
inch of green tissue. Follow with applications, as
needed, to protect the plant throughout the blooming
period, whenever dew or rain threatens to wet leaves for a
6-hour period. Replace highly susceptible cultivars with
resistant cultivars.


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