Symptoms and damage
Tar spot is caused by the fungus Rhytisma acerinum. Spots arise in late
spring or early summer after leaves attain full size. At first the infected
tissue is light green or yellow. Then, during late summer, raised, shining black,
tar-like dots develop within the yellow spots on the upper leaf surfaces. The
lower surface of a leaf beneath a large tar spot turns brown, but the surface
beneath speckled tar spots remains yellow. Leaves with multiple spots may
wither and drop prematurely, but seldom so early or in such quantities as to
threaten the health of the tree.
The fungus survives the winter in
fallen leaves. Rake up and discard the leaves in fall. Spraying three times at
two-week intervals with a copper-based fungicide, mancozeb, or triadimefon
starting when the leaves begin to unfurl in spring will provide control .
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