following question was sent to the P&PDL diagnosticians here
at Purdue University:
Question: Almost every summer the leaves on my maple trees
have black spots on them that look like tar. What is it and what
should I do about it?
spot on maple is not actually "tar" on maple, but rather
a fungal disease. Tar spots on maples are caused by fungi in the
genus Rhytisma. The most common species are Rhytisma
acerinum and R. punctatum.
Symptoms first appear in late spring or early summer as infected
leaves develop light green or yellow-green spots. During mid to
late summer, black tar-like raised structures are formed on the
upper surface of leaves within the yellow spots. R. acerinum causes
spots that are 0.5 to 2 cm in diameter; R. punctatum causes
spots that are smaller (about 1mm in diameter). Spots caused by R.
punctatum are sometimes called speckled tar spots.
Tar spot diseases seldom are detrimental to the overall health
of infected trees. Tar spots may cause premature defoliation, but
are not known to kill trees. Tar spot diseases are best managed
by raking and destroying fallen leaves because the fungi overwinter
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