PPDL Picture of the Week
July 22, 2019
Tar Spot on Maple
Ruhl, Sr. Plant Disease Diagnostician-retired, Botany & Plant Pathology,
Every summer we get questions
about black spots on maple leaves that look like tar. These spots
are not actually "tar" on maple,
but are rather a fungal disease known as tar spot.
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 larger spots that are 0.5
to 2 cm in diameter; R. punctatum causes many small punctate
spots that are smaller (about 1mm in diameter). Spots caused by
R. punctatum are sometimes called speckled tar spots.
tar spot disease may cause premature defoliation, seldom are they
detrimental to the overall health of infected trees. Tar spot diseases
are best managed by raking and destroying fallen leaves since the
fungi overwinter on leaves.