Leaf Diseases of Hardwoods
Rhytisma americanum, is a native tar spot that affects silver and sugar maple in Minnesota.
Tar spot caused by Rhytisma punctatum on maple. This is also a native disease in the US.
Rhytisma acerinum, an introduced pathogen on Norway maple. This pathogen has recently been found in Minnesota.
Tar spot caused by Rhytisma acerinum can cause severe damage to the leaves and premature defoliation.
Section through the black stroma of Rhytisma showing asci and ascospores. A few long, thin ascospores that have broken out of the asci can be seen.