Rhytisma acerinum

Rhytisma acerinum

Hosts: Acer platanoides, saccharum, campestre, negundo

The first symptoms of tar spot on Norway maple in Southern Ontario appeared in late June as small, round, light green, chlorotic spots, 2 mm across. Spots enlarged to 15 mm by mid-August, and developed small black tar-like raised structures on the adaxial surface with a yellow margin. Conidia, which are considered non-infective and possibly spermatizing, appeared as a shiny layer on the black stroma at this time. By early September, the individual spots merged into a circular black spot up to 2 cm across.

In 2007, we found a few tar spots on trees tentatively identified as sugar maple (A. saccharum). We confirmed the identity of these maple trees based on DNA sequencing of a chloroplast gene. The fungal DNA was also sequenced, and it turned out to match R. acerinum, the European species. This was very surprising since the European fungal species should not occur on a native North American maple species. The same trees were visited again in 2008, and samples were collected during the growing season. These also yielded DNA confirmed as belonging to R. acerinum. In 2008, we also collected specimens of A. campestre and A. negundo from Ontario, and other samples of A. saccharum from Quebec. All specimens were infected with tar spot caused by R. acerinum as identified by sequences of the ITS region. This result was not unexpected for A. campestre since both the host and pathogen are European species. However, this was another unexpected result for A negundo, and A. saccharum.

Non-native hosts and control of Rhytisma acerinum causing tar spot of maple