Sporulation and identitiy of tar spot of maple in Canada

Tar spot of maple has been increasing in incidence and severity in the Great Lakes region of eastern North America since the 1990's. The purpose of this work was to examine tar spot on native and imported maple species to determine the fungal species involved. By extracting and sequencing DNA from tar spot samples obtained from Europe and across Canada, we have found that giant tar spot on Norway maple (Acer platanoides) is caused by Rhytisma acerinum, the same fungus found in Europe, whereas native maple species in North America have the large tar spot caused by R. americanum (e.g. on silver maple, A. saccharinum) or the speckled tar spot caused by R. punctatum(on big-leaf maple, A. macrophyllum). We also found that ascospore release from tar spots on Norway maple in southern Ontario occurred over a three-week period, the start of which coincided with full leaf expansion in Norway maple.