Rhytisma acerinum

Hosts: Acer platanoides, campestre [the authors suggest that the similar fungus on Acer pseudoplatanus reported elsewhere as R acerinum may be a distinct species but do not evaluate that themselves]

The fungus on Acer platanoides seems to be limited only to that host species. In many instances where heavily infected A platanoides grow next to native (to North America) red, silver, and sugar (A saccharum) maples, the other host species remain free of disease. Second, the morphology of the fungus on A platanoides differs from that on A rubrum or saccharinum. The former [R acerinum] matures in autumn as relatively large (20+ mm diam) black spots. [It] produces copious amounts of conidia from mid-August until leaf fall in late September.

First symptoms of impending disease are chlorotic [yellow] spots that appear on new foliage in central NY state from mid-June to the first week in July. Within 3 weeks, each chlorotic spot usually holds a dozen or more discrete black masses of fungus tissue each about 1 mm diameter and vaguely reminiscent of spots caused by R punctatum on various maples. Stromatal tissue continues to enlarge (faster on shade leaves than in sun) until the individual masses in each spot coalesce to form a stroma averaging 13 mm in diameter. Some spots are more than 25 mm wide. Furthermore, the stromata [of R acerinum] are much thinner than those [of R americanum], rarely exceeding 0.5 mm. By mid-August, from 6 to 18 individual pycnidia, each discernible by a minute papilla at its ostiole, can be seen from above. As the time of normal leaf drop approaches, developing apothecia also become visible as discrete blisters of bumps in the rind. These rarely coalesce and never assume the sinuous character of [R americanum].

A tar spot resembling the A platanoides pathogen is occasionally found on introduced specimens of a European native, Acer campestre. [genetic evidence is consistent with a monophyly of Rhytisma on Acer campestre and platanoides but doesn't confirm they're the same species]

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.

Rhytisma acerinum

Hosts: Acer platanoides, saccharum

Rhytisma acerinum infects Norway maple causing lesions on leaves and black masses of stromatal tissue. During severe infections, multiple lesions coalesce and remaining leaf tissue becomes necrotic leading to defoliation.

In addition, two sugar maple (A. saccharum) cultivars (‘Sweet Shadow’ and ‘Bailsta’) showed limited infection [by R acerinum] as well as a hybrid (A. truncatum x A. platanoides ‘Keithsform’) which to our knowledge has not yet been reported as a host.