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Species Information
Summary: Rhytisma punctatum is included as an example of the ascomycete order Rhytismatales. Groups of 20-30 black stromata form speckled black patches on maple leaves surrounded by a halo of discoloration. Fruitbodies develop in the stromata of the fallen leaves and the ascospores are released in the spring. The description is derived from Funk(1) except where specified. R. punctatum is very common on Bigleaf Maple.
Distribution includes BC and elsewhere in the Pacific Northwest. The University of British Columbia has collections from BC, OR, NY, VT, and NY, and the University of Washington has collections from WA.
Fruiting body: stromata about 0.1cm across, circular, punctate or speck-like, black, 20-30 aggregated in an infected area of the leaf, each containing a single apothecium or hymenial area, (Funk), islands of small black spots develop on living leaves, surrounded by yellowish tissue - when leaves turn golden and fall, the "tar spots" are surrounded by green tissue that remains photosynthetic - in the spring sexual spores are produced from the overwintered leaves, (Trudell)
Microscopic: spores 30-36 x 1.5-2 microns, filiform [thread-like], slightly broader in upper part, nonseptate, colorless, guttulate; asci 8-spored, 70-80 x 9-10 microns, clavate; paraphyses filiform; conidia 4-6 x 1 microns, allantoid [curved sausage-shaped], colorless; conidiogenous cells enteroblastic, phialidic; conidiophores cylindric, simple or branched; pycnidia forming in stromata singly
Habitat / Range
on Acer macrophyllum (Bigleaf Maple) and Acer spicatum (Mountain Maple), (Funk), islands of black spots form around mid-August (which will produce sexual spores the following spring), (Trudell)
Similar Species
Rhytisma salicinum (Pers.: Fr.) Fr. causes a similar disease in willow in BC and elsewhere: black stromata with raised centers 0.2-0.5cm across form on upper leaf surfaces (Allen(1)). Rhytisma americanum and Rhytisma acerinum also occur in the Pacific Northwest, (Trudell). Rhytisma arbuti produces tarspots on leaves of Arbutus menziesii (Pacific madrone) and Rhododendron (Menziesia) ferruginea (false azalea).
