Willow Tar Spot

not listed

not listed

not listed

Willow Tar Spot is a parasitic fungus infecting several species of willow (Salix). In Minnesota it infects prairie willow, pussy willow, and possibly Bebb’s willow and other willow species. It has not been recorded on crack willow or other willow tree species. It is intollerant of air pollution and is usually found in rural locations.

In the spring the infection appears as a yellow spot on the leaf. These turn first brownish-black with a yellow border, then, in late summer, black with a yellow border. The black spots resemble tar. The spots are solid, not a cluster of small spots. They may be more than 1″ in diameter but are usually smaller, due at least in part to the narrow width of the leaves it infects.

Prairie willow, pussy willow and possibly Bebb’s willow

Spring to fall

Sources

4, 26, 29, 30, 77.

Widespread and locally common

The life cycle of this and most other fungi is pleomorphic. It has both an asexual reproductive phase (anamorph) and a sexual reproductive phase (telemorph). Each phase is often morphologically distinct. In the past, the International Code of Botanical Nomenclature allowed each phase to be assigned a different scientific name. It was incorrect to refer to the anamorph phase by the telemorph name, and vice versa. Some mycologists and molecular biologists consider this practice to be obsolete. Molecular phylogeny allows the accurate placement of a species in any part of their life cycle. On July 30, 2011, at the meeting of the XVIIIth International Botanical Congress in Melbourne, Australia, that practice ended. Prior to that meeting, the early, anamorphic phase of this fungi had been named Melasmia salicina. Both phases are now named Rhytisma salicinum.

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