Rhytisma salicinum (Pers.) Fr.    

Kingdom: Fungi
Phylum: Ascomycota
Subphylum: Pezizomycotina
Class: Leotiomycetes
Order: Rhytismatales

British distribution: Widespread, local.
World distribution: Europe, North America, no doubt elsewhere wherever suitable host willows exist.

A tar-spot disease of sallows (Salix caprea, S. cinerea, S. aurita and hybrids), also on some dwarf willow species (S. repens, S. herbacea). It is apparently unrecorded on the tree willows (Salix section Salix, i.e. S. fragilis and allies).

Rhytisma salicinum  stromata on Salix caprea
Rhytima salicinum: stromatal stage on living leaves of Goat Sallow (Salix caprea, northern form of subsp. caprea), Kindrogan, Perthshire, 1985.

This fungus occurs on sallow/willow leaves in rural areas, the fungus seemingly less tolerant of atmospheric pollution than its well-known relative, R. acerinum, on Sycamore (Acer pseudoplatanus).

Initially seen is usually the stroma, a thickened, blackened region of the leaf tissue, though this is preceded by smaller black spots about 1mm in diameter, the anamorphic (asexual) stage, Melasmia salicina Lév., producing colourless, cylindrical conidia, 5-6 µm in length. This anamorphic stage is described by Grove (1937), pg. 188. It is incorrect to apply the anamorphic name to the stromatic stage.

After leaf-fall, the stromatic area survives over winter and apothecia develop in spring, several per stroma, pushing through splits in the original leaf epidermal tissue to release spores in time to infect the new young leaves of the host.

•   Grove, W.B., (1937). British stem- and leaf-fungi, vol. 2, Cambridge University Press, Cambridge.

© A.J. Silverside
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