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Teaching techniques for mycology: 18. Rhytisma acerinum, cause of tar-spot disease of sycamore leaves

Published online by Cambridge University Press:  17 September 2002

Roland W.S. Weber
Affiliation:
Lehrbereich Biotechnologie, Universität Kaiserslautern, Paul-Ehrlich-Str. 23, 67663 Kaiserslautern, Germany. E-mail: rwsweber@rhrk.uni-kl.de
John Webster
Affiliation:
12 Countess Wear Road, Exeter EX2 6LG, UK. E-mail: J.Webster@exeter.ac.uk
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Abstract

Name of fungus

Teleomorph: Rhytisma acerinum (Pers.) Fr. (order Rhytismatales, family Rhytismataceae)

Anamorph: Melasmia acerina Lév.

Introduction: Features of interest

Tar-spot disease on leaves of sycamore (Acer pseudoplatanus L.) is one of the most easily recognised foliar plant diseases caused by a fungus (Figs 1 and 4). First described by Elias Fries in 1823, knowledge of it had become well-established by the latter half of the 19th century (e.g. Berkeley, 1860; Massee, 1915). The causal fungus, Rhytisma acerinum, occurs in Europe and North America on A. pseudoplatanus throughout its distribution range and also on other species of Acer (Sutton, 1980; Farr et al., 1989), but it is less frequent in urban and industrial areas.

The black tar-spots visible in late summer and autumn are stromata containing many apothecial rudiments, but these only mature to form asci during the winter on fallen leaves (Jones, 1925; Duravetz & Morgan-Jones, 1971). Maturing apothecial stromata develop linear or convoluted ridge-like swellings which raise the thick black layer (clypeus) on the upper surface of the stroma (Fig 1). Eventually, the surface breaks along the ridges (Fig 2), exposing a greyish hymenium (Fig 3) which contains club-shaped asci and filamentous paraphyses with curved or recoiled tips (Figs 8 and 9). The ascospores are unicellular and needle-shaped, with an apical mucilage pad (Fig 10). In Britain and Europe, ascospore discharge from overwintered stromata takes place in March and April, just as the new sycamore leaves unfold.

Type
Original Article
Copyright
© 2002 Cambridge University Press

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