Rhytisma
Rhytisma Fr., K. svenska Vetensk-Akad. Handl., ser. 3 40: 104 (1819)
Facesoffungi number: FoF 07577
Leotiomycetes, Leotiomycetidae, Rhytismatales, Rhytismataceae
Saprobic or parasitic on the host plant. Sexual morph: see Cannon and Minter (1984), Weber and Webster (2002). Asexual morph: Conidiomata black, stromatic, solitary to gregarious or confluent, epidermal, immersed to semi-immersed, circular to irregular in outline, applanate, unilocular or multilocular, glabrous, ostiolate. Ostiole circular, papillate, centrally located. Conidiomatal wall composed of thick-walled, dark brown cells of textura angularis in the upper part, becoming thicker, pale brown cells in the basal part, in which are scattered dark brown sclerotized cells. Conidiophores formed from the inner wall layer cells of basal stroma, hyaline, cylindrical to irregular, branched and septate at the base and irregularly above, smooth-walled. Conidiogenous cells hyaline, enteroblastic, phialidic, cylindrical to lageniform, integrated or discrete, determinate, smooth-walled, with periclinal wall thickened towards apex. Conidia hyaline, cylindrical or slightly tapered to the base, straight or slightly curved, unicellular, smooth-walled, eguttulate (adapted from Sutton 1980, Weber and Webster 2002).
Type species: Rhytisma acerinum (Pers.) Fr., K. svenska Vetensk-Akad. Handl., ser. 3 40: 104 (1819)
Notes: The generic type of Melasmia, M. acerina Lév. was generally considered as asexual morph of R. acerinum (Darker 1967, Sutton 1980, Cannon and Minter 1984). Both sexual and asexual morphs were associated with tar spots on leaves of host plants in family Sapindaceae (Sutton 1980, Weber and Webster 2002, Suto 2009, Wang et al. 2009, Masumoto et al. 2014, 2015). Based on “one fungus = one name” principle, Rhytisma was proposed for conservation over Melasmia (Johnston et al. 2014, Wijayawardene et al. 2017b). The asexual morph of Rhytisma resembles Aoria in having stromatic, glabrous conidiomata as well as the same habit causing tar spots on leaves, but the latter was distinguished from the former by its holoblastic, sympodial conidiogenous cells and ellipsoid conidia (Sutton 1980). There is no molecular data available for Melasmia species. It is necessary to obtain fresh collections and cultures to confirm connection between Rhytisma and Melasmia.
Distribution: worldwide (Wijayawardene et al. 2017).
Rhytisma acerinum (asexual morph, redrawn from Sutton 1980) a Detail of conidiomatal wall. b Conidia. c Conidiophores, conidiogenous cells and developing conidia. d Vertical section of a conidioma.
References:
Li WJ, McKenZie EHC, Liu JK, Bhat DJ, Dai DQ, Caporesi E, Tian Q, Maharachcikumbura SSN, Luo ZL, Shang QJ, Zhang JF, Tangthirasunun N, Karunarathna SC, Xu JC, Hyde KD (2020) Taxonomy and phylogeny of hyaline-spored coelomycetes. Fungal Diversity 100: pages279–801.
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