Placonema (Sacc. & D. Sacc.) Petr., Annls mycol. 19(1/2): 60 (1921)
Facesoffungi number: FoF 07496
Ascomycota, genera incertae sedis
Saprobic on the host plants, such as Arthrostylidium longiflorum, A. racemiflorum, Bambusa sp., Merostachys speciosa (Poaceae) (Nag Raj 1993). Sexual morph: undetermined. Asexual morph: Conidiomata brown to black, stromatic, pycnidial, solitary to gregarious, immersed, elongate to oval in outline, globose to subglobose in section view, unilocular or multilocular, glabrous. Ostiole indistinct, dehiscence by irregular rupture of overlapping tissue. Conidiomatal wall composed of thick-walled, dark brown to pale brown then hyaline cells of textura prismatica to textura angularis. Conidiophores reduced to conidiogenous cells. Conidiogenous cells hyaline, enteroblastic, annellidic, cylindrical to lageniform or irregular, integrated or discrete, indeterminate, smooth-walled, proliferating percurrently, arising from the inner wall layer of conidiomata. Macroconidia hyaline, sometimes pale brown to hyaline, fusiform, ovoid, ellipsoid or naviculate, unicellular, smooth-walled, bearing one filiform, unbranched, apical appendages and an eccentric basal appendage. Microconidia when present, hyaline, acerose, unicellular, smooth-walled (adapted from Morgan-Jones et al. 1972d, Nag Raj 1993).
Type species: Placonema bambusacearum (Sacc., Syd. & P. Syd.) Petr., Annls mycol. 19(1/2): 60 (1921)
Notes: The hyaline, sometimes pale brown to hyaline, fusiform, ovoid, ellipsoid or naviculate, unicellular conidia bearing one filiform, unbranched, apical appendage and an eccentric basal appendage invites a comparison with Annellolacinia. However, Annellolacinia possesses acervular conidiomata with brown to dark brown, subulate, often nodulose, unbranched setae, which differs from the stromatic, pycnidial, glabrous conidiomata in Placonema (Nag Raj 1993). Moreover, Placonema has hyaline, acerose, unicellular microconidia, which are usually absent in Annellolacinia.
Placonema was elevated to generic rank from Placosphaeria (De Not.) Sacc. by Petrak (1921) to accommodate P. bambusacearum. The second species P. napelli was described on Aconitum napellus (Ranunculaceae) from Germany. Sutton (1977b) revised Placonema and transferred P. napelli to a newly introduced genus Neoplaconema because of its distinct conidia and conidiogenous cells morphology. Neoplaconema napelli was separated from Placonema by its phialidic conidiogenous cells with distinct collarettes, and fusiform, subcylindrical to ellipsoidal conidia with an, unbranched or branched appendage. Sutton (1977b) also reduced Shanoria Subram. & K. Ramakr. and Stigmateopsis Bat. to synonyms of Placonema. Nag Raj (1993) re-visited Placonema and provided detailed description and illustration for three species, P. austroamericanum Nag Raj, P. bambusacearum and P. bambusae (Bat.) Nag Raj. The sexual morph of Placonema has been assigned to Phyllachora (Farr 1968), but this connection is based on host association, and has not been confirmed by culture studies or other evidence. Therefore, fresh material of these taxa are needed to resolve the placement of Placonema species and its sexual morph.
Distribution: Brazil, El Salvador, Panama, Paraguay, Venezuela (Nag Raj 1993).
Placonema bambusacearum (redrawn from Morgan-Jones et al. 1972d) a Conidia. b Conidiogenous cells and developing conidia. c Vertical section of conidioma.
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.