Stegonsporium acerophilum
Anamorph: as for S. pyriforme, but conidia (29–)31–37(–40) x (13.5–)15–17.5(–18) µm, pyriform to cylindrical, with (3–)4–5(–6–7) transverse distosepta and often with 1 or rarely 2 longitudinal distosepta in 1–3 cells, light to dark brown, cell lumina lenticular or narrowly ellipsoidal, with a hyaline sheath to ca 2 µm thick.
Teleomorph: as for S. pyriforme, but stromata 0.7–1.5mm diam. Ostioles invisible from the bark surface, fusing within the ectostromatic disc. Ascomata (235–)270–400(–470) µm diam., (110–)140–230(–250) µm high. Paraphyses 3–8 µm diam., collapsing by maturity of the ascospores. Asci (145–)180–235(–255) x 26–32(–35) µm, narrowly clavate or ellipsoidal, the apex with a narrow cylindrical refractive ring, 8-spored. Ascospores (32–)34–40(–44) x (13.5–)14.5–17(–18) µm, ellipsoidal, medium to dark brown, with (3–)5 distosepta and 1 oblique or longitudinal distoseptum in 1 to several cells in 40–50 % of ascospores; with a short subglobose to irregular gelatinous appendage at each end, (2.5–)3–6(–8.5) µm long and (4–)6–10(–13.5) mm diam.
In GB&I, presumed introduced; it has only been reported from RBG Kew.
The common native species Stegonsporium pyriforme has ascospores which rarely have longitudinal septa; that species appears confined to Acer pseudoplatanus.
In GB&I, reported from dead attached branches of Acer saccharum and A. grandidentatum.
In GB&I, only recorded from Surrey.
In GB&I, apparently a weak pathogen that does not cause significant damage to its host. In North America is is considered to be an opportunistic pathogen that may become virulent on trees stressed by insect defoliation (Voglmayer & Jaklitsch 2008).