Boeremia exigua (Desmazières) Aveskamp, de Gruyter & Verkley, 2010
Viburnum opulus “roseum”, Belgium, prov. East Flanders, Wachtebeke, garden Axelsvaardeken © Bernard Declercq
detail with pycnidia
Infection at first forms tiny purple or light brown spots with a purple border in the centre and edge of the leaf. The marginal spots may increase to several cm, and form black, hypophyllous, semi-sunken, subglobose pycnidia, ø 200-300 µm with central ostiolum from which a white tendril of conidia is extruded. Conidia oblong, narrowly ellipsoid to irregular, mostly straight, upper end broadly rounded, (5)6-7(9)x2-2.5(3) µm, biguttulate, rarely triguttulate, sometimes additionally a few minute drops, 1-celled, smooth-walled, hyaline, terminally developing on hyaline, phyalid, conidiogenic cells. (Description Bernard Declercq).
distribution within Europe
Ascochyta viburni Roumeguère, 1884; Boeremia exigua var. viburni (Roumeguère) Aveskamp, de Gruyter & Verkley, 2010; Phoma viburni (Roumeguère) Boerema & Griffin, 1974.
According to the literature (Aveskamp ao) does the genus Boeremia (contrary to Ascochyta) from unicellular conidia. Some exceptionally large conidia may become 1(3)-celled. Apparently Ellis & Ellis described such a collection (or they described it as 1-septate, rather than 0-1-septate because they used the name Ascochyta). Moreover, my collection clearly is hypophyllous, while Ellis & Ellis had an epiphyllous collection at hand (Bernard Declercq in litt.).
Aveskamp, de Gruyter, Woudenberg, ao (2010a), Boerema & Griffin (1974a), Ellis & Ellis (1997a).