Cercospora sp. YMM297B on Phaseolus lunatus L.

Meswaet, Yalemwork, Mangelsdorff, Ralph, Yorou, Nourou S. & Piepenbring, Meike, 2021, Unravelling unexplored diversity of cercosporoid fungi (Mycosphaerellaceae, Mycosphaerellales, Ascomycota) in tropical Africa, MycoKeys 81, pp. 69-138 : 69

Cercospora sp. YMM297B on Phaseolus lunatus L. Fig. 10

Description.

Leaf spots almost lacking to well-developed, amphigenous, subcircular to irregularly angular, 2.5-8 mm diam., reddish brown, later dark brown by abundant caespituli, finally sometimes greyish brown to dark reddish brown, surrounded by dark margins, often with diffuse whitish centres. Caespituli amphigenous, greyish brown to dark brown. Mycelium mainly internal. External hyphae branched, 2-3(-4) μm wide, septate, olivaceous brown to brown, smooth. Stromata lacking or small, up to 20 μm diam., immersed in the mesophyll or in substomatal cavities, subcircular to irregular, olivaceous brown to darker brown. Conidiophores in small and loose fascicles, breaking through the adaxial epidermis of the leaves or penetrating through stomatal openings, sometimes solitarily arising through stomatal openings, erect, straight to sinuous, or somewhat geniculate, unbranched, (13-)17.5-195(-220) × (3.5-)4-5 μm, with 2-6(-8) septa each, occasionally slightly constricted and darker at the septa, brown to dark brown. Conidiogenous cells integrated, terminal, mainly monoblastic; loci 2-3.5 μm wide, thickened and darkened. Conidia solitary, narrowly obclavate to subacicular, straight to curved, (27-)36-148(-164) × (2.5-)3-4(-4.5) μm, with 2-7(-9) somewhat indistinct septa each, hyaline to sub-hyaline, smooth, apex subacute or acute, base truncate to short obconically truncate, 2-3(-3.5) μm wide, hila thickened and darkened.

Specimen examined.

Benin. Borgou: Parakou, Tankaro, c. 360 m a.s.l., 9°23'01"N, 2°30'36"E, on Phaseolus lunatus , 20 Sep 2019, Y. Meswaet and R. Dramani, YMM297B (M-0312654; UNIPAR).

Notes.

The infection of leaves of Phaseolus lunatus by Cercospora sp. YMM297B was associated with the infection by Pseudocercospora griseola . Among the Cercospora spp. known on Phaseolus and Vigna , C. olivascens is morphologically close to Cercospora sp. YMM297B. C. olivascens , however, differs from Cercospora sp. YMM297B by hypophyllous caespituli, no external hyphae, conidiophores that are up to five times geniculate and paler ( Saccardo 1878; Chupp 1954), as well as hyaline conidia. The present specimen from Benin presents amphigenous caespituli, external hyphae, less geniculate and brown to dark brown conidiophores and often sub-hyaline conidia. C. olivascens also differs from the present species by being originally described from Aristolochia clematitis ( Aristolochiaceae ). According to Chupp (1954), this species was wrongly reported on Phaseolus vulgaris by Saccardo (1886). This was confirmed by Crous and Braun (2003). In the ITS phylogeny (see Suppl. material 3), Cercospora sp. YMM297B forms part of a polytomy with a relatively large genetic distance (branch length) in relation to other sequences considered in the analysis. In the tef1 phylogeny (see Suppl. material 4), it is not possible to distinguish this collection from several other Cercospora spp. As the description and sequence data are obtained only from a single specimen, the data are not sufficient for a final conclusion and the description as a new species. A reliable species characterisation is not possible until more collections become available.

Figures

Figure 10. Cercospora sp. on Phaseolus lunatus (YMM 297 B) A fascicle of conidiophor e s emerging through a stomatal opening B solitary conidiophores C conidia. Scale bars: 15 μm (A); 10 μm (B, C).