Verticillium nubilum

Verticillium nubilum

MycoBank: MB225664

Description.

Colonies on PDA after two weeks 2.5–6 cm diam, white at first, later darkening due to the chlamydospores immersed in the agar (Figures 11a, 11b). Aerial mycelium generally abundant, floccose to pruinose, hyphae smooth-walled, (1–) 2–4 µm wide. Conidiophores present (Figure 11c). Conidiogenous cells are phialides (Figure 11d) arranged in whorls along conidiophores (Figure 11c), arising below transverse septum. Whorls consisting of one or more phialides (Figures 11c, 11d). Phialides subulate (Figure 11d). Conidia hyaline, smooth-walled, cylindrical with rounded apices to oval (Figure 11e), allantoid at times, rarely with central septum, (4.5–) 7.5 µm±2.0– µm (–14.5)×(2.0–) 2.5 µm±0.5 µm (–3.5) (l/w = (2.0–) 3.0±0.5 (–5.0), n = 50) (Figure 11e). Chlamydospores present, rounded to elongate, 6–14 µm diam, solitary or in chains of up to 6, straight or curved (Figures 11f, 11g, 11h). Brown-pigmented hyphae present at times (Figures 11h, 11i), generally attached to chlamydospores (Figure 11h).

Types.

Holotype: Missing, not at DBN, IMI, K; Lectotype (designated herein): Illustration from protolog: Figure 5 on Plate 4 in Pethybridge [50], available online from Cyberliber, an Electronic Library for Mycology at http://www.cybertruffle.org.uk/cyberliber/59351/0006/002/p004b.jpg (accessed on October 5, 2011); Epitype (designated herein): Dried culture of Verticillium nubilum strain PD742 (obtained from CBS as CBS 457.51)(UK; soil) deposited at UC (UC 1953894) and NRRL (NRRL 54796).

Specimens examined.

The description was based on V. nubilum strains PD621 (UK; mushroom compost), PD702 (UK; Irish potato), PD741 (UK; soil), PD742 (UK; soil) (Table 1).

Distribution and host range.

Currently known from the UK. Substrates include mushroom compost, Irish potato and soil.

Commentary.

Verticillium nubilum was described by Pethybridge [50] from the surface of a potato tuber attacked by Phytophthora infestans. The protolog of V. nubilum contains descriptions of the V. nubilum morphology and a photograph of chlamydospores, but no reference is made to type material. We inquired at Kew (K), CABI Bioscience (IMI) and Dublin (DBN), none of which has any V. nubilum type material in its possession. Isaac [23] who studied V. nubilum in detail did not mention any herbarium material. We did not find any V. nubilum cultures by Pethybridge in any of the major culture collections (CBS, IMI, DSMZ, ATCC). Thus, in absence of any original fungal material, we designated the illustration from the V. nubilum protolog, Figure 5 on Plate 4 in Pethybridge [50], as the lectotype for V. nubilum.

Isaac [23] studied V. nubilum in detail and submitted several strains to CBS, of which we selected a dried culture of strain PD742 (CBS 457.51) as epitype. Our observations of V. nubilum agreed with the accounts by Pethybridge [50] and Isaac [23]. Pethybridge [50] noted that V. nubilum conidia were larger than those of V. albo-atrum. We found that V. nubilum condia were on average 7.5×2.5 µm (Figure 11c), the largest in Verticillium, with the exception of V. longisporum conidia that were on average 8.5×3.5 µm (Figure 9c). Differing from both Pethybridge [50] and Isaac [23], small numbers of brown-pigmented hyphae not directly associated with chlamydospores were sometimes present (Figure 11i), but these were lighter colored than the resting mycelium in other species (eg Figures 4g, 4h).

All the V. nubilum isolates that we examined formed very few conidia and conidiophores, which prevented us from conclusively assessing conidiophore morphology and dimensions. However, the few conidiophores and phialides that we saw were similar to other Verticillium species, in both morphology and dimensions (Figures 11c, 11d). Verticillium nubilum can be differentiated from other Verticillium species by the near exclusive formation of chlamydospores as resting structure (Figures 11f, 11g, 11h), in combination with the relatively large conidia (Figure 11c), but can be confused with Gibellulopsis nigrescens that forms distinctly smaller chlamydospores [15], [23].