Verticillium albo-atrum

Verticillium albo-atrum

MycoBank: MB199278 (as V. alboatrum)

Description.

Colonies on PDA after two weeks 4.5–5.5 cm diam, white at first, later turning yellow to orange due to the formation of yellow-pigmented hyphae, then darkening due to formation of resting mycelium immersed in the agar medium (Figures 4a, 4b). Aerial mycelium generally abundant, floccose to pruinose, hyphae smooth-walled, (1–) 1.5–4 µm wide. Conidiophores erect or slanted, generally determinate (Figure 4c), branched or unbranched (Figure 4d), formed disjointedly throughout the colonies, hyaline, base brown-pigmented at times, 80–480 µm in length, 3–6 µm wide, narrowing towards the apex to 2–2.5 µm, transversely septate, septa spaced more narrowly towards the apex. Conidiogenous cells are phialides, arranged in 1–4 (–6) whorls along conidiophores (Figures 4c, 4d), arising below transverse septum (Figure 4e). Whorls spaced 20–140 µm apart, closer towards the apex, consisting of (1–) 2–4 (–6) phialides (Figures 4c, 4d, 4e). Apical whorls consisting of one apical and one to several lateral phialides (Figures 4c, 4d, 4e). Phialides subulate, tapering from 1.5–3 µm at the base to 1–1.5 µm at the tip, terminal phialides 40–80 µm long, lateral phialides 20–50 µm long (Figure 4e). Conidia hyaline, smooth-walled, cylindrical with rounded apices to oval (Figure 4f), tapering at times, (3.0–) 6.0 µm±1.5 µm (–10.5)×(2.0–) 3.0 µm±0.5 µm (–6.0) (l/w = (1.1–) 2.0±0.4 (–3.0), n = 86), accumulating at the tip of the phialides (Figures 4c, 4d). After 4 wks, a small number of conidia (generally <1%) with central septum, constricted at the septum at times (Figure 4f). Budding conidia and conidia germinating by formation of a phialide observed (Figure 4f). Resting mycelium present, consisting of brown-pigmented hyphae, up to 7 µm wide, thick-walled, straight or curved, solitary or aggregated, up to 25 µm wide (Figures 4g, 4h). Microsclerotia present, composed of tightly interwoven, torulose brown-pigmented hyphae, rounded or variously shaped, up to 230 µm diam and consisting of rounded to elongate cells, up to 10 µm diam (Figures 4i, 4j, 4k). Yellow-pigmented hyphal cells present at times, up to 5 µm wide (Figure 4l).

Types.

Holotype: Missing, not found at GOET, B, M; Lectotype (designated herein): Illustrations from protolog: Figures of Plate (‘Tafel’) 8 and Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 of Plate 9 in Reinke and Berthold [21], available online at http://books.google.com/books?id=iWgVAAAAYAAJ&dq=Die%20Kr%C3%A4uselkrankheit%20der% 20Kartoffel&pg=PA107#v=onepage&q=Die%20Kr%C3%A4uselkrankheit%20der%20Kartoffel&f=false (accessed on October 5, 2011); Epitype (designated herein): Dried culture of Verticillium albo-atrum strain PD747 (Canada: Prince Edward Island; potato field soil) deposited at UC (UC 1953892), an ex-epitype culture at CBS (CBS 130340) and NRRL (NRRL 54797).

Specimens examined.

The description was based on Verticillium albo-atrum strains PD670 (USA: WI; Irish potato), PD693 (UK; Irish potato), PD746 (Canada: New Brunswick; potato field soil), PD747 (Canada: Prince Edward Island; potato field soil) and PD748 (Canada: Prince Edward Island; potato field soil)(Table S1).

Distribution and host range.

Currently known from Canada, Germany, UK and USA (WI). Substrates include Irish potato and soil collected from Irish potato fields.

Commentary.

Verticillium albo-atrum was described by Reinke and Berthold in 1879 from diseased potato plants collected near Göttingen, Germany [21]. The protolog of V. albo-atrum contains detailed descriptions and drawings, but no reference is made to type material. We inquired at the herbaria of Göttingen (GOET), Berlin (B) and München (M), none of which has any V. albo-atrum type material or any other V. albo-atrum material deposited by Reinke and Berthold. According to Klebahn [33], original cultures of V. albo-atrum are no longer available. We did not find any V. albo-atrum cultures by Reinke and Berthold in any of the major culture collections (CBS, IMI, DSMZ, ATCC). Thus, in absence of any original fungal material, we designated the illustrations from the V. albo-atrum protolog in Plate 8 (Figures 1, 2, 3, 4, 5, 6, 7, 8) and Plate 9 (Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11) in Reinke and Berthold [21] as the lectotype for V. albo-atrum. According to The International Code of Botanical Nomenclature (ICBN) [34] this is permissible when the holotype, all cited or uncited original specimens and all original cultures are missing (Art. 8.4, Art. 9.2, Art. 9.10, Art. 37.4). To serve as an interpretive type, we designated a V. albo-atrum epitype with an ex-epitype culture for molecular studies. Designation of an epitype is permissible according to ICBN when serving the precise application of a name (Article 9.7).

The original description of V. albo-atrum by Reinke and Berthold [21] was based on observations from decaying potato stems, and is congruent with our observations from the V. albo-atrum isolates examined in this study. The exception was the presence of yellow pigment associated with hyphal cells (Figure 4l) not seen by Reinke and Berthold [21]. However, Klebahn [35] reported that V. albo-atrum mycelium on Salep Agar medium was white with a yellow tinge (p. 64), whereas V. dahliae mycelium was described as white (p. 65).

In addition to resting mycelium, Verticillium albo-atrum also forms microsclerotia (Figures 4i, 4j, 4k). Microsclerotia are very ‘small, firm, frequently rounded masses of hyphae with or without the addition of host tissue or soil.’ [36]. The V. albo-atrum microsclerotia were described and illustrated in the protolog on pages 74 and 75, and in Figures 1 and 2 of Plate 9 [21], a translation from the German original is provided by Isaac [22]. The microsclerotia consist of aggregations of brown-pigmented, thick-walled hyphae, no lateral cell divisions are involved in their formation [21]. This is opposed to the microsclerotia of V. dahliae where an increase in width is achieved by the lateral divisions of hyphal cells as described by Klebahn [35] on pages 56 and 57, and illustrated in Klebahn's [35] Figure 8. Microsclerotia were only observed on WA-p and PLYA media, they were absent from strains cultured on PDA. Verticillium albo-atrum has frequently been confused with V. alfalfae and V. nonalfalfae that form resting mycelium but no microsclerotia.

The name ‘V. albo-atrum’ is correct with or without hyphen (Art. 23.1), the hyphenated form is more commonly encountered in the literature.