Verticillium nonalfalfae

Verticillium nonalfalfae

MycoBank: MB563555

Etymology: Known to occur on a variety of hosts, but not Medicago sativa (‘alfalfa’).

Latin diagnosis.

Verticillio alfalfae morphologia simile, sed characteribus sequentiarum nucleidearum distinguendum. Actin positione 16 (C), 63 (A); Elongation factor 1-alpha: positione 148 (C), 179 (G), 190 (C), 248 (G), 316 (G), 332 (G), 342 (T), 414 (G), 470 (T), 473 (C), 494 (G), 513 (T), 541 (C), 580 (G), 595 (T), 597 (C), 639 (T); Glyceraldehyde-3-phosphate dehydrogenase positione 234 (T), 267 (T); Tryptophan synthase positione 471 (C), 534 (C).

Description.

Colonies on PDA after two weeks 3.5–5.5 cm, white at first, later darkening due to the formation of resting mycelium immersed in the agar (Figures 10a, 10b). Aerial mycelium generally abundant, floccose to pruinose, hyphae smooth-walled, 1.5–3 µm wide. Conidiophores erect or slanted (Figures 10c, 10d), generally determinate, branched or unbranched (Figure 10c, 10d), formed disjointedly throughout the colonies, hyaline, 30–710 µm in length, 4.5–6 µm wide, narrowing towards the apex to 2–3 µm, transversely septate, septa spaced more narrowly towards the apex. Conidiogenous cells are phialides (Figure 10e), arranged in (1–) 2–6 whorls along conidiophores (Figures 10d 10e). Whorls spaced 50–160 µm apart, closer towards the apex, consisting of (1–) 2–5 (–7) phialides (Figure 10e), arising below transverse septum. Apical whorls consisting of one apical and one to several lateral phialides (Figures 10d, 10e). Phialides subulate, tapering from 2–3 µm at the base to 1–1.5 µm at the tip, terminal phialides 40–60 µm long, lateral phialides 30–45 µm long (Figure 10e). Conidia hyaline, smooth-walled, cylindrical with rounded apices to oval (Figure 10f), allantoid at times, (4.0–) 6.0 µm±1.0 µm (–10.5)×(2.5–) 3.0 µm±0.5 µm (–3.5) (l/w = (1.3–) 2.0±0.2 (–2.7), n = 80), accumulating at the tip of the phialides (Figure 10d). Resting mycelium present (Figures 10g, 10h, 10i), consisting of brown-pigmented hyphae, up to 9 µm wide, thick-walled, straight or curved, solitary or aggregated (Figures 10g, 10h), torulose at times (Figure 10i).

Types.

Holotype: Dried culture of V. nonalfalfae strain PD592 (Japan: Hokkaidou; Irish potato) deposited at UC (UC 1953898), an ex-holotype culture at CBS (CBS 130339) and NRRL (NRRL 54791).

Specimens examined.

The description was based on V. nonalfalfae strains PD592 (Japan: Hokkaidou; Irish potato), PD616 and PD626 (UK; common hop), PD744 (Cuba; potato field soil), PD745 (Canada: Manitoba; spinach), PD808, PD809 and PD811 (Slovenia; common hop) and PD810 (Slovenia: petunia) (Table S1).

Distribution and host range.

Currently known from Canada, Cuba, Japan, Slovenia and UK. Substrates include common hop, Irish potato, petunia and spinach.

Commentary.

Verticillium nonalfalfae is morphologically indistinguishable from V. alfalfae, but the two species differ in pathogenicity. Verticillium nonalfalfae causes disease on a variety of different hosts, whereas V. alfalfae causes disease mainly on lucerne [44]. Other differences include vegetative compatibility groups [45], mating types (Figure 2), as well as the DNA characters listed in the species descriptions. Verticillium alfalfae and V. nonalfalfae were described as new species because no synonyms of the morphologically similar V. albo-atrum were available (www.indexfungorum.org, accessed on September 30, 2011).

Verticillium nonalfalfae and V. alfalfae have long been recognized as two genetically distinct groups referred to as non-lucerne and lucerne pathotype, respectively [46], [47], [48].

Within Verticillium, V. nonalfalfae and V. alfalfae lack unique, diagnostic morphological characters and were frequently confused with the distantly related V. albo-atrum. The three fungi share an overall similar morphology, including the formation of resting mycelium (Figures 4g, 4h, 5f, 5g, 5h, 10g, 10h, 10i). Verticillium albo-atrum also forms microsclerotia (Figures 4i, 4j, 4k), one-septate, brown-pigmented conidia (Figure 4f), as well as phialides that originate directly from conidia (Figure 4f). However, microsclerotia were only observed on WA-p and on PLYA media, not on PDA medium, and one-septate, brown-pigmented conidia, and conidia germinating by phialide formation are relatively rare. Thus, based on our data, it is not possible to consistently differentiate V. nonalfalfae and V. alfalfae from V. albo-atrum using morphological characters. Verticillium albo-atrum may co-occur with V. nonalfalfae on some hosts, as Keyworth [49] isolated Verticillium strains forming resting mycelium, as well as Verticillium strains forming microsclerotia and resting mycelium simultaneously, from diseased potato plants in Connecticut.