Verticillium dahliae

Verticillium dahliae

MycoBank: MB196942

Description.

Colonies on PDA after two weeks 4–6 cm diam, white at first, later darkening due to the formation of microsclerotia (Figures 6a, 6b). Aerial mycelium generally abundant, floccose, at times sparse and pruinose, or appressed to the agar and appearing water-soaked. Aerial hyphae smooth-walled, (1.5–) 2–4 µm wide, at times containing inflated cells up to 9 µm wide (Figure 6c). Conidiophores erect or slanted, generally determinate (Figure 6d), branched or unbranched (Figure 6e), formed disjointedly throughout the colonies, hyaline, 80–800 µm in length, 3–4 µm wide, narrowing towards the apex, transversely septate, septa spaced more narrowly towards the apex. Conidiogenous cells are phialides (Figure 6f), arranged in (1–) 2–3 (–10) whorls along conidiophores (Figures 6d, 6e), arising below transverse septum (Figure 6f). Whorls spaced 50–100 µm apart, closer towards the apex, consisting of (1–) 2–4 (–6) phialides (Figures 6d, 6e). Apical whorls consisting of one apical and one to several lateral phialides (Figure 6d). At times solitary phialides are formed laterally from vegetative hyphae (Figure 6g). Phialides subulate, tapering from 2–3 µm at the base to 1–2 µm at the tip, terminal phialides 40–60 µm long, lateral phialides 25–50 µm long (Figures 6d, 6e, 6f, 6g). Conidia hyaline, smooth-walled, non-septate, cylindrical with rounded apices to oval (Figure 6h), allantoid or tapering at times, (3.5–) 6.5 µm±1.5 µm (–13.5)×(2.0–) 3.0 µm±0.5 µm (–4.5) (l/w = (1.4–) 2.2±0.3 (–3.4), n = 80), accumulating at the tip of the phialides (Figures 6d, 6e). Microsclerotia immersed in agar, regularly or irregularly distributed throughout the colonies, composed of rounded, brown-pigmented cells up to 13 µm diam, solitary microsclerotia rounded to elongate or irregular in shape, 25–100 µm diam, aggregates of microsclerotia up to 200 µm diam (Figures 6i, 6j, 6k). At times short, brown-pigmented hyphae attached to microsclerotia present (Figure 6k).

Types.

Holotype: Specimen V. dahliae (Germany; Dahlia sp.) at HBG (Figures 3a, 6j) [16]; Epitype (designated herein): Dried culture of Verticillium dahliae strain PD322 (USA: CA, lettuce) deposited at UC (UC 1953893), an ex-epitype culture at CBS (CBS 130341) and NRRL (NRRL 54785).

Specimens examined.

The description was based on Verticillium dahliae strains PD322 (USA: CA; lettuce), PD327 (USA: CA; bell pepper) and PD502 (USA: WI; maple) (Table S1). The V. dahliae holotype specimen was also examined (Figures 3a, 6j).

Distribution and host range.

Currently known from Brazil, Canada, Denmark, France, Germany, Iran, Israel, Italy, Japan, Netherlands, Russia, Spain, Sweden, UK, Ukraine, and USA (CA, ID, IL, OR, TX, WA, WI) [27]. Substrates include Anaheim pepper, annual sunflower, apricot, ash, bell pepper, cabbage, celandine, chili pepper, common flax, eggplant, European smoketree, garden tomato, globe artichoke, horseradish, hybrid strawberry, Icelandic poppy, Irish potato, jalapeno, Japanese maple, lettuce, maple, olive, opium poppy, paprika, pepper, peppermint, pistachio nut, purple coneflower, rape, scentless false mayweed, spinach, stock, sweet almond, udo, upland cotton, and watermelon [27] that represent fourteen different plant families (Aceraceae, Amaranthaceae, Anacardiaceae, Araliaceae, Asteraceae, Brassicaceae, Cucurbitaceae, Fabaceae, Linaceae, Malvaceae, Oleaceae, Papaveraceae, Rosaceae, Solanaceae).

Commentary.

Verticillium dahliae is the type of Verticillium and was described by Klebahn [35] from Dahlia sp. cv. Geiselher in Germany (Figure 3a). Verticillium dahliae is not the oldest species of the genus, but it has the largest impact as a pathogen, is common and genetically relatively homogenous, and has thus been conserved as the type of the genus [16], [34]. Since a viable ex-holotype culture is no longer available [33], and DNA extraction attempts from the holotype specimen failed, we designated a V. dahliae epitype with an ex-epitype culture that serves as an interpretive type for molecular studies.

The original description of V. dahliae by Klebahn [35] was based on material from Dahlia sp., and from cultures on Salep Agar medium which is a mixture of polysaccharides contained in orchid tubers [37]. The composition of Klebahn's medium is unknown, but as a reference, Noël [38] isolated fungal symbionts of orchids using a clear, weak decoction of salep containing 2% agar. We examined the V. dahliae holotype material which contains an approximately 50 cm long stalk of Dahlia sp. ‘Sorte Geiselher’ and several leaves (Figure 3a). The microsclerotia present on the stem (Figure 6j) were similar to the microsclerotia formed in culture (Figure 6i). No conidiophores were observed, these are difficult to detect on Dahlia sp. [35], but are illustrated as part of the protolog [35].

The description of V. dahliae based on V. dahliae strains PD322, PD327 and PD502 agreed with the original description by Klebahn [35] except that we failed to detect strands of erect, hyphal aggregates containing conidia and microsclerotia. Klebahn [35] reported the presence of a slightly wider cell (foot cell) at the base of conidiophores. Since foot cells were absent in culture and we did not inoculate live plants, we were unable to confirm the presence of foot cells in V. dahliae. The dimensions provided by Klebahn [35] for microsclerotia, conidiophores, conidiogenous cells and conidia were at the lower end of the range of dimensions that we observed. Our dimensions were similar to reports in the literature for conidia [22] and microsclerotia [22], [39], [40], [41]. Short brown-pigmented hyphae attached to microsclerotia were illustrated by Klebahn [35], the ones that we observed resemble immature microsclerotia as illustrated by Klebahn [35] and Isaac [22]. Verticillium dahliae resembles V. longisporum but has smaller conidia.