Verticillium alfalfae

General information about Verticillium alfalfae is provided below. Included are a morphological description, comments about morphologically similar species and phylogenetic affiliation. The data is from Inderbitzin et al. (2011) where additional information can be found. Host range data is from Inderbitzin and Subbarao (2014).

Species Description

Colonies on PDA after two weeks 3.5–4.5 cm diam, white at first (Figures 5a, 5b), later darkening due to the formation of resting mycelium immersed in the agar. Aerial mycelium generally abundant, floccose to pruinose, hyphae smooth-walled, 2–3 µm wide. Conidiophores erect or slanted, generally determinate (Figure 5c), branched or unbranched, formed disjointedly throughout the colonies, hyaline, base brown-pigmented at times, enlarged to up to 11 µm at times, 70–570 µm in length, 4.5–6.5 µm wide, narrowing towards the apex to 2–2.5 µm, transversely septate, septa spaced more narrowly towards the apex (Figure 5c). Conidiogenous cells are phialides (Figure 5d), arranged in 1–4 (–6) whorls along conidiophores (Figure 5c). Whorls spaced 30–130 µm apart, closer towards the apex, consisting of (1–) 2–5 (–6) phialides, arising below transverse septum (Figures 5c, 5d). Apical whorls consisting of one apical and one to several lateral phialides (Figures 5c, 5d). Phialides subulate, tapering from 2–3 µm at the base to 1–2.5 µm at the tip, terminal phialides 40–60 µm long, lateral phialides 20–40 µm long (Figure 5d). Conidia hyaline, smooth-walled, cylindrical with rounded apices to oval (Figure 5e), allantoid at times, (4.5–) 6.0 µm±1.0 µm (–11.0)×(2.5–) 3.0 µm±0.5 µm (–4.0) (l/w = (1.4–) 1.9±0.3 (–2.9), n = 68), accumulating at the tip of the phialides (Figure 5c). Resting mycelium present (Figures 5f, 5g, 5h), consisting of brown-pigmented hyphae, up to 9 µm wide, thick-walled (Figures 5f, 5g), straight or curved, solitary or aggregated (Figure 5g), torulose at times.

Morphologically similar species

Verticillium alfalfae produces resting mycelium, as do V. albo-atrum and V. nonalfalfae. Whereas V. nonalfalfae is morphologically indistinguishable from V. alfalfae, V. albo-atrum differs by the formation of microsclerotia and yellow-pigmented hyphae. However, microsclerotia are not formed on PDA, and yellow-pigmented hyphae disappear after multiple transfers in the lab.

Figure 5. Morphological features of Verticillium alfalfae.

5a. Colony of strain PD682 after 24 days on PDA, frontal view. 5b. Colony of strain PD682 after 24 days on PDA, reverse view. 5c. Conidiophore of strain PD682 after 31 days on WA-p. 5d. Phialide of strain PD489 after 30 days on WA-p. 5e. Conidia of strain PD682 after 30 days on WA-p. 5f. Resting mycelium of strain PD489 after 30 days on WA-p. 5g. Aggregated hyphae of resting mycelium in strain PD682 after 73 days on PDA. 5h. Resting mycelium of strain PD683 in the lumen of a thick-walled plant cell after 32 days on WA-p. Scale bar: 5a, 5b = 2 cm; 5c = 50 µm; 5d–5h = 20 µm. Imaging method: 5a, 5b = DS; 5c, 5f–5h = BF; 5d, 5e = DIC.

Phylogenetic position

Verticillium alfalfae belongs to the group of Verticillium that does not produce yellow-pigmented hyphae (group Flavexudans), and is closely related to V. nonalfalfae.

Host range and geographic distribution

These are inferred from GenBank ITS records, and most likely underestimate the current host range and geographic distribution of V. alfalfae.

SubstratesGeographic distribution# plant hosts / # countries
Alfalfa (Medicago sativa) Canada, Iran, Japan, UK, USA (CA,PA)1 / 5