Verticillium longisporum

Verticillium longisporum

Basionym: Verticillium dahliae var. longisporum C. Stark, Gartenbauwissenschaft 26(8): 508 (1961)

MycoBank: MB443108

Description.

Verticillium longisporum was described by Stark [42] and in more detail by Karapapa et al. [24]. We documented colony morphology (Figures 9a, 9b), conidia (Figure 9c) and microsclerotia (Figures 9d, 9e, 9f, 9g). We measured microsclerotia and conidia, and assessed the number of phialides per whorl. Microsclerotia were rounded to elongate, 37–240×25–52 µm (Figures 9d, 9e). Conidia were (5.5–) 8.5 µm±2.5 µm (–15.0)×(2.0–) 3.5 µm±1.0 µm (–6.5) (l/w = (1.6–) 2.5±0.7 (–4.5), n = 29). Whorls consisted of (1–) 2–5 (–6) phialides.

Types.

Holotype: Specimen CBS H-19247 at CBS (Germany: Niedersachsen; horseradish) (Figures 3b, 9g), an ex-holotype culture at CBS (CBS 124.64) included in this study as V. longisporum strain PD687 and submitted to NRRL (NRRL 54793), Stark [42] (p. 509) submitted permanent slides of type material to the Herbarium des Staatsinstitutes für Allgemeine Botanik Hamburg, these slides are missing at HBG.

Specimens examined.

Verticillium longisporum strains PD348 (USA: CA; cauliflower), PD356 (USA: IL; horseradish) and PD687 (Germany: Niedersachsen; horseradish) (Table S1), representing the three lineages of V. longisporum [27], and the holotype specimen CBS H-19247 (Germany: Niedersachsen; horseradish), a dried agar culture (Figures 3b, 9g), were examined in this study.

Distribution and host range.

Currently known from France, Germany, Japan, Sweden and USA (CA, IL). Substrates include birdrape, cabbage, cauliflower, horseradish, radish, rape, sugar beet and wild radish [27].

Commentary.

Verticillium longisporum is a diploid hybrid that originated at least three different times from four different parental lineages in three different species, including V. dahliae, Species A1 and Species D1 (Figure 1) [27]. Verticillium dahliae is the only known parent of V. longisporum, Species A1 and Species D1 have never been collected [27]. The holotype of V. longisporum represented by ex-holotype strain PD687 belongs to V. longisporum lineage A1/D3 that is one of the three lineages of V. longisporum, and V. longisporum is thus polyphyletic [27]. There is general agreement that fungal species should be monophyletic. However, we decided that V. longisporum should remain a polyphyletic species, because it seems impractical to name each lineage of V. longisporum. We currently know of three lineages of V. longisporum that represent three independent hybridization events, but there might be many more. Little is known about fungal hybrids, but in plants, hybrids can evolve frequently over short periods in small areas [43].

We included the ex-holotype isolate V. longisporum strain PD687 in our studies, strain PD687 did not form any microsclerotia. But microsclerotia were present in the holotype that is a dried culture of strain PD687 (CBS 124.64) (Figure 3b). The microsclerotia in the holotype documented in Figure 9g were similar to the ones described by Stark [42] on page 500 for ‘Typ X’ as V. longisporum was referred to prior to its description. Thus, V. longisporum strain PD687 likely lost its ability to produce microsclerotia due to prolonged culturing.

Karapapa et al. [24] compared V. longisporum to the morphologically similar V. dahliae, and found that V. longisporum microsclerotia and conidia were longer than the ones in V. dahliae, and that V. longisporum conidiophores had fewer phialides in each whorl than V. dahliae.

We evaluated those characters and found that for the isolates used in this study grown on PDA, microsclerotia and conidia size might be useful to distinguish V. longisporum from V. dahliae. In Verticillium longisporum strain PD356, the majority of microsclerotia were elongate (Figure 9d), but rounded microsclerotia were still present (Figure 9e), and in some sectors of the colony, rounded microsclerotia were in the majority (Figure 9e). In V. longisporum strain PD348, there were roughly as many elongate microsclerotia as there were rounded microsclerotia. Verticillium dahliae microsclerotia were mostly rounded, but in some areas elongate microsclerotia were prevalent. The short brown-pigmented hyphae that were frequently attached to microsclerotia (Figure 9f) are possibly immature microsclerotia as illustrated by Isaac [22]. Similar structures were seen in this study in V. dahliae (Figure 6k). The third strain of V. longisporum investigated here, the ex-holotype strain PD687 did not form any microsclerotia. Conidia of V. longisporum were on average 8.5×3.5 µm (Figure 9c) and conidia of V. dahliae 6.5×3.0 µm (Figure 6h). However, conidia lengths might also at times be misleading, as the size ranges overlap, standard errors were 2.5 and 1.5 µm, respectively. We found that both V. longisporum and V. dahliae had similar numbers of phialides in each whorl, 2–4 for V. dahliae, and 2–5 for V. longisporum, this is unlike that proposed by Karapapa et al. [24] who reported 4–5 in V. dahliae and mostly 3 in V. longisporum. In our hands, Verticillium longisporum strain PD348 very frequently had 5 phialides per whorl.

Thus, a combination of conidia length and microsclerotia morphology might in many cases yield correct species identifications, but the two characters will also be misleading at times.

Another differentiating character was given by Stark [42]. He found that V. longisporum culture filtrate fluoresced, whereas fluorescence was absent in V. dahliae. We did not investigate fluorescence in the two species.