Alternaria radicicola N. Bessadat & P. Simoneau, 2025

Bessadat, Nabahat, Bataillé-Simoneau, Nelly, Colou, Justine, Hamon, Bruno, Mabrouk, Kihal & Simoneau, Philippe, 2025, New members of Alternaria (Pleosporales, Pleosporaceae) collected from Apiaceae in Algeria, MycoKeys 113, pp. 169-192 : 169-192

Alternaria radicicola N. Bessadat & P. Simoneau sp. nov.

Fig. 4

Etymology.

Name refers to the organ from which the species was isolated, carrot roots.

Type.

Algeria • Oran market on infected roots of Daucus carota . 16 July, 2020, N. Bessadat, ( INH 001054 , holotype), preserved in a metabolically inactive state via deep freezing at INH herbarium, France, using the COMIC technical platform, ex-type cultures ( CBS 149902 , NB 830) .

Description.

On PCA, attaining 75 mm in diameter, velvety (NB 830, NB 936) to flat (NB 794), sometimes with granular appearance by the presence of abundant intra-hyphal, dematiaceous, thick-walled chlamydospores after 8–14 days. Aerial mycelium sparse and submerged hyphae abundant, producing chlamydospores in culture formed from fertile hyphae with 3–5 transverse septa and sometimes one longitudinal. These fructifying elements arising from a distinct radial system of hyphae or near the substrate surface. Conidiophores arising directly from lateral and apical aerial axes, simple, septate, 20–30 × 3.5–5 μm, straight or geniculate at successive sites of conidium production, size of cells decreasing towards apex, rarely branched, cell walls thicker than those of vegetative hyphae, pale brown to brown in color, sometimes swollen at the base, rarely from chlamydospores. Conidiogenous branches, formed on fertile hyphae, 1–2 - celled, 15–20 × 3 µm; the longest conidiophores reaching 31–88 μm in length. Each usually bearing only a single conidium in young cultures and occasionally a short geniculate extension with a second or third conidium. Conidiogenous sites terminal or intercalary, proliferating sympodially, brown. Occasional chain formation of two spores through secondary conidiophores on the tip or basal cells of primary conidia. Conidia mature in broad-cylindrical morphologies, usually narrow ellipsoid, ovoid, or cylindrical with rounded base and apex, as large as 18–26 × 7–10 µm, with 3 (– 4) thickened transverse septa and rarely one longitudinal septum, representing a high percentage of the mature population (Fig. 4 A, B ). Conidia occasionally forming chlamydospores during germination, exhibiting single-celled, spherical or oval brown structures on both ends of conidia, measuring 7–12 µm in diameter (Fig. 4 C ). Another population of conidia with 4–5 (– 6) transverse septa, smooth-walled, cylindrical to obclavate, mostly straight, sometimes curved with middle cells slightly enlarged, pale brown to brown, with rounded basal and apical cells, 30–36 (– 38) × 7–10 (– 12) and 0–1 (– 2) longitudinal septa in 2–3 of the transverse segments. A few conidia with 7 transverse septa, usually without longitudinal septa, 35–39 × 10–11 µm, relatively narrowed cylindrical, not forming secondary conidiophores (Fig. 4 D, E ). Conidiophores emerging from the surface of dead infected plants rigid, brown, clustered, or scattered, with 1–4 (– 6) conidiogenous loci, 25–67 µm long and 3.8–6 (– 7.5) μm thick (Fig. 4 F ). Most conidia with 8–10 (– 11) transverse septa, 44–55 (– 59) × 8–10 µm. On aged parts of the colony, conidia usually clustered in small clumps with 2–4 spores near the tips or laterals of primary conidiophores (Fig. 4 G ).

All isolates producing dark-brown, compound chlamydospores, looking like thickened oval or rounded cell chains with punctate ornamentation, arranged in chains (Fig. 4 H ). Such cells conglomerating and elongating to a pearl necklace or an irregular shape, as large as 15–41 × 20–53 µm with 3–8 cells; these structures enlarging as the colony ages and observed also on six-week inoculated leaf debris in contact with soil. Conidia color before full maturity dark yellow, which deepens to an olive brown, paler towards apex, against which the thickness of major transverse septa contrasts strongly.

Culture characteristics after 7 days — Colonies color and aspect of the holotype strain on PDA (Fig. 4 I ), PCA (Fig. 4 J ), MEA (Fig. 4 K ), and OA (Fig. 4 L ) are provided in Table 2 . This strain grows over a wide range of temperatures with varying growth rates. Among the different temperature and culture media, the best mycelial growth occurs at 25 ° C on PDA and PCA. On all media, the strain grows slowly at 4 ° C and 35 ° C, moderately at 16 ° C, rapidly between 20–30 ° C, and does not grow at 40 ° C.

Additional isolate examined.

Algeria • Oran City, Oran province Market , from the root of Daucus carota . 18 February, 2020, N. Bessadat, ( CBS 149906 , preserved in a metabolically inactive state in the Westerdijk Fungal Biodiversity Institute, Utrecht, the Netherlands). Living culture NB 794 .

Algeria • Mascara City, Tizi province , from leaves of cultivated Daucus carota . 21 December, 2020, N. Bessadat, Living culture NB 936 .

Notes.

Phylogenetic analyses indicated that Alternaria radicicola sp. nov. fell in an individual branch close to A. tellustris (ex-type, CBS 538.83) and A. chlamydosporigena ( CBS 341.71). Although the three species shared identical ITS sequences, there were 8 / 529 differences in gpd, 8 / 833 in rpb 2, and 8 / 199 in tef 1 between isolates of A. radicicola sp. nov. and A. tellustris and 8 / 529 differences in gpd, 11 / 833 in rpb 2, and 7 / 199 in tef 1 between isolates of A. radicicola sp. nov. and A. chlamydosporigena . Although A. radicicola sp. nov. has almost the same conidia size and shape as the closely related A. chlamydosporigena (Table 3 ), it differs from this species by producing chlamydospores in culture that are able to form fertile conidiophores; association of conidiophores with the chlamydospores has never been observed on A. chlamydosporigena ( Simmons 1971; Marin Felix et al. 2019). Other species from section Embellisia exhibited variable conidial size; A. embellisia produces chlamydospores in pairs or chains (up to seven cells) ( Delgado Ortiz et al. 2019), while A. radicicola isolates form shorter chains of 3–4 cells after 7 days under similar incubation conditions. Alternaria tellustris produces obclavate or long ellipsoid conidia smaller than the new species (18–33 × 6–8 vs. 20–38 × 7–12, respectively); obclavate conidia were rare or mainly immature in A. radicicola sp. nov. Chlamydospore production was influenced by temperature and time of incubation. A. radicicola sp. nov. isolates exhibited abundant chlamydospores beyond 7 days when temperatures ranged between 30–35 ° C and 2–3 weeks when incubated at 20–25 ° C. These structures were also observed on infected host leaves after 6 weeks of inoculation.

Figures

Figure 4. Morphology of Alternaria radicicola sp. nov. from Daucus carota: Conidia on PCA for 7 days at 22 ° C (A, B, D); Chlamydospores from germinating conidia (C); Conidia and conidiophores on inoculated coriander leaves at 7 DAI (E, F); Sporulation patterns on PCA for 14 days at 22 ° C (G); Chlamydospores in the surface of vegetative mycelium (H); Colony phenotypes on PDA (I), PCA (J), MEA (K), and OA (L) for 7 days at 25 ° C. Scale bar: 25 µm (A – H); 10 mm (I – L).