Exobasidium

Nagao, Hideyuki, Onishi, Wataru & Takamichi, Orihara, 2023, Leaf Blister on Leucothoe grayana var. venosa Caused by an Exobasidium Species Exobasidium ḆŔḶJÕÒ ¬ ṪÒϮh ‼ įN Ṯƪssậ ‹ Ếữ ›, Bulletin of the Kanagawa Prefectural Museum (Natural Science) 52, pp. 1-5 : 2

Description of Exobasidium species on Leucothoe grayana var. venosa ( Fig. 1)

Spots were produced on the abaxial surface of leaves ( Fig. 1I ). Fresh spots with hymenium appeared pale white to pale yellow. The hymenium often had a farinose appearance. Later, the spots turned brown, formed dry lesions and finally their centers dropped out to form shot holes. Lesions were observed infrequently. The hymenium is composed of basidia with 2 sterigmata and conidia. Basidia are cylindrical, 10.3–35.9 × 4.8–6.9 µm, emerging directly from the host surface. Sterigmata are (2.1–)4.8–6.9(–8.3) × (0.7–)1.3(–2.1) µm, developing outwardly and tapering toward the tip ( Fig. 1A, B ). Basidiospores are ellipsoid, (12.4–)15.2– 20.7 × 4.1–6.9 µm, hyaline, smooth, one–celled when formed, and septate with (2–)3–4(–5) septa ( Fig. 1C ) after formation. Septate basidiospores germinated after 24 h when released onto an agar surface. They were also observed to germinate in hymenia on leaves of the plant. The mode of germination is budding. Direct budding from several cells of the basidiospores was predominant ( Fig. 1D, E ) but short germ tubes sometimes emerged from cells of the basidiospores and produced conidia at the tip of the germ tubes in the hymenium ( Fig. 1F ). The short germ tubes didn’t elongate into pseudohyphae. Colonies on PDA were composed of partially elongated pseudohyphae and conidia. The surface of the colonies was corrugate without a farinose appearance and pink to pale pink, the same as the reverse of the colonies. Colonies were glutinous and did not fix on the agar surface ( Fig. 1G ). Conidia on PDA were lacrimiform, subfusiform or clavulate, 3–6 × 0.7–1.7 µm. Conidia polarly budded or germinated to produce pseudohyphae ( Fig. 1H ).

Specimens examined: JAPAN: Kanagawa Pref., Hakone, near the peak of Mt. Komagatake , 28 July 2021 (KPM-NC 26934) and 30 June 2022 (KPM-NC 26936), W. Ohnishi and H. Nagao .

The sizes of basidia, sterigmata, and basidiospores of E. bisporum Sawada ex Ezuka are 40 × 6–7 µm, 4–6 µm length, and 15–24 × 5–7 µm, respectively ( Ezuka, 1991), which are mostly in the same range of the Exobasidium specimens on L. grayana var. venosa collected in Kanagawa Prefecture ( Table 1 ). The shape of basidiospores of the Exobasidium specimens on L. grayana var. venosa are ellipsoid. Sawada (1950) showed combined spore measurements from L. grayana varieties and V. ovalifolium , whereas Ezuka (1991) reported the spore measurements separately from L. grayana varieties and V. oldhamii . The range of septal number of basidiospores isolated from V. oldhamii was broader (1–7) than those from L. grayana varieties (1–4). The specimens obtained from L. grayana var. venosa showed slightly broader range of septal number (i.e., (2–)3–4(–5)) than E. bisporum . In addition, the mode of germination of basidiospores is budding, the same as that of E. bisporum reported by Ezuka (1991). Thus, the Exobasidium species on this host plant is morphologically close to E. bisporum .

The top three homology search results by NCBI indicated that Exobasidium sp. ON 787635 isolated from Iceland moss, Cetraria islandica , E. bisporum AB 180368 isolated from E. grayana var. oblongifolia , and E. bisporum AB 180364 isolated from E. grayana var. glabra showed 96% (523 bp/546 bp identical), 96% (508 bp/530 bp identical), and 94% (499 bp/532 bp identical) identities, respectively, while the homology search between E. bisporum AB 180368 and E. bisporum AB 180364 showed only 94 % (500 bp/531 bp identical) identity. This homology search result suggests that each E. bisporum isolate from the two varieties of E. grayana are not identical in ITS sequences, and that the ITS region of representative isolate MAFF 247751 is different from E. bisporum .

In conclusion, Exobasidium species on L. grayana var. venosa , MAFF 247751 is morphologically similar to E. bisporum , but genetically different. Therefore, further investigations are required to identify this species.

Figures

Fig. 1. Morphology and symptoms of Exobasidium sp. on Leucothoe grayana var. venosa collected on Mt. Hakone Komagatake, Kanagawa Prefecture (KPM-NC 26934). Basidium with 2 sterigmata (arrows) (A), a sterigma bearing an immature basidiospore (B), ellipsoid basidiospore, one–celled when formed, becoming septate with 3–4 septa (arrows) (C), basidiospore with directly budded conidia (D), or germinated by a short germ tube (arrows) producing a conidium at the tip (E, F). Colony from a single basidiospore of Exobasidium sp. on PDA after 21 days incubation (G); colony was composed of conidia and pseudohyphae (arrows) (H). Chlorotic spot. Arrows indicate the old spots (I). Scale bars:A–F, H = 10 µm, G = 5 mm, I = 20 mm. Photographed by Hideyuki Nagao.