Rhizosphaera
Rhizosphaera L. Mangin & Har., Bull. Soc. mycol. Fr. 23(1): 56 (1907).
Index Fungorum number: IF 9728; Facesoffungi number: FoF 11111, 8 morphological species (Species Fungorum 2022), 5 species with molecular data.
Pathogenic on host. Sexual morph: Unknown. Asexual morph: Mycelium grows within the blighted needles which are greyish brown to reddish brown in colour, groups beneath the stoma and forms a ball-like hyphal mass, brown to greenish brown in colour. Strands grow up from mass of hypha and penetrates the stoma and forms a stalk-like bundle in the stomatal cavity. Outer hypha of the bundle continuously grows up and concentrates to the pycnidial wall. Inner hypha branches sparse and fills the pycnidium. Numerous pycnidia are seen on blighted needles in several regular rows equivalent to those of stomata. Each pycnidium has a white resinous mass on it, that becomes pale to yellowish brown in colour under moist condition. Pycnidia are globular to subglobular, slightly flattened at the lower part, with a stalk- and ball-like hypostroma at their base. Hypostroma is more clearly visible on the host plant, having thick epiderm and wide stomatal cavity. Pycnidial wall comprises a layer of brown hypha. Inner part of pycnidium comprises prosenchymatous brown hyphae when immature. On maturity, conidia arise directly from the wall of hyphae and inside wall of pycnidium, and these inner hyphae are dissolved gradually. Subsequently, mature pycnidium is filled entirely with pycnospores instead of brown hyphae. Ostiole lacking, but pycnidium breaks out irregularly at its upper part. Conidiophores absent, in some cases rest of dissolving hyphal wall is misidentified as conidiophore. Conidia hyaline, unicellular, ovoid, elliptic or cylindric, often somewhat irregular shaped (adapted from Kobayashi 1967).
Type species: Rhizosphaera abietis L. Mangin & Har.
Notes: Rhizosphaera was placed in Sphaeropsidales based on its habitat on blighted needles. Rhizosphaera is unique in the structure of the pycnidium. Rhizosphaera is characterised by stalk-like bundle of brown hyphae which grow up from the hyphal mass gathered below the stoma, and infiltrates through the stoma, then lastly forms a pycnidium on the stoma. Mature pycnidium is characterised by an apparent stalk and hyphal mass at their base. Orton (1915) reported that Rhizosphaera species are the asexual morph of Phaeocryptopus. Humphries et al. (2017) showed that Rhizosphaera pini strains produce colonies with pycnidium-like structures. In the phylogenetic analysis of Hongsanan et al. (2020a), R. pini forms a distinct lineage with Hormonema and Sydowia while some Rhizosphaera species grouped with Plowrightia species. Currently, Rhizosphaera is placed in Dothideales, genera incertae sedis. Molecular markers available for Rhizosphaera are ITS, LSU, SSU and BTUB.
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