Rhizosphaera L. Mangin & Har., Bull. Soc. mycol. Fr. 23(1): 56 (1907)
Index Fungorum number: IF 9728; Facesoffungi number: FoF xxx, 8 morphological species (Species Fungorum 2021), 5 species with molecular data.
Pathogenic on host. Sexual morph: Unknown. Asexual morph: Mycelium grows within the blighted needles which are grayish 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 within the pycnidium. At this phase, numerous small black dots are seen on blighted needles in several regular rows equivalent to those of stomata. Under a hand lens, pycnidium is found to be a small black globule with white resinous mass on it, but becomes pale to yellowish brown in color under moist condition. Pycnidium is globular to sub globular, slightly flattened at the lower part, and has a stalk- and ball-like hypostroma at its base. Hypostroma is more clearly visible on the host plant, having thick epiderm and wide stomatal cavity. Pycnidial wall comprises a layer of brown-colored hypha. Inner part of pycnidium comprises prosenchymatous brown hyphae at 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. Pycnospore is hyaline, unicellular, ovoid, elliptic or cylindric, often somewhat irregular shaped (Adapted from Kobayashi 1967).
Type species: Rhizosphaera abietis L. Mangin & Har.
Notes: Rhizosphaera was introduced by Mangin and Hariot (1907) with R. abietis as type species. Rhizosphaera was placed in Sphaeropsidales based on its habitat, that is inhabiting blighted needles. Rhizosphaera is unique in the structure of the pycinidium. Rhizosphaera is characterized by stalk like bundle of brown hyphae which grows 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 characterized by an apparent stalk and hyphal mass at their base. Orton (1915) reported that Rhizosphaera species is 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. (2020), 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.