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Abstract
Athelia arachnoidea was collected during winter on dead leaves of deciduous trees in Oregon. Minute sclerotia (0.2–1.0 mm) were seen on hymenia, whereas in culture sclerotia were much larger (1.0–5.0 mm). The pathogenicity, morphological characteristics, and temperature growth responses of the anamorphic state, were identical to Rhizoctonia carotae. The relationship between the sclerotial anamorph and the teleomorph was confirmed by DNA sequence of the internal transcribed spacers of the nuclear ribosomal repeat unit. The sequences were homologous among the specimens of A. arachnoidea and the sclerotial anamorph from Oregon, A. arachnoidea and R. carotae isolates from culture collections, and several isolates of R. carotae from carrot cold-storage facilities. Like R. carotae, A. arachnoidea produced abundant crystals of calcium oxalate on the hyphae, was pathogenic on carrot at 3 C, and caused crater rot symptoms equivalent in severity. Sclerotia of A. arachnoidea-R. carotae did not germinate under conditions suitable for eruptive germination in A. rolfsii. The DNA sequence of A. epiphylla from Europe was identical (within 6 bp) to A. arachnoidea-R. carotae, whereas A. epiphylla from North America varied by 34–46 independent base pair changes. Athelia epiphylla and A. arachnoidea formed similar sclerotia. Connecting the Rhizoctonia root pathogen to the Athelia teleomorph links the disparate literatures on the natural history of this fungus and reveals significant insights into the epidemiology of the disease. Fibularhizoctonia gen. nov. is proposed to encompass Rhizoctonia species with clamp connections, and the description of R. carotae and R. centrifuga are emended.