Orchidaceous rhizoctonias in pot cultures of vesicular–arbuscular mycorrhizal fungi

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Abstract
The paper reports that previously undescribed, sterile, septate fungi (Rhizoctonia) with affinity to and attributes of orchid mycorrhizal fungi, commonly occur in pot cultures of vesicular-arbuscular (V-A) mycorrhizal fungi. Seventeen pot cultures of V-A endophytes from several sources were studied. The endophytes included unidentified organisms as well as species of Glomus, Acaulospora and Gigaspora. A Rhizoctonia was present in every pot culture. In different cases, Rhizoctonia isolates were obtained from sporelike cells in intramatrical vesicles, extramatrical hyphae, and chlamydospores or roots of pot culture plants. In pure cultures, the rhizoctonias formed pale or yellow-brown, submerged colonies composed of narrow, irregularly septate hyphae. Monilioid hyphae and terminal or intercalary, spherical chlamydospores about 12 .mu.m in diameter developed in older mycelia. Fruiting experiments by J. H. Warcup indicated that the teleomorph of 3 Rhizoctonia isolates is related to Sebacina vermifera Oberwinkler, a mycorrhizal endophyte of certain Australian terrestrial orchids. Positive tests for symbiotic germination of orchid seed with one isolate are described. Pasture legumes and ryegrass plants were inoculated with mycelia of Rhizoctonia strains in the presence or absence of V-A mycorrhizal fungi. Inoculation affected plant growth only when V-A mycorrhizal fungi were present: in steamed soil containing residual inoculum of a V-A endophyte, the growth response following infection by the V-A endophyte occurred in inoculated plants several weeks earlier than in uninoculated plants; in different natural soils, inoculation increased, decreased or had no effect on growth, depending on the strain of Rhizoctonia used.

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