Orchid Mycorrhizal Fungi Research Articles

Distribution of the Orchid Mycorrhizal Fungus, Rhizoctonia solani, in Relation to Its Host, Pterostylis acuminata, in the Field

The plant pathology model was tested as an explanation of the distribution of orchid mycorrhizal fungi in the field. The mycorrhizal fungus, Rhizoctonia solani Kuhn AG 6, was isolated from adult plants and protocorms of Pterostylis acuminata R.Br. at a study site and from adult plants at five other locations in the Sydney region. In litter and trap seedlings, R. solani AG 6 was found within 50 cm of adult plants of the orchid P. acuminata and was more abundant close to the host plant. In the laboratory, protocorms of P. acuminata became infected with R. solani AG 6, but not with fungi isolated from the other orchids, P. concinna R.Br., P. ophioglossa R.Br. and Caladenia catenata (Smith) Druce, found at the site. It was concluded that the co-distribution model better explains the distribution of fungus and host in this case.

Orchidaceous rhizoctonias in pot cultures of vesicular–arbuscular mycorrhizal fungi

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 culture, 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 μm in diameter developed in older mycelia. Fruiting experiments by J. H. Warcup indicated that the teleomorph of three 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.

Occurrence and intensity of mycorrhizal infections in cultivated orchids in some New Zealand nurseries

Abstract Orchid root samples collected from 4 commercial growers around Hamilton and from the Pukekohe Horticultural Research Station (PHRS), were examined for the intensity of mycorrhizal infections, The orchid roots from commercial growers' glasshouses where orchids had been cultivated for 5-20 years harboured more mycorrhizal fungi than roots collected from PHRS where orchids had been cultivated for about 15 months, The highest infection level recorded in Cymbidium was 88% and in Cattleya was 30%, Comparison of Cymbidium hybrid plants at different ages suggested that mycorrhizal infection increased with the age of the plant. Aerial roots had very low infections, The present study suggests that, once established, naturally occurring orchid mycorrhizal fungi are fairly widespread, and rapidly spread from plant to plant within a glasshouse.

ASYMBIOTIC GERMINATION OF ORCHID SEEDS ON CARBOHYDRAES OF FUNGAL ORIGIN

SUMMARYAsymbiotic germination of two orchid species on sugars found in orchid mycorrhizal fungi was studied. Trehalose and glucose both provided a satisfactory carbon source for growth of non‐photosynthetic orchid protocorms while mannitol did not.

The Relation of Specificity of Orchid Mycorrhizal Fungi to the Problem of Symbiosis

The Relation of Specificity of Orchid Mycorrhizal Fungi to the Problem of Symbiosis