DISTRIBUTION OF THE ORCHID MYCORRHIZAL FUNGUS, RHIZOCTONIA-SOLANI, INRELATION TO ITS HOST, PTEROSTYLIS-ACUMINATA, IN THE FIELD

The plant pathology model was tested as an explanation of the distribu tion of orchid mycorrhizal fungi in the field. The mycorrhizal fungus, Rhizoctonia solani Kuhn AG 6, was isolated from adult plants and prot ocorms of Pterostylis acuminata R.Br. at a study site and from adult p lants 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 t he 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. co ncinna R.Br., P. ophioglossa R.Br. and Caladenia catenata (Smith) Druc e, found at the site. It was concluded that the co-distribution model better explains the distribution of fungus and host in this case.