Rhizoctonia solani and orchid seed

Abstract

Commentary: Roy Watling* This is one of five papers by Dorothy Downie on the Rhizoctonia orchid relationship. Her work was instrumental in stimulating the growth of interest in the intimate and complex relationships between orchids and fungi in the British Isles. The field and experimental approach particularly focused on isolates of Rhizoctonia solani J.G. Ktihn, the agent of silver scurf disease of potatoes and eye-spot of wheat (Downie, 1957), and its relationship with Goodyera repens (L.) R. Br. Her studies were examples to be followed. Indeed they were at her own university of Aberdeen, by Geoff Hadley and his colleagues and students. His studies gradually expanded her work on Goodyera (Downie, 1940), Dactylorchis (Downie, 1959) and Corallorhiza (Downie, 1943) to more searching studies on Dactylorhiza purpurella (Stephenson & T.A. Stephenson) So6 (Hadley & Harvais, 1968), to Coeloglossum viride Hartman (Hadley, 1970) and Platanthera bifolia (L.) L.C. Rich. (Hadley & Ong, 1978). Observations in the laboratory on D. purpurella were conducted by Hadley & Williamson (1971), Hadley & Purves (1974) and Hadley & Ong (1978) as well as on Goodyera repens (Hadley, 1969; 1970; Hadley & Purves, 1974; Hadley & Ong, 1978). Hadley and his team extended the study to enquire as to the kind of dependency the fungus had on the orchid, type of carbohydrate used as a substrate, and the physiology of the orchid/fungus interaction. With the twinning of the University of Aberdeen with Kuala Lumpur the expertise commenced by Downie was transferred to Malaysia where Hadley (1985) and Hadley & Williamson (1972) studied mycorrhizas in tropical orchids. Interest spread further afield stimulating attempts to identify the teleomorphic state of the Rhizoctonia spp. involved in orchid mycorrhiza, as it was becoming quite clear that the R. solani was a complex of different taxa. Thus Warcup & Talbot (1967) demonstrated that the perfect states were to be found in Thanatephorus and also Sebacina and Ceratobasidium. Recently the Rhizctionia complex has been the subject of a monograph from the Royal Botanic Garden, Kew (Roberts, 1999) and Aberdeen isolates are quoted therein. The perfect states were known and circumscribed as jelly fungi well before the link between fungus and orchid had been made being previously well-known on soil, rotten vegetation and wood. It was not until fairly recently was it shown that these fungi had an even more complex role to play in the ecosystem. It came as a great surprise when the fungi studied by Downie turned out also to link orchids to arborescent plants. This phenomenon had been demonstrated in other groups of macrofungi but jelly