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Penetration and Infection of Orchid Protocorms by Thanatephorus cucumeris and Other Rhizoctonia Isolates. Brian Williamson, Postdoctoral Student, Department of Botany, University of Aberdeen, AB9 2UD, Scotland, United Kingdom; Geoffrey Hadley, Lecturer in Mycology, Department of Botany, University of Aberdeen, AB9 2UD, Scotland, United Kingdom. Phytopathology 60:1092-1096. Accepted for publication 18 February 1970. DOI: 10.1094/Phyto-60-1092.

Penetration and infection of protocorms of the orchid Dactylorhiza purpurella by several isolates of Rhizoctonia were observed by a slide culture technique. Eight isolates, including pathogenic strains of Thanatephorus cucumeris (Rhizoctonia solani) and orchid endophytes Tulasnella calospora and Ceratobasidium cornigerum, were all symbiotic to various degrees. Living epidermal hair cells were the only site of symbiotic infection. All isolates penetrated hairs from single hyphae and induced growth and differentiation of protocorms. Pathogenic infection of protocorms occurred infrequently with all isolates, following random penetration of epidermal cells. Compatible (symbiotic) infections frequently became parasitic at different stages of protocorm development. Pathogenicity tests showed that only two isolates of T. cucumeris infected cotyledons and hypocotyls of crucifer seedlings. These isolates grew along the junctions between host epidermal cells, and penetrated from dome-shaped infection cushions.