The mode of infection of a pathogenic uninucleate Rhizoctonia sp. in conifer seedling roots

The mode of infection of a uninucleate Rhizoctonia sp., a root-dieback pathogen in Finnish and Norwegian conifer nurseries, was studied in seedlings of Picea abies (L.) Karst., Finns sylvestris L. and Larix sibirica Ledeb. growing under nonsterile conditions. In all hosts, the pathogen attacked growing tips of primary roots and long laterals. After a 5-month incubation period, the root systems of inoculated pine and spruce were heavily stunted, whereas larch seedlings were less affected In the tip region where protoxylem elements had not yet differentiated, proliferating hyphae formed aggregations on the root surface and within the intercellular spaces of the outer cortex. This gave rise to penetration hyphae that invaded the apical meristem and neighbouring vascular cylinder. On the root surface, at basal root regions, hyphal growth was sparse and the infection was characterized by formation of monilioid hyphae within cortical cells. The hyphae spread intracellularly towards the basal root regions within the vascular cylinder. Considerable hyphal proliferation was observed when the pathogen was grown under membrane-isolated host roots. It is suggested that root exudates induce hyphal proliferation preceding the formation of penetration hyphae. All isolates were induced to fruit in the presence of all hosts.