TAXONOMY, MORPHOLOGY, AND PATHOGENICITY OF RHIZOCTONIA SPECIES FROM FOREST NURSERIES

Abstract

The history of the concept of the genus Rhizoctonia is reviewed. Ten species and one variety of Rhizoctonia were identified from the study of over 60 selected isolates obtained from the forest nurseries in Saskatchewan and Manitoba. These are R. dichotoma, R. endophytica, R. endophytica var. filicata, R. globularis, R. hiemalis, R. callae, R. lilacina, R. repens, R. rubiginosa, R. solani (Ceratobasidium filamentosum), and C. praticolum. The first five of these were previously described by the authors as new. This paper deals with the taxonomic descriptions of the remaining 6 species and the pathogenicity of all 10 species. Of the six species described here, R. callae, R. lilacina, R. repens, and R. rubiginosa are new records for Canada. C. praticolum is distinguished from R. solani in the imperfect stage on cultural and morphological characters. Comparative morphological studies show that chlamydospore characters were maintained consistently within a species, and their value as dependable diagnostic characters is emphasized.The pathogenicity of Rhizoctonia species was tested under aseptic conditions in test tubes and in partially sterile soil on tomato, lettuce, cabbage, beet, dill, caragana, cedar, elm, and pine seedlings. Mycelia of all the species invaded the tissues of all the host seedlings, showing no specialization. Ceratobasidium praticolum and R. dichotoma caused severe damping-off. C. praticolum appears to be by far the most widespread and virulent of all Rhizoctonia species in forest nurseries in Canada. R. solani, R. lilacina, and R. endophytica var. filicata, caused rotting of seeds, roots, and hypocotyls with varying intensity. R. callae, R. endophytica, R. globularis, and R. repens were characterized by production of chlamydospores inside the cortical cells of the roots, completely or partially filling them. This endophytic association affected the root development and general health of the seedlings adversely. Under certain conditions, the endophytes also attacked the aerial parts of the seedlings and caused some damping-off. R. hiemalis and R. rubiginosa were non-pathogenic, though their mycelia penetrated the root tissue.These studies indicate that the Rhizoctonia problem in forest nurseries is a complex one involving several species differing in their virulence and mode of attack.