Abstract

Behavior of Rhizoctonia solani on their hosts was carried out-in many studies when the pathogen was in close proximity with its host. The purpose of this study was focused on the behavior of this fungus on faba bean cotyledons when it was away from the host and the effect of nutritional status on its pathogenicity was also taken into consideration. Observations led to speculation that R. solani feels its host even when it was away from it and begins to assemble its strength to grow toward the host then start to form its pathogenic structures when it reach it. Scanning electron microscopic (SEM) observations which was carried out after 24, 48 and 72 h from exposing the pathogen to the host when the fungus was away from it revealed that when the hyphal pathogen feel the host, they swell and some hyphae begin to differentiate and appear very thick, forming likelihood mycelial strands and such hyphae grow vigorously and vertically toward the host. Starvation of the pathogen was found to be vital factor in R. solani pathogencity. Growing of the fungus on water ager led to dramatic increase in the pathogen. This strength was confirmed by visual determination of disease severity and by determination of polyphenol oxidase activity in infected cotyledons. These results exclude the role of host exudates in the attraction of the R. solani toward its host.

Key words: Scanning electron microscopic (SEM), polyphenol oxidase, disease severity, host exudates.