Abstract
Disease incidence in damping-off of sugarbeet seedlings induced by Rhizoctonia solani AG2-2 declined rapidly after two successive triweekly plantings. However, there was not a great decrease in saprophytic activity by the flax baits until five to seven successive plantings. In a certain combination of soil and isolate, i.e., in Kiyokawa soil inoculated with sclerotia of isolate Rh-65, abnormally slow growth isolates in the early stage (less than half of the hyphal growth of healthy isolates during 5 days on potato dextrose agar) appeared after three to five successive plantings and then gradually accounted for almost the whole number of isolates.
When sclerotia were inoculated into the soil repeatedly with sugarbeet replants, disease incidence in the second successive crop of sugarbeet was extremely low (20%) compared with that of the first crop (60%), and disease incidence of the third crop was almost negligible. Such disease suppressiveness could have been induced only by adding living sclerotia to the soil independently of the presence of the host plant, but not induced by dead sclerotia or by successive replantings.
There was a greater increase in the number of soil microorgaisms in the soil of 11th, but not 2nd successive crop of sugarbeet, than in the soil of non-inoculated and non-planted soil (control). The disease decline was not closely related with the total number of soil microorganisms.
