Decrease in disease severity in sugarbeet in an established Rhizoctonia crown and root rot nursery

Abstract

A disease nursery used for more than 20 years to evaluate resistance of sugarbeet (Beta vulgaris) to crown and root rot caused by Rhizoctonia solani (AG 2-2) has failed in recent years to produce disease suitably severe to discriminate between resistant, partially resistant, and susceptible lines. Experiments were done to determine if biological control of the pathogen was responsible for the reduced disease severity. Highly susceptible to highly resistant sugarbeet lines were planted in the established nursery and in adjacent fields with no prior history as sugarbeet root rot disease nurseries. In each of 3 years, the disease was more severe at the new sites than in the established nursery. The weight of non-inoculated roots was similar at the two sites. Microbiological assays of soil from the sites revealed no quantitative differences in populations of microorganisms antagonistic to R. solani that were consistent with biological control of disease. Greenhouse experiments revealed no difference in seedling growth as a result of the site of soil origin, andautoclaving of soil did not produce the increase in disease severity that would be anticipated with biological control of the pathogen. Soil analyses of likely disease suppressing edaphic factors revealed no consistent differences between sites. Thus, artificial epiphytotics of crown and root rot caused by R. solani were more severe in new sites than in an established disease nursery, but the reasons for this difference remain obscure.