The effects of soil temperature and moisture and certain biological factors on the pathogenicity of Rhizoctonia solani Kuhn to sugar beet seedlings

Abstract

The effect of temperature and moisture and of certain biological factors on the pathogenicity of several isolates of Rhizoctonia solani to sugar beet seedlings tbs studied. Pathogenicity of six isolates of Rhizoctonia solani obtained from seedling and matured sugar beets was established on sugar beet seedlings in the greenhouse. Four of the six isolates were selected for further study on the basis of their virulence and total per cent disease produced on the beets. Two of the isolates were obtained from beet seedlings, the other two were obtained from matured beets. The minimum temperature for growth of these isolates in culture was, found to be near 10° C. The optimum growth for these cultures was at 29° C, and the maximum was near 40° C. Rhizoctonia disease was produced at both temperatures used (15° and 25° C) and at all moisture levels (55, 70, and 85 per cent of the total moisture holding capacity). More disease occurred at 25° C than at 15° C. Moisture level of the soil appeared to have less effect on the incidence of the disease than did temperature. The total per cent disease was greater with isolates obtained from seedling beets, also the average weight per plant and the average length of tops were smaller with these isolates than with isolates from matured beets. A test of the effect of inoculum of varying ages on beet seedlings In different stages of growth revealed that beets are subject to attack by Rhizoctonia at any stage of growth and that the age of the beet is more important than the age of the inoculum in total incidence of disease. Antagonistic action was exhibited by Trichoderma lignorum toward Rhizoctonia in soil and culture tests. Penicillium notatum did not appear to have any effect on Rhizoctonia in soil or culture tests. Streptomyces griseus appeared to increase the pathogenicity of Rhizoctonia in soil tests, but no conclusive results could be obtained in culture tests.