Susceptibility of Rotation Crops to a Root Rot Isolate of Rhizoctonia solani from Sugar Beet and Survival of the Pathogen in Crop Residues. E. G. Ruppel, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Crops Research Laboratory, Colorado State University, Fort Collins 80523. Plant Dis. 69:871-873. Accepted for publication 22 February 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/PD-69-871.

Mean seedling survival in pasteurized soil infested with a sugar beet root rot isolate of Rhizoctonia solani (anastomosis group 2 [AG-2]) ranged from 1.3 to 4.7% for highly susceptible barley, bean, corn, red beet, and soybean plants and from 20.8 to 56.9% for moderately susceptible muskmelon, sorghum, sugar beet, and wheat plants. Pigweed (Amaranthus retroflexus), a common weed in beet fields, had 75% survival. Alfalfa was a nonhost in this study. Lesions from surviving hosts yielded R. solani AG-2 isolates that caused rot of 2-mo-old sugar beet roots. Survival of barley, bean, corn, and sorghum plants 2, 4, and 8 wk old at inoculation ranged from 67.5 to 100% with low levels of inoculum; bean and corn showed a trend toward increased survival with increased age. Of 91 fungal isolates from lesions of surviving barley, bean, corn, and sorghum across all ages, only 16, 41, 53, and 51%, respectively, proved to be R. solani; all but one R. solani isolate were in AG-2, and all AG-2 types rotted 2-mo-old sugar beet roots. Ground residues of infected barley, bean, and sorghum in soil at 20 C yielded R. solani AG-2 after 8 but not 12 wk of incubation; residues of corn yielded the pathogen up to 6 wk. The soil-residue mixtures of bean, corn, and sorghum still were conducive for sugar beet damping-off after 12 wk, even though the pathogen could not be recovered by soil-dilution techniques. No sugar beet damping-off occurred in the barley residue-soil mix after 12 wk. Results indicate that more than pathogen susceptibility must be considered in selecting cropping sequences to control Rhizoctonia root rot in sugar beet and that persistence of the pathogen in crop residues may be dependent on the crop species.