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Influence of Previous Crops on Rhizoctonia Root and Crown Rot of Sugar Beet. C. M. Rush, Texas A&M University, Texas Agricultural Experiment Station, P.O. Drawer 10, Bushland, TX 79012. S. R. Winter, Texas A&M University, Texas Agricultural Experiment Station, P.O. Drawer 10, Bushland, TX 79012. Plant Dis. 74:421-425. Accepted for publication 6 December 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0421.

A field study was conducted to determine the effects of previous crops on Rhizoctonia root and crown rot development in the subsequent sugar beet crop. Alfalfa, cotton, sorghum, sunflower, or wheat, grown in monoculture for 2–3 yr, or fallow ground, preceded sugar beets grown in 1987 and 1988. Disease incidence in the sugar beet crop was monitored by bimonthly counts of dead plants in two 7.6-m lengths of row in each plot. At the end of the season in 1987, sugar beets following alfalfa had the highest incidence of disease, losing 47% of the stand to root rot. Sugar beets on sorghum and winter wheat ground followed with 41 and 38% stand losses, respectively. Sugar beets preceded by cotton, fallow, and sunflower all had significantly less disease, with 32, 22, and 21% losses, respectively. In 1988, results were similar. By season’s end, sugar beets preceded by wheat, sorghum, or alfalfa had 84, 81, or 48% stand losses, respectively. Cotton, fallow, and sunflower were again best for preceding sugar beets, with 30, 22, and 19% stand losses, respectively. Root yield was negatively correlated (P = 0.05) with percent disease, r = –0.96 in 1987 and r = –0.97 in 1988. In both years, sugar beets grown on previously fallow ground had significantly greater root yields than all other treatments except sunflower. Root yields of sugar beets following winter wheat and sorghum were low. However, in both years percent sucrose was highest in sugar beets following wheat. No significant differences were found when sugar beets followed the other crops either year. Previous crops also affected residual soil NO3-N. In general, residual soil NO3-N was lower in alfalfa, sorghum, and winter wheat plots than in cotton, fallow, or sunflower plots, but differences were not always significant. Although previous crops affected yield and root disease development in the subsequent sugar beet crop, many interacting variables, such as disease × yield, NO3-N × yield, and NO3-N × disease complicated interpretation of results.