Susceptibility of Rotation Crops to a Root Rot Isolate ofRhizoctonia solanifrom Sugar Beet and Survival of the Pathogen in Crop Residues

Abstract

temperatures preclude Rhizoctonia Ruppel, E. G. 1985. Susceptibility of rotation crops to a root rot isolate of Rhizoctonia solani from activity, and become resistant with sugar beet and survival of the pathogen in crop residues. Plant Disease 69:871-873. maturity; or 3) the saprophytic persistence of the fungus within crop residues Mean seedling survival in pasteurized soil infested with a sugar beet root rot isolate of Rhizoctonia depends on the crop species involved. solani (anastomosis group 2 [AG-2]) ranged from 1.3 to 4.7% for highly susceptible barley, bean, This study was conducted to examine 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 ore closely the role of rotation crops in fields, had 75% survival. Alfalfa was a nonhost in this study. Lesions from surviving hosts yielded he eemo R. solani AG-2 isolates that caused rot of 2-mo-old sugar beet roots. Survival of barley, bean, corn, in sugar beet. 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 MATERIALS AND METHODS isolates from lesions of surviving barley, bean, corn, and sorghum across all ages, only 16, 41, 53, Preliminary tests indicated that 10 and 51%, respectively, proved to be R. solani; all but one R. solani isolate were in AG-2, and all Colorado sugar beet root isolates of R. AG-2 types rotted 2-mo-old sugar beet roots. Ground residues of infected barley, bean, and solani (all anastomosis group 2 [AG-2]) sorghum in soil at 20 C yielded R. solani AG-2 after 8 but not 12 wk of incubation; residues of corn induced similar rates of damping-off of yielded the pathogen up to 6 wk. The soil-residue mixtures of bean, corn, and sorghum still were several plant species. Thus, isolate R-9 conducive for sugar beet damping-off after 12 wk, even though the pathogen could not be recovered several plan se Ths solate by soil-dilution techniques. No sugar beet damping-off occurred in the barley residue-soil mix after which has been used for several years to 12 wk. Results indicate that more than pathogen susceptibility must be considered in selecting initiate epidemics of root rot in breeding cropping sequences to control Rhizoctonia root rot in sugar beet and that persistence of the nurseries at Fort Collins, CO, was used in pathogen in crop residues may be dependent on the crop species. further studies. Dry, ground, barleygrain inoculum (about 82 propagules of R. solani per gram) was prepared as Crop rotation has been recommended in sugar beet monoculture or 2-yr described by Pierson and Gaskill (12). as a control of sugar beet (Beta vulgaris rotations. Isolate R-9 was tested for its ability to L.) root rot caused by Rhizoctonia solani Ideally, nonhost crop species usually cause seedling damping-off in alfalfa, Kiihn (Thanatephorus cucumeris (Frank) should precede the crop for which barley, bean, corn, muskmelon, pigweed Donk) (10,13). Maxson (10) stated that protection is desired. Maxson (9) (Amaranthus retroflexus L.), red beet, any small-grain crop reduced Rhizoctonia considered sugar beet, potato, alfalfa, sorghum, soybean, sugar beet, and root rot in sugar beet in 3- to 4-yr sweet clover, and bean as crops, wheat. Pasteurized greenhouse soil (three rotations; however, he recommended an whereas he considered small grains and parts soil, one part each of sand and peat alfalfa-potato-sugar beet sequence in corn as non-host He reported moss) was placed in plastic pots (7.5 X 7.5 potato-growing areas. In all the crop 3-50% more root rot in sugar beet cm) to within 1.5 cm of the rim. Twenty sequences tested by Schuster and Harris following host than following nonhost seeds of each test species were evenly (13), either potatoes or beans preceded crops. Greenhouse and field inoculations distributed over the soil surface along sugar beet. Disease reduction was by Schuster and Harris (13) with seven with about 0.8 cm 3 of ground inoculum attributed more to length of the beetfree isolates of R. solani indicated that corn (about 26 propagules per pot). Seeds and period than to the crop species that and bean were nonhosts of the sugar beet inoculum then were covered with an preceded sugar beet. Root rot incidence pathogen, but some isolates were additional 0.5-cm layer of pasteurized was low in 4- or 6-yr rotations and highest pathogenic to potato. In 4- or 6-yr soil. Pots were irrigated immediately and rotations, however, it made little thereafter as needed. Controls consisted Cooeraiv inestgaion o th Agicltual difference in disease incidence whether of seed covered with autoclaved grain