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Root Rot of Sugar Beet in Wyoming Caused by Rhizopus arrhizus. P. C. Vincelli, Department of Plant, Soil & Insect Sciences, University of Wyoming, Laramie 82071. J. C. Burne, Department of Plant, Soil & Insect Sciences, University of Wyoming, Laramie 82071. Plant Dis. 73:518. Accepted for publication 14 March 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0518C.

Severe root rot was observed on sugar beets (Beta vulgaris L. 'Western 0-2') in a field near Worland, Wyoming, in August 1988. Over 50% of the plants were affected in an area of the field approximately 2,500 m2 Isolations from diseased taproots consistently yielded Rhizopus arrhizus Fischer. Healthy sugar beets that were wounded, inoculated with a spore suspension (7 X 106 per milliliter), and incubated at 35 C developed decay within 48 hr. Beets that were wounded but not inoculated did not show symptoms. Reisolation of R. arrhizus completed Koch's postulates. A specimen was deposited in the Cornell University Plant Pathology Herbarium (CUP 61913). The summer of 1988 was much warmer than normal in the Worland area; the daily maximum temperatures for June, July, and August of 33.8, 35.1, and 31.7 C, respectively, were 7.1, 3.2, and 1.4 C above normal. The disease was most severe in an area of the field with poor surface drainage. Larvae of Eumerus sp. (Diptera:Syrphidae) were found within diseased tissue. These larvae are capable of injuring healthy plant tissue and may have provided an infection court for R. arrhizus. These observations are consistent with previous observations on the importance of high temperature, excessive soil moisture, and insect injury in the epidemiology of root rot of sugar beet caused by R. arrhizus (1,2). This is the first report of Rhizopus root rot of sugar beet in Wyoming.

References: (1) A. A. Hildebrand and L. W. Koch. Can. J. Res. 21(c):235, 1943. (2) M. E. Stanghellini and W. C. Kronland. Plant Dis. Rep. 61:255, 1977.