Prevalence of Corynebacterium flaccumfaciens as Incitant of Bacterial Tan Spot of Soybean in Iowa. J. M. Dunleavy, Research Plant Pathologist, USDA, ARS, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. J. W. Keck, Research Associate, K. S. Gobelman, Biological Laboratory Technician, USDA, ARS, and S. Reddy and M. M. Thompson, Research Assistants, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. Plant Dis. 67:1277-1279. Accepted for publication 20 June 1983. 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, 1983. DOI: 10.1094/PD-67-1277.

Bacterial tan spot, caused by Corynebacterium flaccumfaciens, occurred in 51.1% of 818 Iowa soybean fields examined in 1982. The mean number of plants infected per 30 m of row was 3.9. The disease was found in all 95 counties sampled. Greatest prevalence of tan spot occurred in a nine-county area in the southwestern section of the state, where 84.3% of the fields were affected by the disease. The lowest disease prevalence occurred in the nine-county east central section, where 32.1% of the fields were affected. There was no correlation between the percentage of fields affected by tan spot in a particular section of the state and the mean number of infected plants in affected fields. We tested diseased leaves from one field in each county sampled to determine if C. flaccumfaciens produced the symptoms observed. All isolates of the bacterium recovered from these leaves were identified as C. flaccumfaciens on the basis of laboratory tests, were pathogenic to soybean, and produced symptoms of tan spot in a greenhouse test.

Keyword(s): Glycine max.