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Spread of Bacterial Tan Spot of Soybean in the Field. J. M. Dunleavy, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, 317 Bessey Hall, Iowa State University, Ames 50011. Plant Dis. 69:1036-1039. Accepted for publication 4 June 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-69-1036.

The spatial distribution of bacterial tan spot of soybean caused by Corynebacterium flaccumfaciens was observed over time in four field plots in which two soybean plants were inoculated near the center of each plot. There was no evidence of spread by 2 wk after inoculation. By 3 wk after inoculation, secondary spread had occurred to plants neighboring the source, and by 4 wk after inoculation, disease was detected on plants in adjacent rows. Considerable disease was present at that time, and bacterial tan spot was more evident within rows than across rows. Mean size of the four areas of diseased plants 5 wk after inoculation was 19 5m. There was no significant difference between mean disease levels within rows and across rows by 2 and 3 wk after inoculation, but there was a difference (significant at P<0.05) by 4 and 5 wk after inoculation. Bacterial tan spot was not observed in any of the four control plots.

Keyword(s): dissemination.