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Use of Detached Soybean Cotyledons for Testing Pathogenicity of Xanthomonas campestris pv. glycines. Ingyu Hwang, Department of Plant Pathology, University of Illinois at Urbana-Champaign, Urbana, IL 61801. S. M. Lim, and P. D. Shaw. Agricultural Research Service, U.S. Department of Agriculture, and Department of Plant Pathology; and Department of Plant Pathology, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Plant Dis. 76:182-183. Accepted for publication 10 September 1991. 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, 1992. DOI: 10.1094/PD-76-0182.

A rapid technique soybean (Glycine max) cotyledon assay was developed to test the pathogenicity of Xanthomonas campestris pv. glycines, which causes bacterial pustule. Detached soybean cotyledons from 10- to 14-day-old seedlings grown in the greenhouse were surface-sterilized with 0.5% sodium hypochlorite for 5 min and washed with sterile distilled water. The center of the cotyledons were wounded with multiple pins attached to the end of a wooden stick (six pins spaced evenly in an area 6 mm in diameter). Wounds were rubbed gently with cotton swabs dipped in a suspension of pathogenic and nonpathogenic strains of X. c. glycines, and inoculated cotyledons were placed in moist trays and kept in a lighted incubator at 30 C. Within 23 days after inoculation, areas of the cotyledons inoculated with pathogenic strains turned yellowish, whereas those inoculated with nonpathogenic strains did not show any discoloration. Pathogenic and nonpathogenic responses based on detached cotyledon assay were not distinguishable from those based on intact cotyledon assay. Lack of symptom induction on detached cotyledons was equivalent to lack of pathogenicity on leaves. Repeated experiments with this technique resulted in consistent detection of nonpathogenic mutants from 2,000 colonies of the bacterium treated with N-nitro-N-methyl-N-nitrosoguanidine.