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Bacterial Blight (Xanthomonas campestris pv. cyamopsidis) of Guar in Arizona. J. D. Mihail, Department of Plant Pathology, University of Arizona, Tucson 85721. S. M. Alcorn, Department of Plant Pathology, University of Arizona, Tucson 85721. Plant Dis. 69:811. Accepted for publication 25 May 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-811c.

Bacterial blight caused by Xanthomonas campestris pv. cyamopsidis Patel, Dhande & Kulkarni was observed for the first time in Arizona on field-grown guar (Cyamopsis tetragonoloba L.) in experimental plantings in Yuma and Pima counties in September 1981. Although guar was widely planted in 1982, the disease was not observed again until November 1983, after several weeks of unusually heavy, late rains. Field symptoms included small black leaf spots, black streaks on petioles and stems, splitting of stems, and defoliation, corresponding to symptoms observed in Georgia, Maryland, Oklahoma, and Texas. Koch’s postulates were completed with 12 strains of the bacterium in two greenhouse tests. The bacterium, a gram-negative rod with a single, polar flagellum, was able to utilize gelatin, starch, and glucose but gave a negative reaction to indole and nitrate. The bacterium was recovered from 5% of 200 healthy appearing guar seeds collected from naturally infected plants in Yuma County. Bacterial blight is a potential constraint to guar production in Arizona during years of prolonged, late summer rains.