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Characterization of the Bacterium Inciting Chocolate Spot of Corn. Raul de L. D. Ribeiro, Graduate Student, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706; R. D. Durbin(2), D. C. Arny(3), and T. F. Uchytil(3). (2)Research Leader, Plant Disease Resistance Research Unit, ARS, USDA, and Professor, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706; (3)Professor, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706; (4)Microbiologist, Plant Disease Resistance Research Unit, ARS, USDA, Department of Plant Pathology, University of Wisconsin, Madison, WI 53706. Phytopathology 67:1427-1431. Accepted for publication 10 June 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1427.

The bacterium that incites chocolate spot, a leaf blight disease of corn, is a fluorescent pseudomonad in group Ia of Lelliott et al. On the basis of its bacteriological properties and toxin production, this bacterium was shown to differ substantially from strains of Pseudomonas syringae that cause holcus spot of corn, and to conform with P. coronafaciens and with the causal organism of halo blight of timothy. Halo blight symptoms were induced on oats and timothy by inoculation with the corn chocolate spot strains. These strains synthesize three cholorosis-inducing toxins in culture, two of which are structurally identical with the tabtoxins reported for P. coronafaciens and the timothy halo blight bacterium. It is proposed that the casual organism of corn chocolate spot in Wisconsin be designated as P. coronafaciens pathovar zeae to distinguish it from other strains of this nomenspecies which are unable to infect corn and timothy.

Additional keywords: oat halo blight, toxin, timothy.