Sweet Spanish onion (Allium cepa L.) cultivars grown in southern Colorado displayed symptoms of foliar blight and bulb rotting after bulb initiation in early July of 1997, 1998, and 1999. This disease appears identical to that reported from infected onions in Georgia in 1997 (1). Leaf blighting began as whitish to tan lesions, which rapidly coalesced, causing a general wilt, discoloration, and death of affected foliage. A yellow-cream to light orange discoloration progressed into bulbs, resulting in the rotting of neck tissue and between scales. Infection of more than 70% of onion plants exposed to heavy rain and storm damage after bulb initiation occurred in scattered fields in Otero County. Gram negative, rod-shaped, yellow-colored bacteria were consistently recovered from infected foliar and bulb tissues on nutrient agar during this 3-year period. Physiological tests showed that the bacteria utilized glucose in an oxidative and fermentative manner and were catalase positive and oxidase negative. Two strains recovered during 1997 were identified by Microbe Inotech Laboratories (St. Louis, MO) as Pantoea ananas by gas-chromatography fatty acid methyl ester analysis, with similarity indices of 0.70 and 0.79. A literature search determined that the accepted classification is now Pantoea ananatis Serrano (2). To confirm pathogenicity, a 0.5- to 1.0-ml suspension of bacteria (108 CFU/ml sdw) of one of the strains was injected into firm onion bulbs (7.5 to 10.0 cm diameter). After incubation for 14 days at 22°C in enclosed plastic bags in the dark, bulbs were cut in half and scored for visual evidence of yellow to tan discoloration and initial dry rotting prior to reisolation of the pathogen from five of eight inoculated bulbs. No discoloration or disease developed on eight control bulbs injected with water. To our knowledge, this is the first report of P. ananatis from onion grown in Colorado and the western United States.
References: (1) R. D. Gitaitis and J. D. Gay. Plant Dis. 81:1096, 1997. (2) H. G. Truper and L. de Clari. Int. J. System. Bacteriol. 47:908, 1997.