Sweet Spanish onion cultivars (Allium cepa L.) in northern Colorado displayed symptoms of postharvest bulb rot during September to November of 1999. This disease appears identical to that reported from infected onions in California in 1988 and is presumably associated with high temperature stress (1). Mature, firm bulbs harvested from scattered fields in Weld County exhibited a brownish discoloration and breakdown of inner scales. Gram negative, rod-shaped, cream-colored bacteria were consistently recovered from infected bulb tissue on nutrient agar. Physiological tests showed that the bacteria utilized glucose in an oxidative and fermentative manner and were catalase positive and oxidase negative. A representative strain was identified by Microbe Inotech Laboratories (St. Louis, MO) as Enterobacter cloacae (Jordan) Hormaeche & Edwards (2) using Biolog analysis, with a similarity index of 0.81. To confirm pathogenicity, a 0.5- to 1.0-ml suspension of bacteria (108 CFU/ml sdw) was injected into firm onion bulbs (7.5 to 10.0 cm diameter). After incubation for 14 days at 22°C in closed plastic bags in the dark, bulbs were cut in half and evaluated. Tan to brown discoloration and initial dry rot, similar to that observed postharvest, was observed in inoculated bulbs. The pathogen was reisolated from six of eight bulbs inoculated with the representative strain. No discoloration or disease developed on eight control bulbs injected with water. To our knowledge, this is the first report of E. cloacae from onion grown in Colorado.
References: (1) A. L. Bishop and R. M. Davis. Plant Dis. 74:692, 1990. (2) Hormaeche and Edwards. Int. J. System. Bacteriol. 30:293, 1960.