First report of Pantoea agglomeranscausing a leaf blight and bulb rot of onions in Georgia

Between June and September of 2013, sweet onions (Allium cepa L.) in six different counties in Michigan displayed symptoms similar to those caused by infection from Pantoea agglomerans or P. ananatis (Edens et al. 2006; Schwartz and Mohan 2008). Symptoms included leaf and stalk water-soaked or necrotic lesions. In scouted fields, disease incidence progressed throughout the season and reached an incidence of up to 80% or more in some fields. Symptomatic samples of six cultivars were collected from each county for disease identification. Isolations were made using the semiselective onion extract medium (OEM) (Zaid et al. 2012) and single colonies were transferred and maintained on nutrient broth yeast extract (NBY) medium. The colony morphology of the bacterium on OEM medium was pale yellow with a slight greenish pigment in the center, circular to irregular shape, raised elevation, smooth margin, and mucoid texture. On NBY, the colony color was yellow with slightly darker yellow in the center, circular to occasionally slightly irregular, moderately umbonate elevation, with slightly curled margins. Fourteen representative strains were biochemically analyzed through BIOLOG (Hayward, CA) and identified as P. agglomerans (98 to 99% probability). The 16s rDNA was amplified for the same 14 strains using previously described universal primers (De Baere et al. 2004) and sequenced. Sequences were compared with the NCBI database using BLASTn and confirmed as P. agglomerans (97 to 99% identity). To test pathogenicity, five representative strains were tested on onion bulbs in the laboratory and plants in the greenhouse. Fifty microliters of an aqueous bacterial suspension of 108 CFU ml−1 were injected into three 10-week-old plant replicates of onion leaves (n = 15) and 0.5 ml were injected into bulbs (n = 15) using a 12.7 mm-long hypodermic needle and 1-ml gauge syringe. Control bulbs and leaves were injected with sterile water. Inoculated bulbs were maintained at room temperature (23°C) under constant light, whereas plants were maintained in the greenhouse. After 2 weeks of incubation, onion bulbs injected with putative P. agglomerans strains showed rot symptoms at the injection sites, while the water controls had no symptoms. For onion leaves, severe blight occurred 1 week after inoculation. Foliar symptoms were similar to those previously observed in the field. The pathogen was recovered from symptomatic bulbs and leaves, and confirmed as P. agglomerans using morphological characteristics on OEM. The pathogen has been reported to infect onion in Cuba, Israel, and South Africa (Hattingh and Walters 1981). In the United States, it was first reported in 2006 as a causal agent of leaf blight and bulb rot of onions in Georgia (Edens et al. 2006). To the best our knowledge, this is the first official report of P. agglomerans infected onion, as part of the leaf necrotic and “bulb rot complex” in Michigan. The bacterium is one of several bacterial diseases causing major loses of onion industries in Michigan