BACTERIAL-BLIGHT OF SWEET ONION CAUSED BY PSEUDOMONAS-VIRIDIFLAVA IN VIDALIA, GEORGIA

A bacterial blight of sweet spanish onions (Allium cepa) was observed for the first time in the winter and spring of 1990 and again in 1991 in the Vidalia region of Georgia. A fluorescent bacterium, the colonies of which initially were white but turned yellow with age on King's medium B agar, was isolated consistently from necrotic streaks and oval, water-soaked lesions on upper portions of leaves, rotted areas at the base of leaves, and from one or more discolored inner scales of bulbs. Many strains of the onion pathogen were copper-tolerant. The bacterium was negative for oxidase and arginine dihydrolase, was variable for hypersensitive reaction in tobacco, slowly produced acid from sucrose (7-20 days), rotted carrot and potato, incited rust-colored symptoms on snap bean pods, and was ice nucleation active. It degraded sodium polypectate gel at pH 8.5 but not at pH 5.0. The onion pathogen had a relatively simple fatty acid profile. The key features were the presence of delta-cis-9,10,-methylene hexadecanoic acid and that the ratio of alpha-hydroxy lauric acid to lauric acid was greater than 1. Based on these characteristics, the onion pathogen was identified as Pseudomonas viridiflava. Koch's postulates were fulfilled using greenhouse-grown onion plants. Onion strains of the pathogen were also pathogenic on greenhouse-grown snap bean, soybean, and tomato.