An outbreak of fire blight in plums.
An extensive outbreak of shoot blight was observed in a 3-year-old plum orchard in Malheur County, Oregon, USA, during the summer of 1993. All 4 naturally infected cultivars were of Japanese plum, Prunus salicina. All 1200 trees of cv. Fortune and 70 trees of cv. Friar in a 2.4 ha block were infected. The same symptoms were present on 3 trees each of cv. Black Amber and cv. Simka growing in an adjacent block of apple (cv. Jonathan) trees with severe fire blight. The principal symptoms were blighted, young, succulent terminal shoots with brown to black necrotic lesions on the stems and petioles, wilting and shepherds crooks on affected stems, leaf blighting and stem cankers on 1-year-old shoots. Isolations on general (Kings B and sucrose nutrient agar) and selective (Miller-Schroth sorbitol and CCT) media yielded pure cultures of a bacterium. Based on colony characteristics, Gram reaction, pathogenicity to immature pear (cv. Bartlett) fruit, hypersensitive reaction in tobacco leaf, results of Biolog Bacterial Identification System, cellular fatty acid profiles, polymerase chain reaction using specific primers and pathogenicity to apple (cv. Jonathan) and pear (cv. Bartlett) shoots, the bacterium was identified as Erwinia amylovora. Inoculation of plum (cvs. Fortune, Friar and Elephant Heart) shoots with isolates from plum as well as with a known isolate of E. amylovora from apple, caused the same symptoms as observed in original infections. This is the first report of a natural and extensive outbreak of fire blight in Japanese plums.