A New Pathotype of Xanthomonas campestris pv. armoraciae That Causes Bacterial Leaf Spot of Radish

A previously undescribed pathotype of Xanthomonas campestris pv. armoraciae was found in 1995 on radish plants grown on organic soil in north central Ohio. Radish foliage developed numerous small, circular, water-soaked black spots, eventually with yellow halos, on the underside of the leaves, giving the foliage a yellowish, ragged appearance. Spots were also visible on the upper surface of leaves and on petioles. Yellow xanthomonad-like bacteria were consistently isolated from the lesions and confirmed as the causal agent of the disease by fulfilling Koch's postulates. All five strains purified were gram negative, rod shaped, motile, aerobic, oxidase negative, catalase positive, amylolytic, and pectolytic. They were identified as X. campestris pv. armoraciae by fatty acid methyl ester (FAME) analysis (similarity indices [SI] range = 0.21 to 0.39), the Biolog 95 GN reaction (SI range = 0.31 to 0.36), and serological reactions with X. campestris pv. campestris/X. campestris pv. armoraciae-specific monoclonal antibodies X9, X11, X21, A11, and B35 (1). All strains caused bacterial leaf spot on collard, kale, radish, horseradish, and cabbage but not on tomato or pepper. These strains were different from X. campestris pv. raphani, which is pathogenic on kale, radish, cabbage, tomato, and pepper, but not on horseradish. These strains also differed from other previously reported strains of X. campestris pv. armoraciae that do not cause infection on kale and radish (1,2). This is the first report on the existence of a different pathogenic group within X. campestris pv. armoraciae that can cause bacterial spot on kale and radish. References: (1) A. M. Alvarez et al. Phytopathology 84:1449, 1994; (2) H. E. White. Phytopathology 20:653, 1930.