Association of an Unusual Strain ofXanthomonas campestriswith Apple

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Abstract
A yellow, Gram-negative bacterium was isolated form damaged apple explants in tissue culture (TC). Explants in TC frequently became necrotic, the necrosis involving leaf tips or whole aerial portions of the plants. The bacterium was consistently isolated from affected explants and from greenhouse-grown and nursery plants. The strains were recovered from inoculated apple explants in TC. Symptoms and bacterial strain characteristics agreed with those originally observed. Based on growth on diagnostic media, conventional biochemical and physiological tests, and fatty acid analyses the strains represent an unusual form of X. campestris. Greenhouse inoculation tests were negative for pathogenicity to tomato, tobacco, cassava, peach and grape. Apple explants grown in the greenhouse were leaf-inoculated and root-inoculated. Symptoms did not develop on apple; however, the xanthomonad was recovered from leaves 21 and 30 days after leaf inoculation, and from stems 60 days following root inoculation. Similar strains were also isolated from apparently healthy bud material from other uninoculated greenhouse-grown apple plants and bud material sent from a commercial nursery. The strains of Xanthomonas from apple are apparently pathogenic only in TC; however, they are capable of limited colonization of apple tissue under greenhouse conditions without causing apparent symptoms. These strains probably represent an unusual form of X. campestris. These strains differ from other pathovars of X. campestris in their either lack of virulence or low virulence, and ability to colonize apple tissue without causing apparent symptoms.