First and second authors: Instituto Tecnológico Agrario de Castilla y León, Consejería de Agricultura y Ganadería, Junta de Castilla y León, 47071 Valladolid, Spain; and third author: Department of Plant Pathology, University of California, Davis
Common bacterial blight (CBB) disease of the common bean (Phaseolus vulgaris) is caused by Xanthomonas campestris pv. phaseoli and the brown-pigmented variant X. campestris pv. phaseoli var. fuscans. CBB first was described in Castilla y León County, Spain, in 1940, and is now a major constraint on common bean production. In this secondary center of diversity of the common bean, large-seeded Andean cultivars predominate, although medium-seeded Middle American cultivars also are grown. Xanthomonad-like bacteria associated with CBB in Castilla y León were characterized on the basis of carbohydrate metabolism, brown pigment production, genetic analyses (repetitive-element polymerase chain reaction [rep-PCR] and random amplified polymorphic DNA [RAPD]) and pathogenicity on cultivars representing the two common bean gene pools (Andean and Middle American). X. campestris pv. phaseoli was more prevalent (80%) than X. campestris pv. phaseoli var. fuscans (20%). Patterns of carbohydrate metabolism of Spanish CBB bacteria were similar to those of known strains; and only X. campestris pv. phaseoli var. fuscans strains utilized mannitol as a sole carbon source. rep-PCR and RAPD analyses revealed relatively little genetic diversity among Spanish X. campestris pv. phaseoli strains, and these strains were placed together with New World strains into a large cluster. Similar to other New World strains, representative Spanish X. campestris pv. phaseoli strains were highly pathogenic on bean cultivars of both gene pools, showing no gene pool specialization such as that found in certain East African strains. Genetic analyses and pathogenicity tests confirmed and extended previous results, indicating that these East African strains represent distinct xanthomonads that independently evolved to be pathogenic on common bean. X. campestris pv. phaseoli var. fuscans strains were more closely related and genetically distinct from X. campestris pv. phaseoli strains. However, two distinct clusters of X. campestris pv. phaseoli var. fuscans strains were identified, one having the most New World strains and the other having the most African strains. Spanish strains were placed in both clusters, but all strains tested were highly pathogenic on bean cultivars of both gene pools. Together, our results are consistent with multiple introductions of CBB bacteria into Spain. These findings are discussed in terms of breeding for CBB resistance and the overall understanding of the genetic diversity and evolution of CBB bacteria.