Pathogenic variation in Xanthonomas campestris pv. phaseoli, the causal agent of common bacterial blight in Phaseolus beans

A total of 93 isolates of Xanthomonas campestris pv. phaseoli was collected from five countries over a wide range of altitude (500 2320 m above sea level) in eastern Africa. Collections were made from a range of cultivars of the common bean (Phaseolus vulgaris), including known sources of resistance, as well as from wild alternative but symptomless hosts including Senna (Cassia) hirsuta and Digitaria scalarum. From these, 30 isolates were selected for detailed characterization. Although variation was found in parameters including phage type, the production of brown pigment in culture media and in-vitro growth rate, there was little consistent relationship between these characters and pathogenic variation. Studies of pathogenic variation among the 30 isolates on 20 genotypes of P. vulgaris revealed quantitative host non-specific differences (aggressiveness). On tepary bean (P. acutifolius), however, the 30 isolates interacted differentially with the genotypes and eight distinct physiologic races of X. c. pv. phaseoli were defined, suggestive of an underlying gene-for-gene relationship. Despite this apparent gene-for-gene interaction, resistance to common bacterial blight in P. vulgaris, derived from earlier use of P. acutifolius, has apparently remained non-specific and essentially durable. It is suggested that, since levels of resistance already available within P. vulgaris provide adequate protection, bean breeders should resist the temptation to incorporate new complete resistance from P. acutifolius as it would risk destabilizing the host bacterium relationship by introducing race-specific resistance that is more likely to prove transient in agriculture