Durability of resistance in tomato and pepper to xanthomonads causing bacterial spot.

Both hypersensitive and quantitative forms of resistance to the bacterial spot pathogens (Xanthomonas spp.) occur in pepper and tomato. Five resistance genes involved in hypersensitivity in pepper and four in tomato have been identified so far. The corresponding pathogen avirulence genes have been cloned and characterized, and features, including a propensity for accumulating mutations and at times, loss of plasmid-borne avirulence genes, are known to occur. The frequency of these changes affects race composition among pathogen populations and determines the durability of the corresponding plant resistance. At least four different species of Xanthomonas are known to cause bacterial spot, and these can differ in specific avirulence gene content. Quantitative or multigenic resistance has also more recently been researched and appears to be more durable than the hypersensitive resistance. Two recessive genes have been identified that yield a nonhypersensitive form of resistance in pepper and together can provide strong resistance. More emphasis is being given to transfer of quantitative trait resistance to commercial cultivars of both tomato and pepper.