Bacterial spot is one of the most devastating diseases of pepper and tomato. The disease occurs worldwide where pepper and tomato are grown in warm, moist areas. When it occurs soon after transplanting and weather conditions remain favorable for disease development, the results are usually total crop loss. Current chemical control is limited to copper or copper combined with maneb sprays that provide only marginal success thus making the disease very difficult to control once the epidemic is underway. When the disease occurs in commercial pepper fields early in the season, some farmers destroy the entire crop by disking because it is so difficult and economically costly to control once present in the field.

The study of this pathogen also has significantly increased fundamental understanding of host-pathogen interactions and the gene-for-gene model theory at the molecular level. Simply defined, this model states that for each resistance gene in the host there is a corresponding avirulence gene in the pathogen. An avirulence gene (also termed "effector gene") encodes a product that interacts directly, or more likely indirectly, with a product encoded by the corresponding resistance gene in the plant; this interaction triggers resistance. In plants lacking the corresponding resistance gene, effector gene products suppress innate host defenses. In pepper carrying one of these resistance genes, resistance is expressed as a hypersensitive response (Figure 14). A bacterial avirulence gene(s) can be lost or can mutate. This genetic change in the pathogen allows race changes that overcome resistance in the host, and, consequently, the host range of the pathogen is expanded. The known avirulence genes in the bacterial spot pathogens of pepper and tomato have been cloned and the DNA sequence determined for most of them. Their study has improved understanding of pathogen host range and genetic resistance. Recently the complete genome of a strain of the pathogen also was sequenced, leading to a better understanding of the mechanisms that allow a microbe to behave as a plant pathogen.

Figure 14

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by The American Phytopathological Society