Effect of Temperature on Pathogenicity Components of Tomato Bacterial Spot and Competition between Xanthomonas perforans and X. gardneri

Bacterial spot is a major problem for the Brazilian processing tomato industry. The disease is caused by four different species of the genus Xanthomonas. The objectives of this study were to evaluate the effect of temperature on pathogenicity components of bacterial spot caused by each species separately and to assess the competitiveness between X. perforans and X. gardneri in planta. Representative strains of each species were inoculated separately on susceptible tomato plants at 20, 25 and 30 degrees C and the ensuing disease was measured by the percentage of diseased leaf area. Assessment of competitiveness was carried out under greenhouse conditions, by single and combined inoculations of X. perforans and X. gardneri. Colonies were recovered from the inoculations and the frequency of each species was determined. The interactions of temperature and bacterial species for pathogenicity components were significant. At 20 degrees C, X. gardneri induced the highest percentage of diseased leaf area, whereas at 30 degrees C, X. perforans was the most aggressive. Incubation period varied from 5 to 8 days according to the temperature. In the competitiveness assay, only X. perforans was recovered in half of the combined inoculations. Results indicate that temperature has an important role in determining which species will be prevalent in a particular region, especially concerning X. perforans and X. gardneri, since each of these species seemed to be more adaptable to one thermal zone.