RELATIONSHIP OF LEAF SURFACE-POPULATIONS OF STRAINS OF XANTHOMONAS-CAMPESTRIS PV CITRUMELO TO DEVELOPMENT OF CITRUS BACTERIAL SPOT AND PERSISTENCE OF DISEASE SYMPTOMS

Fluctuations of leaf surface populations of Xanthomonas campestris pv. citrumelo, incidence of citrus bacterial spot, and cumulative rainfall adjusted for evapotranspiration were found to be interrelated based on analysis with the Fourier transformation. In cross-correlation analyses, recoverable leaf surface bacteria were correlated best with disease incidence that occurred 25 days later (range for r = 0.86-0.87; 19-32 days). Cumulative rainfall was correlated best with disease incidence that occurred 25 days later (range r = 0.55-0.56; 27-42 days). However, no clear relationship existed between cumulative rainfall and leaf surface bacteria. In a second experiment, rows of Swingle citrumelo seedlings in eight citrus nurseries were inoculated at one end with aggressive, moderately aggressive, or weakly aggressive strains of X. c. citrumelo. Rainstorms with windblown rain were simulated by spraying water at high velocity (24-32 m/s) over the inoculated plants toward the receptor plants down the rows. Leaf surface populations of X. c. citrumelo on receptor plants were recovered immediately after the simulated rainstorms. The initial populations were correlated (r = 0.679-0.960) with disease incidence assessments on these plants 21 days after the simulated rainstorm. The slope of the bacterial deposition gradient was positively related to eventual disease development and negatively to strain aggressiveness (the steeper the gradient, the less the aggressiveness). In some of the nurseries, disease incidence decreased over time at approximately the same rate irrespective of bacterial strain aggressiveness. This decrease was due to the continued growth of plants, which added new healthy susceptible tissue; to the lack of disease development or spread; and to the disease-induced defoliation. In a third experiment, persistence of disease symptoms caused by each aggressiveness type was examined over time on Swingle citrumelo and Duncan grapefruit under simulated citrus grove conditions. Disease decrease was nearly linear. Rates (r) of disease decrease, from linearized-transformed data, were -0.0054, -0.0061, and -0.0067 for Swingle and -0.0067, -0.0055, -0.0018 for grapefruit, for aggressive, moderately aggressive, and weakly aggressive strains, respectively. Based on lack of persistence of disease symptoms (i.e., polycyclic disease development) in citrus groves, even on a susceptible host, and on the resistance of commercial scion cultivars, citrus bacterial spot in Florida should be considered a minor disease of citrus.