THE EFFECT OF WIND SPEED ON DISPERSAL OF SPLASH-BORNE XANTHOMONAS CITRI subsp. CITRI AT DIFFERENT HEIGHTS AND DISTANCES DOWNWIND OF CANKER-INFECTED GRAPEFRUIT TREES

Xanthomonas citri subsp. citri (Xcc), which causes citrus canker, is a major pathogen of grapefruit and other canker-susceptible citrus species and cultivars grown in Florida and elsewhere. It is dispersed by rain splash, and wind promotes the dispersal of the pathogen. The aim of this study was to explore the relationships between wind speed and the number of bacteria dispersed downwind in rain splash at different heights and distances. Wind (up to 20 m sec-1) was generated using a fan, and rain simulated using sprayer nozzles. A negative exponential function described the relationship between height and the bacteria flux density (BFD, bacteria cm-2 min-1), with a rapid decline in mean BFD of Xcc collected at greater heights (P = <0.0001-0.3838, R2= 0.56 - 1.00). A power function described the relationship between distance and BFD of Xcc collected at most heights and wind speeds (P=0.049 - <0.0001, R2 = 0.78- 1.00). A multiple regression analysis based on wind speed, height and distance suggested predictability of the proportion of the total BFD of Xcc collected downwind of the simulated wind-rain system (F=49, P<0.0001, R2= 0.46). Crop management options to reduce sources of inoculum and the wind speed in orchards are discussed.