PREDISPOSITION OF CITRUS FOLIAGE TO INFECTION WITH XANTHOMONAS CITRI subsp CITRI

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc), is a serious disease of susceptible citrus in Florida and other citrus-growing areas of the world. The effect of leaf preconditioning as a route for entry of the bacteria is poorly characterized. A series of experiments were designed to investigate the effects of wind and rain (to simulate a storm), high humidity (>90%) and mild abrasion with sand (to simulate wind-blown sand and debris) in predisposing citrus foliage to infection with Xcc. Exposure of leaves of Swingle citrumelo seedlings to wind and rain [16 m sec(-1) (58 km h(-1), 36 mph) and 235 mm h(-1) (9.25 ins h(-1)), respectively] for 15 or 30 min caused significant injury and disease incidence was twice as high and severity was ten times greater than on leaves on seedlings of unexposed control plants. The points of attachment of the lamina to the petioles were particularly susceptible to wind-induced injury with up to 25% showing symptoms compared to 0% for the unexposed control. Over 80% of injured leaves had lesions associated with the site of injury. There was little or no effect of humidity >90% for 1.5 or 2.5 h on disease incidence or severity compared to the unexposed control. Mild leaf abrasion of grapefruit seedlings with sand increased incidence and severity of disease two-fold. Ways to reduce leaf injury by minimizing wind speed in orchards may contribute to reducing canker incidence and severity.