Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 28, 1997
Publication Date: N/A
Interpretive Summary: An increase in citrus bacterial spot was noted in some citrus nurseries. Upon investigation, this increase seemed to be related to the use, by some citrus nurseries, of commercial spray adjuvants that had been added to various spray tank mixes of agrochemicals. To examine this problem, field trials were set up to examine the effect of various spray adjuvants on bacterial disease epidemics in simulated citrus nurseries. It was determined that one class of chemical adjuvants known as surfactants increases the number, size, and incidence of bacterial lesions on susceptible citrus nursery plants, whereas other spray adjuvants had little effect. To our knowledge this is the first report of a spray adjuvant causing an increase in disease.
Technical Abstract: The effect of commercial chemical adjuvants on the spread of Xanthomonas axonopodis pv. citrumelo in nursery plots of citrus rootstock trees by simulated wind-blown rain was studied. Individual rows were treated with a water control, or commercial adjuvants including a penetrant-surfactant, an antitranspirant, a surfactant, or one of three formulations of a spreader-binder. Bacterial dispersal gradients in all rows were similar and extended the full 7m of the nursery rows. The penetrant-surfactant and its surfactant component significantly increased the total number of lesions per plant and mean lesion diameter compared to the water control. The disease gradient slopes associated with the penetrant-surfactant, and its surfactant component, were significantly flatter and more extensive than the water control, whereas its penetrant component, the antitranspirant, and two spreader-binder adjuvants tested were not generally significantly different from the water control. Lesion size and number were also increased by a surfactant product and the surfactant component of the penetrant-surfactant, but generally not by the penetrant component of the penetrant-surfactant, the antitranspirant or the three spreader-binder formulations. These results suggest that surfactants which cause stomatal flooding may enhance infection and exacerbate citrus bacterial epidemics.