Spatial and temporal analyses of bacterial blight of onion caused by Xanthomonas axonopodis pv. allii

Bacterial blight of onion is a severe disease, which emerged over the past decade in several onion-producing areas. This disease currently is observed in both the Old and New Worlds. Although the causative agent, Xanthomonas axonopodis pv. allii, is potentially seedborne, seed trans-mission and significance of seedbome initial inoculum for the development of bacterial blight of onion previously has not been assessed. This article describes experimental work designed to evaluate the biological importance of seedborne X. axonopodis pv. alui as an initial inoculum source, and examine the temporal and spatial dynamics of the disease. Over 3 years, outbreaks of bacterial blight of onion always were induced in experimental plots sown with naturally contaminated seed lots, with a contamination rate determined as 0.04%. Analyses of disease patterns indicated a likely seedbome origin for the inoculum associated with the early stages of epidemics. Spatial analyses performed with several statistical methods indicated aggregated patterns of disease incidence data. Primary foci enlarged over time, and a few distinct secondary foci some-times were established after occurrence of wind-driven rains (with gusts up to 15 m s-1). Distances between primary and secondary foci ranged from less than 1 m (satellite foci) to 25 m. It remains possible that long-distance dispersal of inoculum was at least partly involved in the later stages of epidemics. (Résumé d'auteur