Effect of Nitrogen Fertilization and Seed Contamination on Epiphytic Populations of Xanthomonas axonopodis pv. allii and Development of Xanthomonas Leaf Blight of Onion

Abstract

Xanthomonas leaf blight of onion, caused by Xanthomonas axonopodis pv. allii, is a yield-limiting disease in Colorado, for which few effective management strategies are available. The effects of rates of nitrogen fertilization and levels of seed contamination by X. axonopodis pv. allii on epiphytic populations of the pathogen and subsequent disease development were determined in field studies over two years. Epiphytic populations of a rifampicin-resistant mutant were quantified from bulked, asymptomatic leaf tissue using a leaf wash assay. There was no significant relationship between nitrogen fertilization treatment and epiphytic population development, but excessive nitrogen (> 200 kg/ha) fertilization increased disease severity 27 to 50% compared with non-fertilized and low (112 kg/ha) nitrogen fertilizer treatments in 2004. Seed inoculated with various levels of X. axonopodis pv. allii did not result in persistent epiphytic populations of the bacterium or development of symptoms, even at 10⁷ CFU/g of seed. The results of these studies suggest moderate rates of nitrogen fertilization may reduce severity of Xanthomonas leaf blight compared to higher rates, and that seed contamination by X. axonopodis pv. allii may be of limited importance for disease development under the semi-arid conditions of furrow-irrigated onion production in Colorado. Accepted for publication 13 March 2005. Published 31 March 2005.