Epiphytic Pseudomonas syringae on Dry Beans Treated with Copper-Based Bactericides

Bacterial brown spot (caused by Pseudomonas syringae pv. syringae van Hall) and halo blight (caused by P. syringae pv. phaseolicola (Burk.) Young, Dye, and Wilkie) are two important diseases of dry beans (Phaseolus vulgaris L.) in Colorado and other parts of the world (14). These bacterial pathogens are of particular interest for the following reasons. Though there is potential for using epiphytic population size to predict risk of disease (12,19,20), little is actually known about how P. syringae spreads as an epiphyte or the relationship between natural leaflet populations and populations on flowers, pods, and seeds. There is concern that P. syringae populations may develop increased resistance to copper-based pesticides. A range of tolerance levels has been found in P. syringae pv. syringae from cherry orchards (29) and nurseries in the Pacific Northwest (26), for example, and plasmid-encoded copper resistance is being found more widely in several genera of phytopathogenic bacteria (5). Laboratory experiments indicate that plasmids conferring copper resistance can be transferred between pathovars of P. syringae (1,2). Since both P. syringae pv. syringae and P. syringae pv. phaseolicola overwinter in seed and plant debris and infect volunteer beans the following year, resistant populations could survive from season to season and jeopardize future management efforts with bactericides and other integrated pest management approaches such as planting certified seed, varietal selection, and crop rotation, among others (18). The objectives of this research were to (i) estimate the effectiveness of different formulations of copper-based bactericides for reducing populations of epiphytic bacteria, (ii) examine the relationship between epiphytic populations on leaves and flowers, and (iii) determine levels of copper resistance present in P. syringae populations during exposure to copper-based bactericide regimes.