Relation of Temperature and Rainfall to Development of Xanthomonas and Pantoea Leaf Blights of Onion in Colorado.

During the 1996 to 1999 growing seasons, some areas of Colorado's onion-growing regions experienced a complex of bacterial diseases including leaf blights caused by Xanthomonas campestris and Pantoea ananatis. Crop losses varied depending on adverse weather (associated with rain, storm, and temperature patterns) and stage of onion plant development. Environmental conditions during vegetative development had no significant association with the initial appearance or subsequent intensity of disease. Both pathogens were active at average high temperatures that ranged from 28 to 35°C during bulbing. Multiple regression models were developed to predict the initial appearance (growing degree day [GDD]) and subsequent Xanthomonas leaf blight intensity (final proportion of disease [FPD]) using macroclimatic meteorological conditions, including July average daily high temperature (T), August cumulative rainfall (P), and cumulative rainfall in July and August (P). Initial disease appearance and disease intensity were described by GDD = -6,153.43 + 215.50T - 0.92P and FPD = 222.79 - 6.92T + 0.52P, respectively. Pantoea leaf blight initial appearance was strongly associated with July average daily temperatures (T) and was described by GDD = -5,930.43 + 289.07T. Results are discussed in relation to an integrated pest management strategy in Colorado.