Applications of Germicidal Ultraviolet Light as a Tool for Fire Blight Management () in Apple Plantings.

Nighttime applications of germicidal UV light (UV-C) have been used to suppress several fungal diseases of plants, but less is known of UV-C's potential to suppress bacterial plant pathogens. Fire blight of apple and pear, caused by the bacterium , is difficult to suppress using cultural practices, antibiotics, and host resistance. We therefore investigated the potential of UV-C as an additional means to manage the disease. Laboratory assays confirmed that in vitro exposure of cultures to UV-C at doses ranging from 0 to 400 J/m in the absence of visible light was more than 200% as effective as cultures exposed to visible light after the same UV-C treatments. In a 2-year orchard study, we demonstrated that with only two nighttime applications of UV-C at 200 J/m made at bloom resulted in an incidence of blossom blight and shoot blight equivalent to the results viewed when antibiotic and biopesticide commercial standards were applied. In vitro dose-response studies indicated consistency in pathogen response to suppressive UV-C doses, including pathogen isolates that were resistant to streptomycin. Based on these results, UV-C may be useful in managing bacterial populations with antibiotic resistance. Concurrent measurements of host growth after UV-C applications indicated that the dose required to suppress had no significant ( > 0.05) effects on foliar growth, shoot extension, internode length, or fruit finish but substantially reduced epiphytic populations of on host tissues.