Applications of Germicidal Ultraviolet Light as A Tool for Fire Blight Management (Erwinia amylovora) in Apple

Nighttime applications of germicidal UV light (UV-C) have been used to suppress several fungal diseases of plants, but less is known of the potential of UV-C to suppress bacterial plant pathogens. Fire blight of apple and pear, caused by the bacterium Erwinia amylovora, is difficult to suppress using cultural practices, antibiotics, and host resistance, and 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 E. amylovora to UV-C at doses ranging from 0 to 400 J/m2 in the absence of visible light were more than doubly effective compared to the same doses when the cultures were exposed to visible light after UV exposure. In a 2-yr orchard study, we demonstrated that as few as two nighttime applications of UV-C at 200 J/m2 made at bloom resulted in an incidence of blossom blight and shoot blight equivalent to antibiotic and biopesticide commercial standards. In vitro dose-response studies indicated uniformity of pathogen response to suppressive UV-C doses, including pathogen isolates that were resistant to streptomycin. Thus, 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 E. amylovora had no significant (P > 0.05) effects on foliar growth, shoot extension, internode length, or fruit finish, but substantially reduced epiphytic populations of E. amylovora on host tissues.