Comparative in vitro and field efficacy of ultraviolet light (UV-C) applied during light or darkness to suppress the apple scab pathogen ( Venturia inaequalis )

Apple scab, caused by the ascomycete fungus Venturia inaequalis , if left untreated or managed ineffectively, can result in velvety olive-brown lesions on fruit of apple (Malus × domestica) and up to 70% yield losses. Feasibility of V. inaequalis suppression using synthetic fungicides alone, upon which widespread plantings of scab-susceptible cultivars are largely dependent, is threatened by fungicide resistance and increasing restriction of fungicide use. Ultraviolet light (UV-C) has effectively controlled diverse microbial pathogens in multiple pathosystems as an alternative method. We explored the potential of UV-C alone and UV-C integrated programs, with day and nighttime applications, to manage apple scab, and evaluated the timing of applications based on decision support systems (DSSs). In vitro spore germination assays in which UV-C exposure of the pathogen followed by incubation in visible light (to simulate daytime application) required doses of UV-C higher than those followed by darkness (to simulate nighttime application) to result in the same level of suppression of spore germination. Half maximal effective concentration (EC-50) of conidial germination for light incubation was achieved at 400 J/m 2 , where dark incubation EC-50 was achieved below 200 J/m 2 . Field experiments yielded commercially relevant and equivalent levels of suppression of the apple scab pathogen over four years on highly susceptible cultivars under high disease pressure following nighttime UV-C treatment at 200 J/m2 and daytime treatment at 400 J/m2, without consequent phytotoxicity. The biological and commercial standard fungicide treatments were not significantly different (P > 0.05) from UV-C treatments in the incidence of apple scab symptoms on leaves and fruit. The use of titanium dioxide as an adjuvant did not increase the efficacy of either daytime or nighttime UV-C treatments. The programs that alternated UV-C with single-site fungicides resulted in lower apple scab incidence during high disease pressure years. This study demonstrates the effectiveness of UV-C applications to manage apple scab in a manner comparable to commercial fungicide programs.