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Design, Operation, and Efficacy of an Apparatus Using Ultraviolet Light to Suppress Powdery Mildew of Strawberry in Open Field Production Systems

Rodrigo Borba Onofre: University of Florida - Gulf Coast Research and Education Center

<div>Strawberry powdery mildew caused by <em>Podosphaera aphanis</em> can be destructive in both covered and open field production. Fungicides are the principal means used to suppress this disease. Ultraviolet light (UV) has effectively suppressed diverse powdery mildews in several greenhouse and tunnel production trials. Over two growing seasons, we exposed open field plots of mildew-susceptible strawberry plants to UV light (254 nm, FWHM < 5 nm) approximately 1 hr after sunset either once or twice weekly. The apparatus used was tractor-drawn, and consisted of a dense reflectorized hemicylindrical array of multiple UV fluorescent lamps. Total UV energy at the plant canopy surface plant was approximately 84.5 or 169 J m<sup>-2</sup> at tractor speeds of 4.5 km h<sup>-1</sup> and 2.25 km h<sup>-1</sup>, respectively. Comparative fungicide treatments consisted of cyflufenamid alternated with quinoxyfen every two weeks. In the 2016-17 growing season, UV reduced foliar mildew up to 99% compared to the nontreated control, and provided greater suppression than fungicide treatments. The 2017-18 trials are ongoing, but treatment separations after one month indicate that UV is again providing suppression equal to or better than biweekly fungicide treatments. Deleterious effects of UV exposures have not been noted on plant growth or yield. Effects of UV treatments on <em>Botrytis cinerea</em> and <em>Colletotrichum acutatum</em> will also be assessed in 2017-18.</div>