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Suppression of cucumber powdery mildew by UV-B is affected by background light quality.
A. SUTHAPARAN (1), A. Stensvand (2), K. A. Solhaug (2), S. Torre (1), K. Telfer (1), A. Ruud (1), L. Cadle-Davidson (3), L. Mortensen (1), D. M. Gadoury (4), R. C. Seem (4), H. R. Gislerod (1). (1) Norwegian University of Life Sciences, Aas, Norway; (2) Bioforsk, Aas, Norway; (3) USDA-ARS, Grape Genetics Research Unit, Geneva, NY, U.S.A.; (4) New York State Agricultural Experiment Station, Geneva, NY, U.S.A.

Brief (5-10 min) exposure to UV-B radiation (280-300 nm) at 1 W m<sup>2</sup> suppressed powdery mildew (<i>Podosphaera xanthii</i>) on <i>Cucumis sativus</i>. The effect was enhanced by red light (600-660 nm), but offset by blue light (420-500 nm) and UV-A (300-420 nm). Compared to untreated controls, 2 h red light from specific light emitting diodes (LEDs) reduced disease severity by 25.4%, and by 98 % when combined with brief exposure to UV-B. We hypothesized that UV-B damaged fungal DNA, that blue light and UV-A were involved in DNA repair, and that red light exacerbated damage to DNA. We confirmed the expression of genes similar to phytochrome, cryptochrome, white collar, and photolyase (light-dependent DNA repair) using next generation sequencing of <i>Erysiphe necator</i>. Sequence and expression analysis of these light-responsive genes from <i>P. xanthii</i> will be reported. Our findings may explain how powdery mildews, which are principally external to the host and lack protective pigmentation, nonetheless thrive despite their lack of obvious protection from UV radiation. Our results suggest that efficacy of UV-B treatments against powdery mildews will be greatly enhanced by applying them during the night period, thereby circumventing the counteracting effects of blue light and UV-A, particularly when UV-B is used in combination with red light. Brief night exposure to UV-B, and/or inexpensive and specific LEDs may provide additional tools to suppress powdery mildews in diverse crops.<p><p>Keywords: Fungus, Vegetables

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