Suppression of cucumber powdery mildew by UV-B is affected by background light quality A. Suthaparan, A. Stensvand, K. A. Solhaug, S. Torre, K. H. Telfer, A. K. Ruud, L. Cadle-Davidson, L. M. Mortensen, D. M. Gadoury, R. C. Seem, H. R. Gislerød

Light & ultraviolet Light Visible part of the electromagnetic spectrum to human eye From 400 nm to 790 nm Ultraviolet Short wavelength part of the electromagnetic spectrum Divided into UV-A (315- 400 nm) UV-B (280-315 nm) UV-C (200-280 nm) Vacuum UV (100-200 nm) 200 Wavelength (nm) 400 600 800

Why light & ultraviolet? Norway is situated in Northern Europe Very low solar irradiance during winter months Supplemental lighting is common practice Photo - Unknown

Effect of red light on conidial release of rose powdery mildew (Plant Dis 94:1105-1110)

Effect of 5 min with UV-B applied in the dark period on severity of rose powdery mildew 9 d after inoculation (Plant Dis doi: 01-12-0094-RE)

Effect of UV-B if the short wavelength part was filtered out by cellulose diacetate (remove wavelengths below 295 nm) Rose powdery mildew 9 d after inoculation (Plant Dis doi: 01-12-0094-RE)

Powdery mildew in cucumber Three different pathogens can cause powdery mildew Podosphaera xanthii (syn. Sphaerotheca fuliginea) Golovinomyces cichoracearum (syn. Erysiphe cichoracearum) Leveillula taurica

Spectral quality of radiation sources used in the experiments Spectral width (nm) Peak (nm) UV-B 280 - 400 313 UV-A 300 - 420 340 Blue 420 - 500 450 Red 600 - 660 635

Effect of UV-B on cucumber powdery mildew

Effect of UV-B on severity of powdery mildew 9 d after inoculation

0 min 5 min 15 min 10 min Effect of UV-B on development of P. xanthii 6 d after inoculation

Effect of background light on efficiency of UV-B

16 h W 16 h W + UV-B UV-A UV-B + Blue UV-B + Blue Red UV-B +Red Effect of background light on the efficiency of UV-B on cucumber powdery mildew; assessed at 3 d intervals over 12 d (bars = AUDPC)

Mechanisms of red light and UV-B Indirectly via induced resistance Directly upon the pathogen Systemic induced resistance Locally induced resistance Presence of photoreceptors UV-B induced DNA damage and its light dependent repair X X

Photoreceptor genes in powdery mildew fungi among classes of photoreceptors UVR8 Phototropins ZEITHLUPE White collar VIVID Cryptochromes Cry-DASH Photolyase Phy A-E FPH 1, 2 and E BPH CBPH BRh BovR Tryptophan FAD/FMN FAD Retinal Bilin

Increasing wavelength (nm) Conclusion 280-295 The shortest wavelength part of UV-B (280-295 nm) inhibits all developmental stages of powdery mildew 635-660 Orange red light (635-660 nm) at low intensity reduces sporulation, at high intensity it also reduces colony growth UV-A Blue Far-red Increasing wavelength (nm) 400 600 200 800