Shedding the Light on Powdery Mildew: The Use of Optical Irradiation in Management of the Disease.

Ultraviolet (UV) irradiation below 300 nm may control powdery mildew in numerous crops. Depending on disease pressure, wavelength, and crop growth stage, one to three applications of 100-200 J/m2 per week at night are as effective or better than the best fungicides. Higher doses may harm the plants and reduce yields. Although red light alone or in combination with UV has a suppressive effect on powdery mildew, concomitant or subsequent exposure to blue light or UV-A strongly reduces the efficacy of UV treatments. To be effective, direct exposure of the pathogen/infection sites to UV/red light is important, but there are clear indications for the involvement of induced resistance in the host. Other pathogens and pests are susceptible to UV, but the effective dose may be phytotoxic. Although there are certain limitations, this technology is gradually becoming more used in both protected and open-field commercial production systems.