Irradiation with UV-B fluorescent bulbs suppresses strawberry powdery mildew

Abstract

Strawberry powdery mildew [caused by Sphaerotheca aphanis (Wallr.) U. Braun] is a serious disease of strawberry (Fragaria × ananassa). It is difficult to control this disease using fungicides, because S. aphanis is adept at developing fungicide resistance. However, UV-B lights have been reported to induce resistance in strawberry plants against powdery mildew. In this study, we investigated the ability of a new type of UV-B fluorescent bulb (recently developed by the Panasonic Lighting Devices Co., Ltd., Japan) to suppress strawberry powdery mildew under greenhouse conditions. Strawberry plants were irradiated daily with UV-B (light intensity 4.0-13.4 μW cm(-2)) for 3 h during the night from the time of transplanting to the end of cultivation. We found that the number of fruits infected with powdery mildew declined by 84.3% relative to the control treatment (no irradiation). UV-B radiation (light intensity 4.3-15.3 μW cm(-2)) for 4 d week-1 for 3 h during the night lowered the percentage of infection by 54.8% relative to controls. Because daily UV-B irradiation significantly suppressed the mildew, we investigated whether incorporating a daily UV-B radiation regime could reduce the required frequency of fungicide application. Under a daily 3-h, UV-B radiation regime (intensity 8.0-18.8 μW cm(-2)), the percentage of powdery mildew infection on strawberry fruits was 0.1%, even when fungicide spraying was reduced to four times (compared with 16 times in the control). Our results indicate that using a UV-B lighting system for inhibiting powdery mildew in strawberry plants could substantially reduce fruit loss and the cost of fungicide spraying.