Flashes of UV-C light stimulate defences of Vitis vinifera L. cv. Chardonnay against Erysiphe necator in greenhouse and vineyard conditions

Using detached leaves, UV-C light in the form of 1-sec flashes has recently been shown to stimulate defences of several plants against different pathogenes better than 1 min exposures under greenhouse conditions. In the present work, the pathological tests were conducted using undetached leaves under greenhouse and vineyard conditions. In a first trial, two flashes of UV-C light were applied to plants of Vitis vinifera L. cv. Chardonnay grown under greenhouse conditions, at an interval of 10 days. Plants were inoculated with Erysiphe necator two days after the last light treatment. After 18 days of inoculation, the symptom severity on leaves was reduced by 60 % when compared with the untreated control. In a second trial, flashes of UV-C light were applied to grapevine Chardonnay plants under field conditions in the South-East of France, every 10 days from the 18th of April until the 10th of July 2019. The symptom severity resulting from natural contaminations by Erysiphe necator was reduced by 42 % in leaves on the 4th of July 2019 and by 65 % in clusters on the 25th of July 2019. In a third trial, we observed that UV-C light did not have any effect on net photosynthesis, maximal net photosynthesis, dark respiration, maximal quantum efficiency of photosystem II, the performance index of Strasser and, generally, any parameter derived from induction curves of maximal chlorophyll fluorescence. It was concluded that flashes of UV-C light have true potential for stimulating plant defences against Erysiphe necator under vineyard conditions and, therefore, help in reducing fungicide use.