Résumé
Using detached leaves, UV-C light in the form of 1-s flashes has recently been shown to stimulate defenses of several plants against different pathogens 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. ‘Chardonnay’ grown under greenhouse conditions, at an interval of 10 days. Plants were inoculated with Erysiphe necator 2 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 southeast of France every 10 days from 18 April until 10 July 2019. The symptom severity resulting from natural contaminations by E. necator was reduced by 42% in leaves on 4 July 2019 and by 65% in clusters on 25 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 defenses against E. necator under vineyard conditions and, therefore, help in reducing fungicide use.