Flashes of UV-C light improve tolerance to powdery mildew of rose plants

Abstract: Flashes of UV-C light (FUL) represent an innovative process for stimulating plant
defences against a large range of diseases in a large number of crops. Their effects on severity
of disease symptoms by powdery mildew were investigated on cultivars ‘Invitation’ (very
susceptible), ‘Giardina’ (susceptible) and ‘London eye’ (not susceptible) of rose plants grown
in soilless greenhouse conditions. Additional measurements of leaf gas exchanges, chlorophyll fluorescence, yield and quality, were made. FUL were provided by amalgam lamps, delivering 600 J/m2in one second at the upper-height level of the crop. FUL were applied every 10 days from March to July 2021. Disease was allowed to develop naturally, and first symptoms were observed from March 2021. Infection was rated on a 0 to 3 scale. FUL reduced more than three times the proportion of severely affected plants in ‘Invitation’ and suppressed them totally in ‘Giardina’ and ‘London eye’. More than 80 % plants had a score of 1 or less in FUL-treated ‘Giardina’ plants and ca. 97 % in ‘London eye’. In all the cultivars, we observed only limited negative effect of FUL on leaf gas exchanges and chlorophyll fluorescence parameters. FUL treatments reduced dramatically the proportion of downgraded cut roses in all cultivars and therefore increased the proportion of marketable cut roses from +15 % in ‘London eye’ to + 82 % in ‘Invitation’. Our observations are discussed in the light of what is known about the effect of FUL on the salicylic signaling pathway.