S14 - Session O4 - Flashes of UV-C light : a powerful new method for stimulating plant defenses against powdery mildew of tomato plants grown in soilless greenhouse conditions

Authors: Laurent Urban *, Jawad Aarrouf, Félicie Lauri, Marine Rigault, Raphaël Tisiot

Flashes of UV-C light represent an innovative process for stimulating plant defenses against a large range of diseases in a large number of crops. The effects of flashes of UV-C light on severity of disease symptoms by powdery mildew were investigated on 'Cauralina' (very susceptible), 'Clodano' (susceptible) and 'Brioso' (not susceptible) tomato plants grown in standard soilless greenhouse conditions. UV-C light was provided by amalgam lamps transported by a trolley for delivering 600 J m-2 in one second at the medium-height level of the fully developed crop. Four treatments were made, every 10 days from May 10 th to June 25 th 2021. Disease was allowed to develop naturally, and first symptoms were observed around mid-May. Severity was assessed from pictures taken on leaflets until it reached 100 %. Leaf gas exchanges were measured every 10 days. Chlorophyll fluorescence (ChlF) parameters were derived from induction curves of maximal ChlF taken on leaves. We observed no negative effect of UV-C treatments on either leaf net assimilation rate (Anet) or maximal Anet. We even observed positive effects on Anet after treatment two for 'Cauralina' and after treatment one for 'Clodano', and on SPAD data after the first and the second treatments for 'Brioso'. Generally speaking there were no negative effects of UV-C treatments on ChlF parameters. Disease severity was strongly decreased in 'Cauralina' until June 2 nd and 'Clodano' until June 21 st . In 'Brioso' there was still a substantial positive effect observable on July 19 th . Our observations clearly show that flashes of UV-C light have the potential of decreasing symptoms of powdery mildew on tomato leaves without entailing any substantial or durable negative effect on photosynthesis. Our results are discussed in the light of other observations made as part of previous trials on tomato, strawberry and grapevine.