Aims We tested here the effect of low nitrogen (N) availability and of UV-C radiations (a physical agent) on the susceptibility of Lactuca sativa L. towards Botrytis cinerea (BC 87) and Sclerotinia minor (SM). Methods We assessed first that a dose of UV-C radiation of 0.85 kJ. m−2 is effective in triggering a positive effect without entailing any negative effect on plants. We then tested the effect of this dose on 62-day-old lettuce plants grown at three N concentrations: 2 (low), 10 (normal) and 20 mM (high). Plants were inoculated 24 h after exposure to UV-C radiation and monitored for 4 days. Results Analysis of lesion areas showed that leaf susceptibility towards Botrytis cinerea (BC 87) and Sclerotinia minor (SM) substantially increased with the increasing N concentrations. Results also showed that the UV-C treatment can reduce MDA and H2O2 concentrations and lesion areas. Reduction of lesion areas reached 67 % on day 4 in the Sclerotinia minor (SM)-10 mM N trial and 52 % in the Botrytis cinerea (BC 87)-20 mM N trial. The UV-C treatment also resulted in an increase in the concentration of total phenolics in leaves which could be observed on days 1, 2 and 4 in Sclerotinia minor SM trial and days 1 and 2 in Botrytis cinerea BC 87 trial. Conclusions Taken together, our observations suggest that UV-C irradiation stimulate the capacity of lettuce plants to resist against Sclerotinia minor (SM) and Botrytis cinerea (BC 87) and that phenolic compounds may be involved in this resistance.