Preharvest UV-C affects lettuce resistance to Xanthomonas campestris pv. vitians and quality

A series of independent trials were conducted to evaluate the use of UV-C radiation for controlling Xanthomonas campestris pv. vitians (Xcv), the causative agent of bacterial leaf spot in lettuce, and to assess the impact of this treatment on lettuce yield. The first trial involved repeated applications of a pre-established hormetic dose of UV-C (0.4 kJ/m²), for a total dose of 1.6 kJ/m², which represented a single treatment cycle. Lettuce plants were inoculated with a virulent strain of Xcv 48 h after this treatment cycle. The second trial, which took place after Xcv inoculation, involved additional UV-C treatment cycles (0.5, 1.0 and 1.75 cycles), corresponding to total doses of 2.4, 3.2 and 4.4 kJ/m² respectively. In the third trial, three weekly UV-C treatments were given at 48 h intervals using the standard 0.4 kJ/m² dose, beginning the third week after seeding and continuing until the lettuce reached the commercial stage and was harvested. By the time the mature lettuce plants were harvested, they had undergone five treatment cycles, representing a total dose of 8 kJ/m². A single UV-C treatment cycle resulted in a 90 %, 30 % or 10 % reduction in the susceptibility of the growing lettuce plants to Xcv compared with the control group, depending on when symptoms were evaluated. The application of additional treatment cycles (0.5, 1.0 and 1.75 cycles) resulted in a reduction of 30 %, 45 % and 50 % in susceptibility, respectively, in the treated lettuce plants relative to the controls. The fresh yield of lettuce treated with 2.75 cycles of UV-C (4.4 kJ/m²) was similar to that of the control lettuce; this treatment also resulted in a significant decrease (20 %) in core length, an agronomic trait associated with increased yield when lettuce is processed as fresh-cut ready-to-eat product. UV-C-treated lettuce had a significantly greater dry matter content and higher total mineral concentrations than the control lettuce. These results indicate that repeated preharvest UV-C treatments (0.4 kJ/m²) improve the tolerance of lettuce to Xcv without negative effects.