Solar photo-inactivation of phytopathogens by trace level hydrogen peroxide and titanium dioxide photocatalysis

Plant pathogenic bacteria in recirculated greenhouse water were inactivated by two distinct photochemical approaches: photo-inactivation in the presence of 0.005% to 0.01% hydrogen peroxide (H2O2), and photocatalytic inactivation with 0.01% titanium dioxide (TiO2). In both processes photo-inactivation is achieved by exposure to sunlight. Total inactivation, with 6–8 log units decrease in viable counts, was achieved in the study of the phytopathogensErwinia carotovora (E.c.), Clavibacter michiganensis (C.m.) andPseudomonas syringae pv.tomato (P.t.) by 10 to 30 min solar irradiation, in the presence of 0.15 to 0.3 mM (50–100 mgl −1) H2O2. Different responses of the examined pathogens towards TiO2 photo-inactivation were noticed. Whereas 10 min of solar illumination in the presence of both 100 mgl −1 H2O2 and 100 mgl −1 TiO2 resulted in total inactivation ofP.t. andE.c., this treatment had no effect onC.m. However, with traces of H2O2 (e.g. 50–100 mgl −1), and in the absence of TiO2,C.m. was deactivated by 20 min of solar irradiation.P.t. was fully inactivated in the dark by H2O2 at 3,000 mgl −1 (0.3%), but not with H2O2 at ≤ 1000 mgl −1. Also, no inactivation occurred with solar illumination in the absence of H2O2. The mechanism of the bactericidal photoreaction and the special significance of plant pathogen inactivation by natural sunlight in the presence of trace levels of H2O2 is discussed.