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. http://www.phytoparasitica.org posting May 20, 2005.