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

Abstract

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 phytopathogens Erwinia carotovora (E.c.), Clavibacter michiganensis (C.m.) and Pseudomonas syringae pv. tomato (P.t.) by 10 to 30 min solar irradiation, in the presence of 0.15 to 0.3 mM (50-100 mg l-1) H 2O2. Different responses of the examined pathogens towards TiO2 photo-inactivation were noticed. Whereas 10 min of solar illumination in the presence of both 100 mg l-1 H2O 2 and 100 mg l-1 TiO2 resulted in total inactivation of P.t. and E.c., this treatment had no effect on C.m. However, with traces of H2O2 (e.g. 50-100 mg l-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 H2O 2 at 3,000 mg l-1 (0.3%), but not with H2O 2 at ≤1000 mg l-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 H 2O2 is discussed.