Effect of ultraviolet disinfection of hydroponic solutions on Pythium root rot and non-target bacteria

Dispersal of root pathogens is a major concern in closed hydroponic cultures. To limit dispersal, ultraviolet (UV) disinfection technology has been used to remove pathogens but its effect on non-target bacterial populations is largely unknown. In this study, the circulating solution was disinfected with various UV doses (i.e., 19, 38, 59, 88 mJ cm(-2)) before recirculation. At the beginning of the experiment, the hydroponic solution was inoculated with Pythium aphanidermatum at 6.7 +/- 1.5 CFU mL(-1). Four months later the population density of P. aphanidermatum reached 1030 CFU mL(-1) in the control and 1028, 970, 610, and 521 CFU mL(-1) in the solutions treated by the above UV doses. In all UV-treated solutions, significant (P < 0.05) reduction of both Pythium and bacterial populations was noted. For the former, its reduction did not result in decrease of Pythium root rot nor increase of tomato fruit yield. For the latter, its reduction showed a concomitant decrease of the populations in the rhizosphere. The bacterial populations in the rhizosphere were 6.77x10(9) CFU mg(-1) fresh roots at the beginning of the experiment and were 7.89x10(8), 9.93x10(7), 7.33x10(7), and 3.51x10(7) CFU mg(-)1 fresh roots at the end of the experiment in the control, UV38, UV59, and UV88 treatments, respectively. The bacterial density also decreased with time in the control (UV0) although at a low rate. The results suggest that the attempt to control Pythium root rot by UV irradiation of recirculating solutions to remove P. aphanidermatum also affects the non-target bacterial populations in the rhizosphere. The interaction between the target pathogen and non-target bacterial flora in UV-treated hydroponics needs further investigation.