The production of essential commodities (O2, H2 O, and edible biomass) and removal of CO2 by higher plants in bioregenerative life support systems would be seriously limited by the occurrence of disease epidemics. Among several treatment possibilities is ultraviolet (UV) radiation, which is one of the preferred sterilization techniques due to cost considerations and observed effectiveness against pathogens in hydroponic systems. Doses of 20 to 40 mW.s/cm2 as estimated in a laboratory flowthrough apparatus inactivated 99.99% of Pythium aphanidermatum, a common pathogen of hydroponic crops. Inactivation increased logarithmically with UV radiation dose. NCER (Net Carbon Exchange Rate) provides an indirect method to determine the effectiveness of UV in reducing Pythium infection, by measuring any changes in primary plant productivity. Observations demonstrated changes in NCER of lettuce infected with Pythium, showing loss of gas exchange capacity and reduced productivity in terms of CO2 scrubbing, transpired H2 O and edible biomass. Current trials to determine the extent of protection offered by UV in minimizing photosynthetic loss due to pathogen infection in chrysanthemum in a recirculating hydroponic system are underway.