Heat pasteurization is an effective water treatment to address the emerging plant pathogen issue associated with increased water recycling practices in the ornamental horticulture industry. The current protocol that recommends treating water at 95"C for 30 s, however, faces two major challenges: its energy cost and environmental footprint. We hypothesized that temperature required to inactivate major pathogens in re-circulated water may be substantially lowered from 95"C with extended exposure time. The goal of this study was to test this hypothesis and make this water decontamination technology economically more attractive while reducing its environmental impact. Specific objectives were to (1) examine the effect of water temperature on the survival of Phytophthora and bacterial species, two major groups of plant pathogens in water recycling systems, and (2) elucidate the underlying biological mechanisms by which plant pathogens are killed at those temperatures. Lab assays were performed to determine the survival of zoospores and chlamydospores of P. nicotianae, and oospores of P. pini as well as seven bacterial species after heat treatments at given periods of time. Greenhouse experiments were conducted to determine the applicability of the lab assay data to the real world using annual vinca (Catharanthus roseus) and P. nicotianae as a model system. The results of these studies indicated that the water temperature required to eliminate Phytophthora and bacterial species can be lowered to 48"C from 95"C if treatment time extends to 24 h. Two major steps were taken to elucidate the underlying biological mechanisms. Firstly, a scheme based on the DNA fingerprint and sequence analysis was developed for characterizing bacterial species in irrigation water, after comparing two typing strategies, three sample concentration methods, and evaluating conditions in denaturing gradient gel electrophoresis (DGGE) profiling. Bacterial species detected by culture-dependent and -independent strategies were rather different. The greater bacterial diversity was detected when water samples were concentrated by using both methods than centrifugation or filtration alone. As for DGGE profiling, 40 to 60% denaturant concentrations at 70 V for 16 h revealed the highest bacterial diversity. Secondly, water samples were taken from an irrigation reservoir in a local nursery and analyzed for bacterial diversity following heat treatments at 42 and 48"C. After these heat treatments "-proteobacteria, "-proteobacteria, and Firmicutes became dominant which presents a substantial shift of bacterial community structure compared to those in the control water at 25"C. Among the dominant in treated water were Bacillus, Pseudomonas, Paenibacillus, Brevibacillus, and Lysobacter species, which may have potential biocontrol activities against plant pathogens. This study provided the scientific basis for developing a more energy-efficient and environmentally sound heat pasteurization protocol for water decontamination.