Heterotrophic bacteria present in recycled greenhouse irrigation water (RIW) were characterized and then evaluated for their effect on Pythium aphanidermatum, P. cryptoirregulare, and P. irregulare. Nutrient agar (NA) and R2A agar were used to isolate copiotrophic and oligotrophic bacteria. Bacterial isolates recovered from RIW were categorized according to whether they inhibited Pythium growth, attached to hyphae, or enhanced Pythium growth in the three Pythium species used. Three bacterial isolates were selected to determine whether their in vitro interactions with Pythium aphanidermatum, the most pathogenic of the three species used, influenced disease development in the greenhouse. An isolate of Sphingobium sp. that inhibited Pythium, Pseudomonas sp. that attached to hyphae, and Cupriavidus sp. that enhanced the growth of P. aphanidermatum in vitro were used in greenhouse experiments to examine their effects on disease development in geranium (Pelargonium ×hortorum ‘White Orbit’) grown in pasteurized potting mix in ebb and flood irrigation systems. Disease progress curves evaluating the effect of each bacterium indicate that they did not suppress or enhance disease development (P = 0.05). Thus, the effects that the bacterial isolates had in vitro differed from their effects under greenhouse conditions.