EFFECT OF HETEROTROPHIC BACTERIAL COMMUNITIES ON PYTHIUM SPP. IN RECYCLED IRRIGATION WATER

A main concern in commercial greenhouses is the harboring of Pythium species in recycled irrigation water and their spread into susceptible crops. Pythium species are among the most damaging pathogens of horticultural crops causing damping-off of seedlings as well as root and stem rots in ornamental plants. Despite frequent attempts to recover Pythium from recycled water, success in isolation is sporadic. This raises the question of possible microbial suppression of Pythium present in recycled irrigation water. The present study focused on the deleterious effect of microbial communities in recycled irrigation water on the development of Pythium aphanidermatum, P. irregulare, and P. cryptoirregulare. Microscopic observations of the interaction between microorganisms in recycled irrigation water and Pythium species indicated a deleterious effect on Pythium development. The amount of sporangia produced in the presence of the microbial community was negatively affected when compared to water samples with no microorganisms. In addition, research was conducted to evaluate heterotrophic bacteria isolated from recycled irrigation water that reduced the growth in vitro of the three highly pathogenic Pythium species. We isolated a variety of bacteria some of which inhibited the three Pythium spp. growth, others bound to their hyphae, and still others seemed to enhance the growth of the Pythium species. Known biocontrol agents against Pythium (Pseudomonas and Bacillus) and some potentially new biocontrol agents (Acidovorax, Microbacterium, and Chryseobacterium) were identified as isolates inhibiting the growth of P. aphanidermatum, P. irregulare, and P. cryptoirregulare in vitro. The use of biological control agents to control Pythium spp. is not new. However, in most cases, commercially available beneficial organisms are from other environments. Utilization of naturally occurring suppressive bacterial communities in recycled irrigation water may provide an environmentally friendly ecologically sound biological control strategy. We observed that Pythium grew slower when bacterial isolates bound to their hyphae. This phenomenon was previously observed but is not fully understood. These isolates were identified as Acinetobacter, Enterobacter, Pantoea, and Pseudomonas species. Surprisingly, we also isolated