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Pythium aphanidermatum root rot in hydroponically grown lettuce and the effect of chemical and biological agents on its control

Experiments were conducted to isolate, identify, and monitor populations of Pythium spp. in recirculating nutrient solutions at two commercial greenhouses and to evaluate the ability of 11 chemical and biological agents to control root rot of lettuce caused by Pythium aphanidermatum under conditions similar to those of commercial greenhouses. The reverse dot blot technique was used for identification of Pythium spp. isolated from lettuce roots and the nutrient solutions. The population of P. aphanidermatum in nutrient storage tanks at both locations were fairly consistent ranging from l x 103 to 13 x 103 colony-forming units (CFU)/100 L from March 11 to July 21, 1998. Treatment with Boost (strain BACT-0 of Bacillus subtilis) at 1 x 109 and 1 x 1011 CFU/L, Promo (a nutrient supplement) at 1 mL/L. and Agrol (Amway) at 0.02 mL/L significantly increased dry lettuce and root weights and reduced the disease ratings compared with the control. These results suggest that the applications of Boost at 1 x 109 CFU/L, Promo at 1 mL/L, and Agrol at 0.02 mL/L of nutrient solution have the ability to increase lettuce production in the presence of P. aphanidermatum in recirculating nutrient systems.