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 greenh ouses and to evaluate the ability of 11 chemical and biological agents to c ontrol root rot of lettuce caused by Pythium aphanidermatum under condition s similar to those of commercial greenhouses. The reverse dot blot techniqu e was used for identification of Pythium spp. isolated from lettuce roots a nd the nutrient solutions. The population of P. aphanidermatum in nutrient storage tanks at both locations were fairly consistent ranging from 1 x 10( 3) to 13 x 10(3) colony-forming units (CFU)/100 L from March 11 to July 21, 1998. Treatment with Boost (strain BACT-O of Bacillus subtilis) at 1 x 10( 9) and 1 x 10(11) 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 su ggest that the applications of Boost at 1 x 10(9) CFU/L. Promo at 1 mL/L, a nd Agrol at 0.02 mL/L of nutrient solution have the ability to increase let tuce production in the presence of P. aphanidermatum in recirculating nutri ent systems.