Bait method to detect Pythium species that grow at high temperatures in hydroponic solutions

Pythium helicoides, P. aphanidermatum and P. myriotylum are important pathogens that cause root rot of several crops in hydroponic culture and in ebb-and-flow irrigation systems. These species belong to a group of Pythium species that can grow at temperatures higher than 40C. We developed a method for baiting these high-temperature Pythium species and evaluated its practicality to monitor their presence in nutrient solutions. Seeds of cucumber, tomato, radish, hemp, perilla and millet and leaves of bent grass and rose were tested as baits in hydroponic systems. Hemp, perilla and radish seeds and bent grass and rose leaves were more effective than the other baits for Pythium zoospores, and bent grass leaves were the most effective. In a sensitivity test, bent grass leaf traps (BLTs) detected three Pythium species after only a 1 day exposure to suspensions of 40 zoospores per liter of water, and the frequency of detection increased with zoospore density and with baiting period. A temperature of 38C was optimum for the selective reisolation of the high-temperature Pythium species from the BLTs. The BLT was also tested with inoculated and noninoculated miniature roses that shared a recirculating nutrient solution. The pathogen was detected in the nutrient solution 23 days before the disease spread to the noninoculated roses. In addition, P. helicoides was detected 30 days before the disease was evident in a commercial greenhouse. The baiting method described here will be useful for monitoring high-temperature Pythium species in recirculating hydroponic culture systems.