Interspecific Variation in Virulence Among Pythium Species in Tobacco Transplant Production Greenhouses

Pythium pathogens commonly inhibit tobacco transplant production in float-bed style hydroponic greenhouses, but disease symptoms often vary. Results from 2018 to 2020 Petri dish and greenhouse "mini-bay" virulence assays found that the 12 Pythium spp. isolated in a 2017 tobacco greenhouse survey fell into three categories: aggressive, weak, or nonpathogens. P. myriotylum and P. coloratum were aggressive pathogens, always suppressing seedling stands and causing disease damage compared with the untreated control (P < 0.05). Seedling stands were almost always lower and disease incidence and severity were almost always greater for P. myriotylum versus the other Pythium species tested (P < 0.05). One of two isolates of P. dissotocum was also an aggressive pathogen, whereas the second isolate was a weak pathogen, rarely reducing plant stands or increasing damage compared with the untreated control (P < 0.05). P. irregulare usually reduced seedling stands more than the other weak pathogens (P. aristosporum, P. catenulatum, P. inflatum, P. porphyrae, and P. torulosum) but caused less disease less consistently than P. myriotylum, P. coloratum, and the more aggressive P. dissotocum isolate (P < 0.05). P. adhaerens, P. attrantheridium, and P. pectinolyticum were nonpathogens, failing to cause symptoms on tobacco seeds or seedlings. Variation in virulence among Pythium spp. was consistent across the virulence categories but sometimes varied among species within categories and was also usually consistent across inoculation dates. These results suggest variance in virulence among Pythium biotypes is likely a significant factor responsible for variability in Pythium disease in tobacco greenhouses.