Pythium spp. Associated with Root Rot and Stunting of Winter Wheat in North Carolina

In eastern North Carolina, mild to severe stunting and root rot have reduced yields of winter wheat, especially during years with abundant rainfall. Causal agents of root rot of wheat in this region were previously identified as Pythium irregulare, P. vanterpoolii, and P. spinosum. To investigate species prevalence, 114 isolates of Pythium were obtained from symptomatic wheat plants collected in eight counties. Twelve species were recovered, with P. irregulare (32%), P. vanterpoolii (17%), and P. spinosum (16%) the most common. Pathogenicity screens were performed with selected isolates of each species, and slight to severe necrosis of young roots was observed. The aggressiveness of five isolates each of P. irregulare, P. vanterpoolii, and P. spinosum was compared on a single cultivar of wheat at 14°C, and very aggressive isolates were found within all species. In vitro growth of these isolates was measured at 14 and 20°C, and all isolates grew faster at the warmer temperature. The effects of varying temperatures and rates of nitrogen on root rot caused by Pythium spp. alone or in combination were investigated. All inoculation treatments caused severe root rot under all conditions tested, and disease was more severe at 12 and 14°C compared with 18 and 20°C; however, there was no effect of nitrogen application.