Pythium Root Rot of Wheat is not Suppressed by Indigenous 2,4-Diacetylphloroglucinol-Producing Pseudomonads in Take-All Decline Soils.

Abstract

Take-all, caused by Gaeumannomyces tritici and Pythium root rot, caused by a complex of Pythium spp., are important diseases of wheat in Washington state, USA, and they often occur on the same plant. During wheat monoculture, a natural biocontrol of take-all known as take-all decline (TAD) develops from a buildup in the rhizosphere of 2,4-diacetylphloroglucinol (DAPG)-producing pseudomonads belonging to the Pseudomonas fluorescens complex. To determine if TAD also suppresses Pythium root rot, wheat seedlings were grown in two TAD soils and their homologous non-cropped virgin (conducive) soils amended with inoculum of Pythium abappressorium, P. (Globisporangium) irregulare group I, P. (Globisporangium) irregulare group IV or P. (Globisporangium) ultimum. TAD soils alone or mixed 1:9 with pasteurized conducive soil did not consistently reduce symptoms of Pythium root rot as compared to their homologous virgin (conducive) soils. In TAD and conducive soils, plant height, length of the first true leaf, number of root tips, and root lengths were similar. Wheat grown in Pythium-infested and non-infested TAD soils supported rhizosphere populations of DAPG-producing pseudomonads above 10⁷ CFU g^-1 of root. Mycelial growth of the Pythium spp. was inhibited in vitro by both DAPG-producers and synthetic DAPG. However, concentrations of DAPG required to inhibit mycelial growth of the Pythium spp. ranged from 15 to 30 µg mL^-1, which is 5-10 times greater than the dose reported to inhibit the take-all pathogen. In conclusion, Washington TAD soils do not suppress Pythium root rot of wheat.