Role of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens in the suppression of take-all and Pythium root rot of wheat

Take-all decline (TAD) is a field phenomenon in which a spontaneous decline of take-all occurs during continuous wheat or barley monoculture and after an outbreak of the disease. Strains of Pseudomonas fluorescens that produce the antibiotic 2,4-diacetylphloroglucinol (DAPG) are not only the major determinant of the suppressiveness of take-all but also in the suppression of other soilborne diseases in many different soils. Soils from fields with a history of crop monoculture, crop rotation or adjacent uncultivated areas were cycled to wheat to "activate" the population of indigenous DAPG-producers. Indigenous populations of DAPG-producing P. fluorescens were detected only in soils from fields with a history of wheat, pea or flax monoculture. In soils from fields under crop rotation or non-cropped areas, DAPG producers were below the detection level (log 3.26 CFU/g root) in the rhizosphere of wheat grown in the soils. To assess the role of DAPG producers in the suppression of take-all, soils that had undergone wheat or pea monoculture, and a non-cropped soil were infested with the take-all pathogen and then the take-all severity on wheat grown in the soils was determined. A greater suppression of take-all was observed in wheat seedlings grown in soils from continuous wheat or pea monoculture fields than in soil from non-cropped fields. Pasteurization of the pea monoculture soil resulted in a loss of DAPG producers and concurrent inability to suppress take-all. Strains of DAPG-producing P. fluorescens inhibited in vitro mycelial growth of Pythium spp.; however, TAD soils from Lind and Quincy, harboring large populations of indigenous DAPG-producing P. fluorescens , were not suppressive to Pythium root rot. Pythium root rot severity was similar on "Penawawa" wheat seedlings grown in soils conducive or suppressive to take-all infested with Pythium spp. Pythium spp. were less sensitive to synthesized DAPG than the take-all pathogen, which may explain the lack of Pythium suppression. In greenhouse experiments, wheat seeds treated with different DAPG-producing P. fluorescens genotypes or the recombinant strains Z30-97 or Z34-97, able to produce phenazine-1-carboxylic acid and DAPG, did not reduce Pythium root rot caused by P. ultimum or P. irregulare in wheat seedlings.