Take-all, caused by Gaeumannomyces graminis var. tritici (Ggt), develops where wheat is grown under moist conditions. It is the most important root disease of wheat worldwide. Take-all decline (TAD) is the spontaneous decrease in take-all incidence and severity induced by continuous wheat or barley monoculture after a severe outbreak of the disease. The build-up of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens (phlD+) during monoculture is responsible for TAD. More than 177 Ggt isolates were isolated from fields with different cropping histories. Sensitivity of Ggt to 2,4-DAPG was not correlated with cropping history, indicating that wheat monoculture does not lead to the emergence of 2,4-DAPG insensitive isolates of the pathogen. Gaeumannomyces-Phialophora complex were isolated from three reduced-till TAD fields in Eastern Washington and identified as Phialophora sp. Phialophora sp. showed less sensitivity to 2,4-DAPG than did isolates of Ggt. The Phialophora isolates may be contributing some level of take-all suppression in the field. A genome-wide deletion library of Saccharomyces cerevisiae was screened for sensitivity to 2,4-DAPG. The screen identified 231 mutants with increased sensitivity to 2,4-DAPG. The targeted mutants were impaired in three cellular physiological functions: membrane function, reactive oxygen regulation, and cellular homeostasis. 2,4-DAPG showed a chemical-genetic fitness data profile similar to the profiles of menthol, sodium azide, and hydrogen peroxide. These findings suggest that 2,4-DAPG inhibits S. cerevisiae, and presumably other fungi, through multiple targets at basic cellular processes. Accumulation of 2,4-DAPG was greatest at two weeks after planting and decreased over the next four weeks. The half life of 2,4-DAPG introduced artificially onto wheat roots with adhering rhizosphere soil was 6 hours. When wheat seedlings were grown on plates amended with 2,4-DAPG (10 [mu]g ml-1) roots showed stunting and brownish necrosis. Levels of 2,4-DAPG sufficient to cause phytotoxicity may in some cases accumulate on roots.A combination of P. fluorescens Q8r1-96 and host plant that supports moderate amounts of 2,4-DAPG accumulation showed the best suppression of take-all. Combinations that resulted in the greatest amount of 2,4-DAPG accumulation, failed to control the disease. This result indicated that a suitable amount of 2,4-DAPG is needed to maximize the biocontrol effect
