Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2009
Publication Date: 5/1/2009
Citation: Kwak, Y., Bakker, P.A., Glandorf, D., Topham, J., Paulitz, T.C., Weller, D.M. 2009. Diversity, virulence and 2,4-diacetylphloroglucinol sensitivity of Gaeumannomyces graminis var. tritici isolates from Washington State. Phytopathology Vol. 99, No. 5, p. 472-479. Interpretive Summary: Biological control is the application or stimulation of antagonistic microorganisms for the control of plant pathogens. Some of the most effective biocontrol agents of soilborne pathogens are strains of the bacterium Pseudomonas fluorescens that produce the antifungal metabolite 2,4-diacetylphloroglucinol (2,4-DAPG). 2,4-DAPG producers can be applied as seed or soil treatments and are responsible for the natural biocontrol of take-all disease when wheat is grown continuously in a field. 2,4-DAPG has a broad spectrum of activity against some of the most important soilborne pathogens. Previous studies have shown that 2,4-DAPG produced by P. fluorescens directly inhibits pathogens on roots. In this study, we compared the sensitivity of the take-all pathogen of wheat, isolated from wheat-monoculture and non-monoculture fields, to determine if Gaeumannomyces graminis var. tritici develops tolerance to 2,4-DAPG because of exposure to the antibiotic over many years. The results of this research are very important because they demonstrated that tolerance to 2,4-DAPG does not develop in natural populations of the take-all pathogen. This is one of the first studies addressing the potential for emergence of resistance in a target pathogen to an introduced or indigenous biocontrol agent.
Technical Abstract: We determined whether isolates of the take-all pathogen Gaeumannomyces graminis var. tritici become less sensitive to 2,4-DAPG during wheat monoculture as a result of exposure to the antibiotic over multiple growing seasons. Over 177 isolates of G. graminis var. tritici were baited from roots of native grasses collected from non-cropped fields and from roots of wheat from fields with different cropping histories near Lind, Ritzville, Pullman and Almota, WA. Isolates were characterized using morphological traits, G. graminis variety-specific PCR and pathogenicity tests. The sensitivity of G. graminis var. tritici isolates to 2,4-DAPG was determined by measuring radial growth of each isolate. The 90% effective does value (ED90) was 3.1 to 4.4 µg ml-1 for 2,4-DAPG sensitive isolates, 4.5 to 6.1 µg ml-1 for moderately sensitive isolates, and 6.2 to 11.1 ml-1 for less sensitive isolates. Sensitivity of G. graminis var. tritici isolates to 2,4-DAPG was normally distributed in all fields and was not correlated with geographic origin or cropping history of the field. There was no correlation between virulence on wheat and geographical origin, or virulence and sensitivity to 2,4-DAPG. These results indicate that G. graminis var. tritici does not become less sensitive to 2,4-DAPG during extended wheat monoculture.