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Applied and Environmental Microbiology, October 2001, p. 4414-4425, Vol. 67, No. 10
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.10.4414-4425.2001

Changes in Populations of Rhizosphere Bacteria Associated with Take-All Disease of Wheat

Brian B. McSpadden Gardener1,2,* and David M. Weller1

Root Disease and Biological Control Research Unit, USDA Agricultural Research Service, Pullman, Washington,1 and Department of Plant Pathology, The Ohio State University, Wooster, Ohio2

Received 3 May 2001/Accepted 8 July 2001

Take-all, caused by Gaeumannomyces graminis var. tritici, is one of the most important fungal diseases of wheat worldwide. Knowing that microbe-based suppression of the disease occurs in monoculture wheat fields following severe outbreaks of take-all, we analyzed the changes in rhizosphere bacterial communities following infection by the take-all pathogen. Several bacterial populations were more abundant on diseased plants than on healthy plants, as indicated by higher counts on a Pseudomonas-selective medium and a higher fluorescence signal in terminal restriction fragment length polymorphism analyses of amplified 16S ribosomal DNA (rDNA). Amplified rDNA restriction analysis (ARDRA) of the most abundant cultured populations showed a shift in dominance from Pseudomonas to Chryseobacterium species in the rhizosphere of diseased plants. Fluorescence-tagged ARDRA of uncultured rhizosphere washes revealed an increase in ribotypes corresponding to several bacterial genera, including those subsequently identified by partial 16S sequencing as belonging to species of alpha-, beta-, and gamma-proteobacteria, sphingobacteria, and flavobacteria. The functional significance of some of these populations was investigated in vitro. Of those isolated, only a small subset of the most abundant Pseudomonas spp. and a phlD+ Pseudomonas sp. showed any significant ability to inhibit G. graminis var. tritici directly. When cultured strains were mixed with the inhibitory phlD+ Pseudomonas strain, the Chryseobacterium isolates showed the least capacity to inhibit this antagonist of the pathogen, indicating that increases in Chryseobacterium populations may facilitate the suppression of take-all by 2,4-diacetylphloroglucinol-producing phlD+ pseudomonads.

* Corresponding author. Mailing address: Department of Plant Pathology, The Ohio State University, OARDC, 1680 Madison Ave., Wooster, OH 44691-4096. Phone: (330) 202-3565. Fax: (330) 263-3841. E-mail: bbmg+{at}osu.edu.

Applied and Environmental Microbiology, October 2001, p. 4414-4425, Vol. 67, No. 10
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.10.4414-4425.2001


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