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Ecology and Epidemiology

Colonization of Wheat Roots by a Fluorescent Pseudomonad Suppressive to Take-All. D. M. Weller, Research plant pathologist, U.S. Department of Agriculture, Agricultural Research Service, Washington State University, Pullman, 99164; Phytopathology 73:1548-1553. Accepted for publication 4 May 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-1548.

Pseudomonas fluorescens strain 2-79 is suppressive to take-all of wheat caused by Gaeumannomyces graminis var. tritici when applied as a wheat seed treatment. A strain resistant to rifampin and nalidixic acid, 2-79RN_10, was used to study the colonization of wheat roots. Winter wheat was treated with 10⁸ colony-forming units (CFU) of the bacterium per seed and sown in the field in October, 1980. A population greater than 10⁶ CFU of 2-79RN₁₀ per 0.1 g of root occurred up to 1 mo after planting. The bacteria were present on the entire length of the root (~ 7 cm long at 1 mo) including near the root tip. The population of the introduced strain declined through the late fall and winter to 2.8 x 10³ CFU per 0.1 g of root in early March (plants tillered but still dormant). With renewed growth of the plants in the spring, the population of 2-79RN₁₀ increased tenfold on the roots of plants with take-all and remained stable until the wheat was mature. The population of strain 2-79RN₁₀ remained higher in the spring and summer on roots infected with G. graminis var. tritici than on roots not infected with the pathogen. Strain 2-79 aggressively competed with native bacteria on the wheat roots. In the fall, cells of the introduced strain comprised up to 100% of the population of fluorescent pseudomonads on seminal roots. However, in the spring 2-79RN₁₀ generally comprised less than 10% of the fluorescent pseudomonads on crown roots.