Colonization of Wheat Roots by a Fluorescent Pseudomonad Suppressive to Take-All

Abstract

Pseudomonas fluorescens strain 2-79 is suppressive to take-all of wheat [Triticum aestivum] caused by Gaeumannomyces graminis var. tritici when applied as a wheat seed treatment. A strain resistant to rifampin and nalidixic acid, 2-79RN10, 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 Oct., 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 (.apprx. 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 .times. 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 10-fold 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.