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Population Dynamics of Bacillus sp. L324-92R₁₂ and Pseudomonas fluorescens 2-79RN₁₀ in the Rhizosphere of Wheat

Bacillus sp. L324-92 is suppressive to three root diseases of wheat, namely take-all caused by Gaeumannomyces graminis var. tritici, Rhizoctonia root rot caused by Rhizoctonia solani AG8, and Pythium root rot caused by several Pythium species. Populations of strain L324-92R₁₂, a rifampicin-resistant mutant of L324-92 applied as a seed treatment, were monitored in the rhizosphere and spermosphere of wheat and compared with populations of Pseudomonas fluorescens 2-79RN₁₀, a known, rhizosphere-competent, biocontrol agent. In growth chamber studies, the population sizes of L324-92R₁₂ on roots of wheat were approximately 1,000-fold smaller than those of 2-79RN₁₀ at 5 days after planting, but, thereafter, they increased while those of 2-79RN₁₀ decreased until the two were equal in size at 45 days after planting. In the field with winter wheat, the population sizes of L324-92R₁₂ on roots were at least 10-fold smaller than those of 2-79RN₁₀ during the fall (November 1993) and early spring (March 1994). Thereafter, the population of L324-92R₁₂ remained constant or increased slightly, while the population of 2-79RN₁₀ decreased until the two were roughly the same at 10⁴ to 10⁵ CFU/plant over the period of 150 days (April 1994) until 285 days (harvest) after planting. In growth chamber studies, strain L324-92R₁₂ remained confined to root sections within 3.5 cm below the seed, whereas 2-79RN₁₀ was recovered from all root sections ranging from 0.5 to 6.5 cm below the seed. In the field on winter wheat, both strains were recovered from root sections down to 5.0 to 6.5 cm below the seed at 75 days after planting (mid December), but only 2-79RN₁₀ was recovered at this depth at 90 days after planting. Both strains were recovered from the seed remnants 6 months after planting in the field. Both strains also were recovered from inside the roots and shoots, but population sizes of strain 279RN₁₀ were greater than those of L324 92R₁₂.