FIG. 4.

Long-term bioassay: biological control of take-all of wheat by P. fluorescens strains Q2-87, 1M1-96, and Q8r1-96. Strains Q2-87, 1M1-96, and Q8r1-96 were introduced into raw Quincy virgin soil to a final density of approximately 100 CFU g of soil⁻¹, and wheat was grown for eight successive cycles of 3 weeks each. The population dynamics of each of the strains is shown in Fig. 3. After eight successive cycles, an oat grain inoculum of the take-all pathogen was introduced to a final density of 0.2% (wt/wt). Wheat plants were grown for 3 weeks in the infested soils, after which take-all severity was rated on a 0-to-8 scale, and the shoot height was determined (Table 4). Wheat grown in Quincy virgin (conducive to take-all) and Quincy TAD (suppressive to take-all) soils for eight successive growth cycles of 3 weeks each served as controls. 1, Quincy virgin; 2, Quincy virgin plus Q2-87; 3, Quincy virgin plus 1M1-96; 4, Quincy virgin plus Q8r1-96; 5, Quincy TAD.