Microbiological assays
At the R1 growth stage (42 and 38 DAS for GR1 and GR2, respectively), shoots were clipped close to the ground, roots were carefully removed from soil and soil tightly adhering to the root surface (rhizoplane) was rigorously removed using a camel-hair brush and retained as ‘rhizosphere soil.’ A subsample of the cleaned roots was washed under running water and gently blotted on paper towels in preparation for microbiological assays. Roots were surface sterilized in 1Æ25% sodium hypochlorite for 2 min followed by rinsing three times in sterile water and blotted dry on sterile paper towels. Fusarium colonization of the surface-sterilized soybean root segments (8, 2-cm segments per agar plate) was assessed by the root-plating procedure of Le´vesque et al. (1993) on Fusarium-selective agar medium (Nash and Snyder 1962). After 5-day incubation at 25C, the number of fungal colonies developing on root segments was recorded.