EFFECTS OF SEED FACTORS ON SPERMOSPHERE AND RHIZOSPHERE COLONIZATION OF COTTON BY BACILLUS-SUBTILIS GB03

Cotton seed factors, including seed-surface pH, cultivar, and presence of a seed-treatment fungicide, were investigated for their effect on spermosphere and rhizosphere colonization by the commercial biological control agent Bacillus subtilis strain GB03 (GB03). Commercial (nonne utralized) cotton seed is traditionally acid-delinted and in these stu dies was found to have surface pH values ranging from 1.90 to 3.50. Fo llowing seed treatment with GB03, spermosphere populations of GB03 wer e up to 5-10-fold lower on nonneutralized (commercial acid-delinted) c otton seed than on neutralized seed grown in sterile sand or in field soil, respectively; however, rhizosphere populations of GB03 on 2-wk-o ld cotton root systems did not differ significantly. GB03 populations 24 h after planting on nonneutralized seed did not significantly incre ase from initial seed populations at the time of planting, and fewer v egetative cells were recovered on nonneutralized seed than on neutrali zed seed. Spermosphere and radicle populations of GB03 on seed of four cotton cultivars differed significantly, although whether the differe nce was due to genotype is unclear. Seed treatment with metalaxyl/pent achloronitrobenzene did not affect spermosphere colonization by GB03; however, treatment with these fungicides resulted in an increase of up to 1 log unit in GB03 rhizosphere populations compared to the level f or no fungicide treatment. The spermosphere environment can affect the root colonization potential of bacteria applied as seed treatments to cotton seed, thereby possibly affecting the biological control or pla nt growth-promoting potential of the inoculants.