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 (nonneutralized) cotton seed is traditionally acid-delinted and in these studies was found to have surface pH values ranging from 1.90 to 3.50. Following seed treatment with GB03, spermosphere populations of GB03 were up to 5-10-fold lower on nonneutralized (commercial acid-delinted) cotton seed than on neutralized seed grown in sterile sand or in field soil, respectively; however, rhizosphere populations of GB03 on 2-wk-old cotton root systems did not differ significantly. GB03 populations 24 h after planting on nonneutralized seed did not significantly increase from initial seed populations at the time of planting, and fewer vegetative cells were recovered on nonneutralized seed than on neutralized seed. Spermosphere and radicle populations of GB03 on seed of four cotton cultivars differed significantly, although whether the difference was due to genotype is unclear. Seed treatment with metalaxyl/pentachloronitrobenzene 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 for 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 plant growth-promoting potential of the inoculants.