POPULATION BIOLOGY OF BACILLUS-CEREUS UW85 IN THE RHIZOSPHERE OF FIELD-GROWN SOYBEANS

We studied the spatial and temporal distribution and abundance of the biocontrol agent Bacillus cereus UW85 in the rhizosphere of soybeans during two growing seasons in the field. For this study, we used a neomycin-resistant mutant, UW85n1, derived from UW85. UW85n1 spores were applied to seeds and by 24 h after planting 72% of the spores had germinated. UW85n1 colonized emerging radicles, but populations were 10-50-fold smaller on radicles than they were on seeds at planting. UW85n1 population sizes in the rhizosphere of 10-35-day old plants were largest near the crown of the plant and smaller further from the crown. For example, in a 1989 experiment 21 days after planting, UW85n1 populations were 158-fold larger 0-2 cm below the crown than they were 7-8 cm below the crown. UW85n1 population sizes were largest right after planting, and by seed harvest they decreased to levels comparable to indigenous B. cereus populations on untreated roots; however, at harvest the entire B. cereus population on treated roots was neomycin resistant, whereas on untreated roots <3% was neomycin resistant. The proportion of the heterotrophic bacterial population that was represented by UW85n1 was generally largest right after planting and decreased at subsequent sampling times. Our results indicate that UW85n1 grew and spread on soybean roots, and it persisted in the rhizosphere until seed harvest.