Spatio-temporal distribution of naturally occurring Bacillus spp. and other bacteria on the phylloplane of soybean under field conditions

Bacterial populations were surveyed on phylloplane of field-grown soybean every 20 days in southern Buenos Aires, Argentina. The crop was disease-free and no pesticides were applied. Out of 523 bacterial isolates (mainly Gram positive bacteria) from upper and lower leaves, 134 of them were Bacillus spp. Bacillus pumilus was the dominant species followed by Bacillus subtilis. The proportion of Bacillus spp. decreased from 80% of all bacterial isolates in early stages to 0% at harvest, whereas the diversity of Bacillus spp. decreased from nine species at day 45, to one species at day 133, shortly before harvest. Among the isolates, a significant increase in the percentage of nitrifiers with plant aging was observed on lower and upper trifoliates. In contrast, proteolytic bacteria were more abundant on lower than on upper trifoliates at all sampling dates. The consistent recovery of B. pumilus from soybean phylloplane during most of the crop cycle indicated this species as an interesting candidate for future plant microbial management and biocontrol studies.