Spatial and temporal population dynamics of a phyllosphere colonizing Bacillus subtilis biological control agent of sugar beet cercospora leaf spot

Abstract

Abstract This study examines the spatial and temporal variation of populations of the biological control agent (BCA) BacB, a Bacillus subtilis isolate, in the field and growth chamber in the presence of the fungus, Cercospora beticola , the causal agent of Cercospora leaf spot of sugarbeet. The use of the selective BCA support substrate β-glucan, applied at 0, 0.5, and 1.0% of the spray solution, did not influence differences in total population numbers (spores + vegetative cells) of a spontaneous rifampicin resistant isolate of BacB (Rif+) over a 14 day spray period. BacB Rif+, applied as a spore formulation, declined from 10,000 CFU/cm 2 on day 0.5–100 CFU/cm 2 on day 14 at the three levels of β-glucan tested. Distribution of BacB Rif+ populations was modeled on a leaf scale, with and without β-glucan. Higher populations of vegetative cells were more likely at 14 days with 1% β-glucan than with 0% β-glucan. BacB populations were more aggregated without β-glucan than with the nutrient substrate. There was no correlation between BacB density and Cercospora leaf spot disease severity, indicating that neither antibiosis nor parasitism is likely an important mechanism of disease control.