Polysaccharide production benefits dry storage survival of the biocontrol agent Pseudomonas fluorescens S11:P:12 effective against several maladies of stored potatoes

Abstract

Pseudomonas fluorescens S11:P:12 (NRRL B-21133) is a biological control agent able to suppress several potato diseases and sprouting. Notably, it produces a polysaccharide during liquid cultivation, and the objective of this work was to determine the role of this material in the bio-control process. First, the polysaccharide was isolated, purified and identified as marginalan, which accumulated to ~3.3 g/L in cultures. The bioactivity of isolated marginalan applied alone or in combination with washed cells of strain S11:P:12 was tested in potato bioassays of dry rot and pink rot suppressiveness and sprout inhibition. Since the formulation and storage of a dried biocontrol product is preferred for commercial use, the impact of marginalan on cell survival during drying and storage was also studied. Washed bacteria formulated with 0–6.6 g/L polysaccharide were either applied to Hyflo granules, then slowly dried for 24 h with airflow at 50–60% relative humidity, or in 1-µL droplets placed in replicate wells of a micro-plate, then quickly dried for 1 h in a biohazard hood. Both Hyflo and micro-plate dry storage results indicated that marginalan significantly reduced cell death after drying, such that the final stable viable cell density was 2.5–5 orders of magnitude greater, respectively, than if no marginalan were included with cells. Marginalan had no significant impact on disease or sprout suppression by strain S11:P:12, and its main benefit to biocontrol was viable cell preservation during drying and storage. When marginalan was formulated with other selected P. fluorescens strains, its benefits to drying and storage survival were again evident (especially after 4°C instead of 25°C storage), but its effects were more subtle than for strain S11:P:12, and dry rot suppression was not impacted.