Combining proteolytic and phloroglucinol‐producing bacteria for improved biocontrol of <i>Pythium</i> ‐mediated damping‐off of sugar beet

Pseudomonas fluorescens F113 and Stenotrophomonas maltophilia W81 protect sugar beet from Pythium ‐mediated damping‐off through production of the antifungal secondary metabolite 2,4‐diacetylphloroglucinol and extracellular proteolytic activity, respectively. In this study, the two biocontrol strains were combined in a consortium, with the objective of improving upon the level of protection achieved when using each strain singly. Growth and in vitro production of 2,4‐diacetylphloroglucinol by F113 and extracellular lytic enzymes by W81 were not affected when inoculated in combination. The abilities of W81 and F113 to colonize the rhizosphere of sugar beet were essentially similar when the two strains were applied singly or coinoculated onto seeds in a 1 : 1 ratio, both in natural soil microcosms and under field conditions. Concomitantly, single inoculation with W81 or F113 effectively prevented colonization of sugar beet seeds by Pythium spp. in soil microcosms, without the necessity for combining both strains. However, this parity was not reflected in seed emergence where the combination of W81 and F113 significantly enhanced final sugar beet stands (to the level achieved with chemical pesticides) under microcosm conditions at 28 days after sowing. In a field experiment, the only inoculation treatment capable of conferring effective protection of sugar beet was that in which W81 and F113 were coinoculated, and this treatment proved equivalent to the use of chemical fungicides. In conclusion, when compared with single inoculations of either biocontrol strain, the combined use of a phloroglucinol‐producing P. fluorescens and a proteolytic S. maltophilia improved protection of sugar beet against Pythium ‐mediated damping‐off.