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.
. Sugarbeet seeds used by farmers are often pelleted using an EB TM -based mix. During the pelleting process, the seeds are dried immediately after application of the mix. In this work, the effects of inoculum preparation and formulation on survival and biocontrol efficacy of Pseudomonas fluorescens F113Rif were investigated using a 1:1 EB TM /vermiculite mix and sugarbeet seeds pelleted with this material. Growing F113Rif for 3 d (28°C) within the EB TM /vermiculite mix amended with nutrients (sucrose asparagine broth), instead of adding the cells to the unamended mix immediately before drying the mix or the pelleted sugarbeet seeds, resulted in improved survival of the strain in the mix or on the seeds, respectively, during subsequent storage. A slower drying (20 h instead of 3 h) of the F113Rif-inoculated EB TM /vermiculite mix to 11% w/w water content enhanced strain survival in the mix during storage, but the drying conditions studied had no effect on inoculant survival on the seed during storage when pelleted seeds were dried to 10% w/w water content. Biological control of damping-off disease of sugarbeet (caused by Pythium spp.) in soil microcosms was achieved when F113Rif was inoculated in the unamended mix 3 d before pelleting the seeds, but not when nutrient-amended mix was used. Inoculum preparation and drying of the formulation are key factors to consider when optimizing the use of a commercial EB TM /vermiculite seed formulation for delivery of a biocontrol Pseudomonas inoculant.
Antifungal extracts from four strains of bacteria that were selected for their ability to inhibit fungal turfgrass pathogens were compared for in vitro activity. The cell extract from Pseudomonas aureofaciens Tx-1 (ATCC 55670) exhibited the greatest antifungal activity against selected turfgrass pathogens. Purification of the extract yielded a single active component that was identified as phenazine-1 carboxylic acid (PCA). Minimum inhibitory concentrations of PCA to tested fungal pathogens ranged from 10 to 25 µg/ml. In greenhouse studies, PCA provided management of dollar spot on creeping bentgrass equal to that of the commercial fungicides triadimefon and chlorothalonil at equivalent rates of active ingredient. Phytotoxic effects were observed on creeping bentgrass in greenhouse but not field evaluations of PCA at the rate of 0.48 g/m2. At the end of 2 years of field study, PCA applied every 14 days at 0.15 g/m2 provided dollar spot management on creeping bentgrass equal to that of chlorothalonil applied every 10 days at the label rate of 0.48 g/m2.
Abstract. Pseudomonas fluorescens F113, which produces the antimicrobial compound 2,4-diacetylphloroglucinol is a prospective biocontrol agent. Soil enzyme activities were used to investigate the ecological impact of strain F113 in the rhizosphere of field-grown sugar beet. There were distinct trends in rhizosphere enzyme activities in relation to soil chemistry (studied by electro-ultrafiltration). The activities of enzymes from the phosphorus cycle (acid phosphatase, alkaline phosphatase and phosphodiesterase) and of arylsulphatase were negatively correlated with the amount of readily available P, whereas urease activity was positively correlated with the latter. Significant correlations between electro-ultrafiltration nutrient levels and enzyme activity in the rhizosphere were obtained, highlighting the usefulness of enzyme assays to document variations in soil nutrient cycling. Contrary to previous microcosm studies, which did not investigate plants grown to maturity, the biocontrol inoculant had no effect on enzyme activity or on soil chemistry in the rhizosphere. The results show the importance of homogenous soil microcosm systems, used in previous work, in risk assessment studies, where inherent soil variability is minimised, and where an effect of the pseudomonad on soil enzymology could be detected.
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