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Zinc Improves Biocontrol of Fusarium Crown and Root Rot of Tomato by Pseudomonas fluorescens and Represses the Production of Pathogen Metabolites Inhibitory to Bacterial Antibiotic Biosynthesis

Crown and root rot of tomato caused by Fusarium oxysporum f. sp. radicis-lycopersici is an increasing problem in Europe, Israel, Japan, and North America. The biocontrol agent Pseudomonas fluorescens strain CHA0 provides only moderate control of this disease. A one-time amendment of zinc EDTA at 33 μg of Zn²⁺/ml to hydroponic nutrient solution in soilless rockwool culture did not reduce disease when used alone, but did reduce disease by 25% in the presence of CHA0. In in vitro studies with the pathogen, zinc at concentrations as low as 10 μg/ml abolished production of the phytotoxin fusaric acid, a Fusarium pathogenicity factor, and increased production of microconidia over 100-fold, but reduced total biomass. Copper EDTA at 33 μg of Cu²⁺/ml had a similar effect as zinc on the pathogen in vitro; it reduced disease when used alone, and increased the biocontrol activity of CHA0 in soilless culture. Ammonium-molybdate neither improved the biocontrol activity of CHA0 nor affected production of fusaric acid or microconidia. Strain CHA0 did not degrade fusaric acid. Fusaric acid at concentrations as low as 0.12 μg/ml repressed production by CHA0 of the antibiotic 2,4-diacetylphloroglucinol, a key factor in the biocontrol activity of this strain. Production of pyoluteorin by CHA0 was also reduced, but production of hydrogen cyanide and protease was not affected, suggesting that fusaric acid affects biosynthesis at a regulatory level downstream of gacA and apdA genes. Fusaric acid did not affect the recovery of preformed antibiotics nor did it affect bacterial growth even at concentrations as high as 200 μg/ml. When microbial meta-bolite production was measured in the rockwool bioassay, zinc amendments reduced fusaric acid production and enhanced 2,4-diacetylphloro-glucinol production. We suggest that zinc, which did not alleviate the repression of antibiotic biosynthesis by fusaric acid, improved biocontrol activity by reducing fusaric acid production by the pathogen, which resulted in increased antibiotic production by the biocontrol agent. This demonstrates that pathogens can have a direct negative impact on the mechanism(s) of biocontrol agents.