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. radic is-lycopersici is an increasing problem in Europe, Israel, Japan, and North America. The biocontrol agent Pseudomonas fluorescens strain CHA 0 provides only moderate control of this disease. A one-time amendment of zinc EDTA at 33 mu g 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 studie s with the pathogen, zinc at concentrations as low as 10 mu g/ml aboli shed production of the phytotoxin fusaric acid, a Fusarium pathogenici ty factor, and increased production of microconidia over 100-fold, but reduced total biomass. Copper EDTA at 33 mu g of Cu2+/ml had a simila r effect as zinc on the pathogen in vitro; it reduced disease when use d alone, and increased the biocontrol activity of CHA0 in soilless cul ture. Ammonium-molybdate neither improved the biocontrol activity of C HA0 nor affected production of fusaric acid or microconidia. Strain CH A0 did not degrade fusaric acid. Fusaric acid at concentrations as low as 0.12 mu g/ml repressed production by CHA0 of the antibiotic 2,4-di acetylphloroglucinol, a key factor in the biocontrol activity of this strain, Production of pyoluteorin by CHA0 was also reduced, but produc tion of hydrogen cyanide and protease was not affected, suggesting tha t fusaric acid affects biosynthesis at a regulatory level downstream o f gacA and apdA genes. Fusaric acid did not affect the recovery of pre formed antibiotics nor did it affect bacterial growth even at concentr ations as high as 200 mu g/ml. When microbial metabolite production wa s measured in the rockwool bioassay, zinc amendments reduced fusaric a cid production and enhanced 2,4-diacetylphloroglucinol production. We suggest that zinc, which did not alleviate the repression of antibioti c biosynthesis by fusaric acid, improved biocontrol activity by reduci ng fusaric acid production by the pathogen, which resulted in increase d antibiotic production by the biocontrol agent. This demonstrates tha t pathogens can have a direct negative impact on the mechanism(s) of b iocontrol agents.