Effects of increased nitrate availability on the control of plant pathogenic fungi by the soil bacterium Bacillus subtilis

In wet soils, low oxygen conditions often develop that favour disease devel opment by many soil-borne plant pathogens. The introduction of a biocontrol agent, to suppress disease development, would require that the agent remai ns metabolically active under such conditions. Denitrifying bacteria can ma intain this metabolic activity by switching to nitrate respiration. In the rhizosphere, plant roots not only supply carbon as an electron donor, but a lso cause a localised lowering of oxygen concentrations, conditions favoura ble for nitrate respiration. Two strains of Bacillus subtilis, showing stro ng inhibition of a number of pathogenic fungi on agar plates, and the capac ity to grow under anoxic and anaerobic conditions when provided with nitrat e, were used to study the possible involvement of nitrate respiration in fu ngal disease control. The effect of the addition of nitrate: on the activit y of these antagonistic strains was studied under anoxic conditions using t he sealed plate method of Fiddaman and Rossal [Fiddaman, P.J., Rossal, S., 1995. Plant Pathol. 44, 695-703]. The assay tests the activity, measured as a reduction in fungal growth, of antifungal volatiles (AFV) produced by th e bacteria. The in vitro experiments showed that antagonism by the B. subti lis strains towards Fusarium oxysporum varied under anoxic conditions, depe nding on the nitrate availability and agar used as a growth medium. AFV act ivity was increased by the presence of nitrate in the medium at concentrati ons of 10 mM or more. Nitrate respiration may therefore have an important r ole in the control of fungal root diseases by allowing denitrifying soil-bo rne bacteria to remain metabolically active in wet soils with low oxygen co ncentrations. (C) 2000 Elsevier Science B.V. All rights reserved.