Effects of Culture Fluids and Preinduction of Chitinase Production on Biocontrol of Bipolaris Leaf Spot by Stenotrophomonas maltophilia C3

Control of leaf spot, caused by Bipolaris sorokiniana, on tall fescue and perennial ryegrass by Stenotrophomonas maltophilia C3 was enhanced in growth chamber and field experiments by application of bacterial cells in culture fluids as compared to phosphate buffer. C3 population levels on leaves were up to 0.8 log units higher when applied with culture fluid than with phosphate buffer. Although fluids alone were inhibitory to conidial germination and leaf spot development, there was a synergistic effect when combined with C3 cells. Fluids from broth cultures with chitin as the carbon source were more inhibitory than those containing glucose, and the suppressiveness of a culture fluid was related to the age of the culture from which it was collected. Both of these effects were associated with the production of high levels of chitinase (EC 3.2.1.14), protease (EC 3.4.21-24), β-1,3-glucanase (EC 3.2.1.58), and lipase (EC 3.1.1.3) in the fluid. Culture fluids had a durable protective effect, inhibiting disease development even when applied 9 days before inoculation, and had a therapeutic effect if applied within 3 days after inoculation. When chitin was applied with C3 cells preinduced or noninduced for chitinase production (i.e., grown on chitin-containing or chitin-lacking media, respectively) biocontrol efficacy was significantly increased over either cell type without chitin, but the addition of chitin did not always increase colonization by C3. In general, preinduced cells were more effective than noninduced cells. The most effective field treatment was the combination of chitin with induced cells in culture fluid.