Biocontrol of Plant Diseases Laboratory, Plant Sciences Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705
Engelkes, C. A., Nuclo, R. L., and Fravel, D. R. 1997. Effect of carbon, nitrogen, and C:N ratio on growth, sporulation, and biocontrol efficacy of Talaromyces flavus. Phytopathology 87:500-505.Five-day biomass production by the biocontrol fungus Talaromyces flavus was measured in a liquid basal medium, pH 5.5, containing each of 37 carbon (C) sources with a single nitrogen (N) source, and each of 42 N sources with a single C source. In general, production of biomass was greatest on complex sugars such as polysaccharides (32 g/liter of medium) and β-glucosides (2.4 g/liter of medium), and was least on monosaccharides (1.3 g/liter of medium). Ascospore production at 6 weeks on solid basal medium with the same amount of these same 37°C sources was greatest on oligosaccharides (2.9 × 108 spores per 5.5-cm-diameter petri dish), and least on polysaccharides and monosaccharides (1.6 and 1.4 × 108 spores per 5.5-cm-diameter petri dish, respectively). For C sources, there was no correlation between production of ascospores and hyphal dry weight. The various N sources yielded 0 to 109 ascospores per 5.5-cm-diameter petri dish and 10-4 to 10-5 g of hyphal dry weight per milliliter. In general, N sources that resulted in the greatest number of ascospores also yielded the greatest hyphal dry weights. For the two C and two N sources tested, the number of ascospores increased as the ratio of C to N increased from 5:1 to 30:1. This effect was most obvious as the C:N ratio increased from 5:1 to 15:1. At low C:N ratios (<15:1), treatments with hypoxanthine as a N source resulted in significantly greater production of biomass than treatments with ammonium tartrate; no difference was observed at C:N ratios ≥15:1. Incidence of Verticillium wilt was 50% lower for eggplants drenched with ascospores grown on potato dextrose agar (PDA) compared with eggplants either nondrenched or drenched with ascospores grown on media with hypoxanthine plus lactose or maltose. Thus, C and N sources that slightly increased ascospore production of T. flavus reduced efficacy of biocontrol of Verticillium wilt compared with ascospores produced on PDA.