| Switchgrass head smut caused by the pathogenic fungus Tilletia maclaganii (Berk.) Clint. is a systemic, perennial disease that affects switchgrass (Panicum virgatum L.), a crop with strong potential for production of biomass for energy in temperate climates. Switchgrass head smut replaces the entire caryopsis of all spikelets on affected panicles with dense spore-masses and is associated with significant biomass yield loss due to plant stunting. Prior to this work, little was known about the life cycle of T. maclaganii or how to manage it. To identify its infection pathways we conducted inoculation experiments on switchgrass in the greenhouse and laboratory. Infection was observed with both seed/seedling inoculation and inoculation of adult plants. Infection rates were low in the seed/seedling inoculations (<30% smutted seedlings), whereas inoculation resulted in higher infection rates (up to 64% of tillers smutted). We also conducted two greenhouse trials and one multi-year field trial to evaluate susceptibility of ten switchgrass cultivars. Across all three trials two lowland cultivars (Kanlow and BoMaster) were significantly (P < 0.05) less susceptible than the upland cultivars included in the trials. Our results suggest a strong potential for improving the head smut resistance of switchgrass cultivars through cross-breeding and recurrent phenotypic selection within existing cultivar populations. We evaluated four commercial seed treatment fungicides (Raxil 2.6 FS, 42-S Thiram, Dividend, and Penflufen FS 240) for their efficacy at suppressing germination of T. maclaganii teliospores in a seedless microtiter plate assay. T. maclaganii was most sensitive to Dividend in the microtiter plate assay, with an EC50 of 1.96 mg a.i./L. Seed fungicides could be useful in reducing the spread of pathogen to new locations. Finally, since the only way to definitively diagnose a switchgrass plant as being infected with T. maclaganii was to wait for it to produce smutted panicles, we designed a PCR screening protocol for detecting T. maclaganii DNA extracted from both teliospores and infected switchgrass tissue. The species-specific primers we developed for T. maclaganii were sensitive to a threshold of 1 pg DNA/muL and selectively only amplified T. maclaganii DNA in a panel of nine fungal species and switchgrass. |
