Rust Diseases on Switchgrass (\u3ci\u3ePanicum virgatum\u3c/i\u3e)

Switchgrass (Panicum virgatum) is a perennial C4 grass native to the central prairies of North America. Recent development of switchgrass as a sustainable biofuel feedstock has raised interest in diseases that could impact switchgrass, especially when grown under monoculture conditions. Among the known diseases of switchgrass, rust diseases are the most widespread and could potentially impact yield and biomass quality of new switchgrass cultivars. Two species of rust fungi, Uromyces graminicola and Puccinia emaculata, are known to infect switchgrass, with P. emaculata being the primary or sole rust pathogen on switchgrass in southern US states. The relative importance of each of the two species in the North Central region, however, was unknown. In this study, the rust fungi on switchgrass samples collected from various locations in Nebraska, Illinois, Indiana, Missouri and Wisconsin were identified on the basis of teliospore morphology and DNA sequence analysis. Both U. graminicola and P. emaculata were found in samples from Nebraska and Wisconsin, whereas P. emaculata only was found in samples from the other states. Among 22 switchgrass strains evaluated in a Nebraska varietal experiment, lowland types were found to be more resistant to rust than upland types. Most upland switchgrass strains were infected by both rust species, while most lowland switchgrasses were infected by P. emaculata only. The results indicate that development of new biofuel switchgrass cultivars, particularly those intended for the northern plains, must include improved resistance against both rust species. As a new tool to facilitate future switchgrass research and resistance breeding efforts, a diagnostic polymerase chain reaction (PCR) system was developed whereby the two rust species can be distinguished in individual infection lesions and in infected leaves. The system utilized three sets of primers, one set (UgF and UgR) designed in this study specifically for U. graminicola on the basis of its internal transcribed spacer sequence, and the other two being previously-reported primers diagnostic for P. emaculata and for all rust fungi. Advisor: Gary Y. Yue