First Report of Puccinia emaculata Infection on Switchgrass in Virginia

During the spring of 2007, approximately 2,000 switchgrass plants, representing 168 core switchgrass germplasm, were established in a field nursery at the Virginia Tech Kentland Farm Research Center in Montgomery County, VA. These germplasms were originally obtained from the USDA Plant Genetic Resources Conservation Unit (Griffin, GA) and included both lowland and upland ecotypes. After planting, the switchgrass plants were allowed to establish for 3 years. In the summers of 2009, 2010, and 2011, a substantial infection of a foliar rust pathogen was observed on more than 90% of switchgrass lines in the field. The infected leaves had dark brown pustules that were arranged in a linear fashion between the veins and mainly located on the adaxial surface of the leaf. We observed the infected leaf samples under a microscope and isolated the urediniospores. The urediniospores were brown, round, and averaged 26.08 ± 1.67 μm long and 24.65 ± 1.66 μm wide. Teliospores were observed late in the summers of 2009 and 2010. The teliospores were two-celled, oblong to ellipsoid in shape, and averaged 32.23 ± 3.07 μm in length. The apical cell width averaged 17.6 ± 1.83 μm and the basal cell width averaged 15.08 ± 1.75 μm. The morphology of both the urediniospores and teliospores were similar to Puccinia emaculata Schw. (P. emaculata) (2,3). Using previously established rust disease scoring methods (1), the infected switchgrass lines were evaluated and scored for infection severity on a 0 to 9 scale based on the percentage of leaf area infected, with 0 being either highly resistant or escaping infection and 9 being highly susceptible. The majority of upland switchgrass cultivars, including Caddo, Cave-in-rock, Blackwell, Sunburst, Pathfinder, and Dacotah, were moderately to highly susceptible to this rust pathogen and on average, scored between 7 and 8. However, the majority of lowland switchgrass cultivars, including Alamo, Kanlow, TEM-SEC, TEM-SLC, and TEM-LoDorm, were moderately to highly resistant and scored on average between 3 and 4. This result is consistent with previous reports that show that lowland ecotypes are more resistant to rust diseases (1). To further validate the identity of the rust pathogen, we designed two primers (5'-CCAGTAACGGCGAGTGAAGAG-3' and 5'-CGACTTCCATGGCCACCGTGCGGCTGTCT-3') based on the 18S rDNA sequence of P. emaculata (3). DNA was extracted from bulk infected leaf material for PCR amplification. The 1.2 kb PCR product was isolated and sent for DNA sequencing. The DNA sequence was 98% identical to the 18S rDNA sequence of P. emaculata (EU915294.1). To our knowledge, this study represents the first report of rust on a wide range of various switchgrass cultivars in Virginia. References: (1) D. M. Gustafson et al. Crop Sci. 43:755, 2003. (2) R. L. Hirsch et al. Plant Dis. 94:381, 2010. (3) J. Zale et al. Plant Dis. 92:1710, 2008.