BSA is firstly developed by Michelmore et al. (1991) and has becomes a rapid mapping strategy suitable for monogenic or major qualitative traits. Two bulked DNA samples are generated from a segregating population that originated from a single cross. Each pool, or bulk, contains individuals (the number of individuals in each bulk varied between 10 and 20 plants) that are identical for a particular trait (e. g., disease resistant or susceptible) or genomic region but arbitrary at all unlinked regions. The two bulks are therefore genetically dissimilar in the selected region but seemingly heterozygous at all other regions. Thus, the two bulks can be made for any genomic region and from any segregating population, and they can be screened for differences the same way as near isogenic lines (NILs) (Michelmore et al. 1991). This approach is applicable both in those species where selfing is possible and in those that are obligatorily outbreeding like switchgrass. Since its invention, BSA technique has been widely used in different species for single gene and QTL analysis (Van Leeuwen et al. 2012).

Diseases had been reported to affect switchgrass biomass yields in southern Iowa (Gravert and Munkvold 2002). Thomsen et al. (2008) conducted a study in naturally infected condition caused by smut, and found biomass yield loss of an upland cultivar ‘Cave-in-Rock’ varying from 0.6% to 40.1% among fields. Other major diseases affecting biomass yields are rust (Gustafson et al. 2003) and leaf spots (Krupinsky et al. 2004). Little information is available regarding disease resistance of switchgrass cultivars. Gustafson et al. (2003) used four switchgrass populations (two were derived from cultivars ‘Summer’ and ‘Sunburst’, and the other two developed by breeder of Nebraska and Oklahoma, respectively), and evaluated rust resistance at two locations (Aurora and Kimball, SD) in two years. Significant variations for rust resistance were observed among and within populations, suggesting genetic improvement in rust resistance may be effective through selection (Gustafson et al. 2003). In an ongoing project, we found obvious differences for leaf rust resistance in a segregated population derived from selfing of ‘NL94 LYE 16Х13’. Considering that bulks screened method had been utilized for mapping resistance gene (s) initially in lettuce (Michelmore et al. 1991), and then successfully expanded in other plant species (Michelmore 1995), it can be expectedly moved to switchgrass to identify markers linked with major resistant genes. A gene mapping project of switchgrass rust resistance is on the way (Y. Q. Wu, unpublished).