Research on pest and disease control on switchgrass grown for bioenergy has been limited. A few diseases have been described in switchgrass, and some may have potential for significantly affecting production (Sanderson et al. 2012). When or if switchgrass begins to be used in large-scale monocultures, more disease pressures may emerge (Parrish and Fike 2005).

Pathogens and Their Treatments

Rust (Puccinia spp.) have been reported on switchgrass cultivars (Zale et al. 2008; Hirsch et al. 2010) and cultivars of northern origin appear more susceptible (Cassida et al. 2005b). However, heritability exists for rust resistance (Gustafson et al. 2003), and differences in resistance among accessions were noted and characterized early on by Cornelius and Johnston (1941). This variation provides breeders an opportunity to make improvements in yield, but we note that the level of resistance displayed by an accession or cultivar may be affected by growing conditions—and thus by their location of planting. E. g., in on-going research in Virginia—which typically has humid conditions—"rust-resistant" southern lowlands have sometimes displayed greater rust infestation during exceptionally dry periods, while their "rust-susceptible" counterparts displayed few such symptoms (B. Zhao, personal communication).

Switchgrass is a host for numerous fungal species in the United States; 65 were catalogued by Farr et al. in 2004. Wide-spread interest in and plantings of the species likely are drivers in the more than 150 species that have been catalogued at the time of this writing (Farr et al. 2012). Most reports provide no evidence of fungal pathogenicity, although such reports, too, are increasing (as, e. g., Vanky 2004; Crouch et al. 2009; Waxman and Bergstrom 2011b).

Several Bipolaris (Cochliobolus) species have been reported on switchgrass in recent years (Krupinsky et al. 2004; Tomaso-Peterson and Balbalian 2010; Vu et al. 2011a, b; Waxman and Bergstrom 2011a). Previous reports of B. sorokiniana (Sacc.) Shoemaker indicate this fungus is widespread and attacks a wide range of grasses (Braverman 1986; Sivanesan 1987; Roane and Roane 1997; Gravert et al. 2000; Farr et al. 2004). The recent reports of spot blotch attacks by B. sorkiniana on switchgrass come from diverse regions of North America including Mississippi, New York, and Tennessee (Tomaso — Peterson and Balbalian 2010; Waxman and Bergstrom 2011a; Vu et al. 2011b) and studies by Vu et al. (2011b) suggest the disease is seed borne. Whether these recent, geographically dispersed accounts of B. sorkiniana infestations reflect the natural or anthropogenic spread of this disease—or simply a greater interest given the relevance to expanded plantings—is unclear.

Leaf spot caused by B. oryzae has also been reported for switchgrass grown in North Dakota (Krupinsky et al. 2004) and the fungus was suggested as the causative agent for disease observed in West Virginia (Belesky and Fedders 1995), again demonstrating the widespread nature of these diseases. Spot blotches from Bipolaris spp. generally cause moderate to severe leaf tissue damage and Zeiders (1984) suggested these have potential to be the most important switchgrass diseases in future. Although fertility (low soil phosphorus) was empirically associated with the B. oryzae outbreak observed by Zeiders (1984), genetic variability for resistance was also evident among switchgrasses. Such resistance may be of increasing importance with further domestication of switchgrass and expansion of plantings.

Disease-related yield declines have been reported in extensive, long­term plantings in southern Iowa, USA (Gravert et al. 2000). A smut caused by Tilletia maclagani (Berk.) G. P. Clinton was found in 15 of 17 fields surveyed and considered the likely cause of yield reductions. Fifty to 82% of the area in production was infested with the fungus and the relationship between percent of smut-infected tillers and overall yield reduction was very close. Subsequent yield loss estimates due to T. maclaganii in 10 Iowa fields ranged from 1.7 to 40.1%, with 38 to 82% reductions in tiller mass (Thomsen et al.

2008) . T. maclaganii outbreaks have also occurred in New York and Texas (Carris et al. 2008; Layton and Bergstrom 2011).

Switchgrass seeds coated with fungicides have been used in humid climates to increase seedling emergence. It is unclear, however, if fungicide application limits the symbiotic relationship between switchgrass and arbuscular mycorrhizal fungi (Parrish and Fike 2005). Interrante et al. (2011) reported that in Oklahoma and Georgia USA, Proceed [prothioconazole + tebuconazole + metalaxyl] fungicide mixture applied alone did not improve switchgrass seedling counts or establishment.

Reports of viral infections of switchgrass are more limited than for fungal diseases. Panicum mosaic virus (PMV) was first reported in switchgrass in 1957 (Sill and Pickett 1957) but nearly all subsequent research on PMV disease effects have studied the virus as a disease agent in other species. Switchgrass has susceptibility to some strains of barley yellow dwarf virus (Garrett et al. 2004), which are transmitted by aphids. A range of switchgrass populations (near wild-type to highly selected cultivars) was tested to determine the factors most associated with infection. Interestingly, plant growth rate was a stronger predictor of aphid-transmitted virus disease susceptibility than foliar digestibility (Schrotenboer et al. 2011). The authors suggested that selection for biofuel crops should account for increasing virus susceptibility to avoid potential disease vulnerability. In a similar vein, Thomsen et al. (2008) noted that research on management approaches to deal with such issues are of critical importance for the long­term success of switchgrass for biomass production systems.