Characterization of the Rust Fungus, Puccinia emaculata, and Evaluation of Genetic Variability for Rust Resistance in Switchgrass Populations

Abstract

Several fungal pathogens have been identified on ornamental and native stands of switchgrass (Panicum virgatum L.). Diseases of switchgrass, particularly rust, have been largely neglected and are likely to become the major limiting factor to biomass yield and quality, especially when monocultured over a large acreage. Based on teliospore morphology and internal transcribed spacer-based diagnostic primers, the rust pathogen collected from switchgrass research fields in Oklahoma was identified as Puccinia emaculata. Furthermore, to identify genetically diverse source(s) of rust resistance, several switchgrass genotypes from both upland (cv. 'Summer' and 'Cave-in-Rock') and lowland (cv. 'Alamo' and 'Kanlow') ecotypes were evaluated in Ardmore, Oklahoma during 2008 and 2009 and in growth chamber assays. Field and growth chamber evaluations revealed a high degree of genetic variation within and among switchgrass cultivars. In general, Alamo and Kanlow showed moderate resistance to P. emaculata, while Summer was highly susceptible. Distinct ecotypic variations for reactions to rust were also prevalent with the lowlands maintaining a high level of resistance. These results suggest the potential for improvement of rust resistance via the selection of resistant individuals from currently available cultivars. Further, the selection pressure on the pathogen would also be reduced by employing several rust resistant cultivars in production-scale situations.

The Genetic Basis of Upland/Lowland Ecotype Divergence in Switchgrass (Panicum virgatum)

“…Upland and lowland ecotypes of switchgrass are characterized by divergence in a variety of traits including differences in flowering time, growth architecture, physiological characteristics, and disease susceptibility (McMillan 1964, 1965, 1967; Casler et al 2011; Uppalapati et al 2013). Most traits we measured showed the expected trait differentiation between the upland and lowland ecotypes based on the mean values of each of the grandparents, including height (lowland > upland), tiller mass (lowland > upland), SLA (upland > lowland), rust resistance (lowland > upland), flowering time (lowland > upland), and leaf number (lowland > upland) (Figure S2).…”

“…It is well known that the lowland switchgrass ecotype is more resistant to rust than the upland ecotype (Hopkins et al 1995; Uppalapati et al 2013). Fungal pathogens are generally moisture- and temperature-sensitive and often require high relative humidity for infection and sporulation (Harvell et al 2002).…”

“…Numerous other transplant and field trials have demonstrated phenotypic and physiological differences between the two ecotypes (Wullschleger et al 1996; Casler and Vogel 2004; Cassida et al 2005a,b; Barney et al 2009; Yang et al 2009; Cortese et al 2010). For example, resistance to rust fungus infection has been found to be heritable and lowland populations are typically more resistant than upland populations (Eberhart and Newell 1959; Uppalapati et al 2013). …”

Identification, characterization, and gene expression analysis of nucleotide binding site (NB)-type resistance gene homologues in switchgrass

“…The causal agent of switchgrass rust, Puccinia emaculata Schw., is widespread and has been reported in Tennessee [7], Arkansas [8], Virginia [9], and Mississippi [10]. The majority of switchgrass cultivars, including lowland and upland ecotypes, have been shown to be moderate to highly susceptible to this rust pathogen [11, 12]. …”

Multi-Year Pathogen Survey of Biofuel Switchgrass Breeding Plots Reveals High Prevalence of Infections by Panicum mosaic virus and Its Satellite Virus

“…Current switchgrass cultivars deployed in the field for forage and erosion control are typically described as tolerant to or unaffected by most types of microbial pathogens (Bouton 2007;Vogel 2004), yet our knowledge of pathogens of switchgrass is limited. Several species of plant-pathogenic fungi including Bipolaris oryzae (leaf spot), B. spicifera (spot blotch and root rot), Colletotrichum graminicola (leaf spot), C. navitas (anthracnose), Elsinoe panici (leaf spot), Puccinia emaculata (rust), Rhizoctonia cerealis (sharp eyespot), Tilletia pulcherrima (bunt), and Uromyces graminicola (rust) have been isolated and identified as disease-causing agents of switchgrass (Carris et al 2008;Crouch et al 2009;Etheridge et al 2001;Krupinsky et al 2004;Tiffany and Knaphus 1995;Uppalapati et al 2013;Vu et al 2011). Thus far, confirmed reports of plantpathogenic bacteria and nematodes of switchgrass are limited or circumstantial (Cassida et al 2005;Mekete et al 2010).…”