Genetic resistance of switchgrass to rust evaluated in a composite upland x lowland population in lab and field settings

Authors: Edme, Serge; Palmer, Nathan - Nate; Sarath, Gautam; MUHLE, ANTHONY - Former ARS Employee; Mitchell, Robert - Rob; YUEN, GARY - University Of Nebraska

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2022
Publication Date: 12/10/2022
Citation: Edme, S.J., Palmer, N.A., Sarath, G., Muhle, A., Mitchell, R., Yuen, G. 2022. Genetic resistance of switchgrass to rust evaluated in a composite upland x lowland population in lab and field settings. Agronomy. 12,3137. https://doi.org/10.3390/agronomy12123137.
DOI: https://doi.org/10.3390/agronomy12123137

Interpretive Summary: Switchgrass is being developed as a forage and bioenergy crop for the U.S. However, rust, caused by a fungus named Puccinia novopanici, threatens the ability of future producers to obtain high biomass yield and quality of switchgrass. Genetic resistance to rust is a critical tool to control the disease. This study investigated to what degree genetic factors are able to control and prevent rust symptoms in populations of switchgrass. Two generations of a switchgrass population were studied in the greenhouse in 2020 and the field in 2021 and 2022, near Mead, NE. A rating going from 0 (no rust) to 4 (50+ of leaf area covered) was applied to discriminate families and progeny for resistance or susceptibility to fungal infections. The study detected ample and significant genetic variation in the switchgrass populations and satisfactory progress can be made by selection when analyzing the data collected in 2021, but not in 2022. In 2022, Nebraska experienced severe drought and high heat conditions, which were not conducive to the development of the rust fungus on the plants. This study suggested that reducing the incidence of rust in switchgrass may be readily attainable through breeding.

Technical Abstract: Maintaining low levels of rust incidence (caused by Puccinia novopanici) in switchgrass (Panicum virgatum L.) breeding populations is a priority for the USDA-ARS program engaged in improving cultivars for high biomass yield and quality. Essential to this goal is the unbiased and accurate estimation of genetic parameters to predict the merits of parents and progeny. Spores of the fungus were inoculated in greenhouse-grown seedling progeny of 31 halfsib families in generation 2 (Gen 2) of a composite Summer x Kanlow population for evaluation of rust incidence on the leaves with a 0-9 rating scale. Two parents were later chosen to cross and develop a linkage mapping population as Gen 3. The Gen 2, 3, and Kanlow seedlings were transplanted into the field in early June 2020 located near Mead, NE and laid out as a replicated row-column design with six blocks of single-row plots of 5 plants each. The field trial was rated in September 2021 and 2022 with a 0-4 scale. Lab and field data were subjected to univariate linear mixed models via the restricted maximum likelihood to extract the variance components needed to predict the breeding values. The additive genetic variation was substantial (P<0.01), enough to result in high heritability estimates ranging from 0.42±14 to 0.73±0.09 at the individual and family-mean levels. This result implies that rust resistance is under strong genetic control to use mass selection for obtaining satisfactory gains. A possible rust-incidence x year interaction was detected with a Spearman correlation of breeding values of -0.38, caused by significant rank changes of the Gen 3 genotypes in 2022 (a high heat and drought year). Genetic gains were predicted to reduce rust incidence scores by at least 2 points on the rating scale when selecting backwards and by 1 point when selecting individual candidates as parents of the next generation. Faster gains (31 and 59%) were realized relative to the second generation by respectively selecting the top 10% of the families in Gen 3 or the top 10% of genotypes within this group.