1a. Objectives (from AD-416)
The long-term objective of this project is to improve the genetics of peanut for disease resistance and the oleic acid content of oil. Objective 1. Develop peanut germplasm that is high-oleic in nature with improved resistance to Sclerotinia blight and southern blight. Objective 2. Develop molecular markers for peanut associated with resistance to Sclerotinia blight and southern blight. Objective 3. Develop improved methodology to characterize the reaction of Sclerotinia minor and Sclerotium rolfsii on inoculated peanut germplasm and breeding lines under greenhouse conditions.
1b. Approach (from AD-416)
Parental lines being used in such crosses include Arachis hypogaea L. cultivars, advanced breeding lines, and plant introductions (PIs) with demonstrated Sclerotinia disease resistance and high oleic acid content. New and existing potential parent lines with high oleic acid content are continually tested in the greenhouse and field plots for resistance to Sclerotinia blight and southern blight and are readily available for use in the peanut breeding program. Included in our annual screening of germplasm for disease resistance are cultivars, breeding lines, and germplasm accessions. Also, collaborators include the curator of the U.S. peanut germplasm collection as well as other breeders who are continually evaluating accessions for value added traits. Molecular markers for Sclerotinia resistance will be identified and verified by phonotypic reaction.
3. Progress Report
The long-term objective of this project is to improve the genetics of peanut for disease resistance and the oleic acid content of oil. Substantial progress has been made toward that end. Advanced breeding lines with improved genetics for the high oleic trait as well as resistance to Sclerotinia blight are now in their second year of performance trials. Analysis of the results of year 1 trials allowed the identification of breeding lines for further advancement, and also resulted in the removal of poor performing lines from the program. Additionally, we developed a new non-destructive technique using capillary electrophoresis to determine the concentration of oleic and linoleic acid in a single peanut seed. This technique will allow rapid screening of early generation breeding lines for those individual seed possessing the desired high oleic acid trait. Progress was also made toward placing the molecular marker for Sclerotinia blight resistance on the tetraploid peanut map. Genotypic and phenotypic data from one mapping population was gathered and another population is currently being analyzed. The marker was also used to examine members of the US Peanut Germplasm Mini-Core Collection for possible new sources of resistance to Sclerotinia blight. Progress was also made on defining the post inoculation relative humidity for Sclerotinia minor. This is important in improving our inoculation method to identify disease resistance under greenhouse conditions. Also, progress was made to show that stem inoculation with S. minor is more reliable to predict physiologic resistance in peanut.
1. Molecular Marker for Sclerotinia Minor Resistance in Peanut Identifies New Potential Sources for Resistance: The US Peanut Mini-Core Collection is composed of approximately 100 peanut plant introductions (PIs) that are representative of the entire US Peanut Germplasm Collection (composed of over 9000 peanut PIs). Using a marker that identified peanut lines with potential resistance to Sclerotinia blight, the peanut mini-core collection was examined. Thirty-nine PIs, spanning runner, Valencia, and Spanish market-types, were identified as potential new sources of Sclerotinia blight resistance. Since no reports of field evaluation of the mini-core collection for Sclerotinia blight resistance or susceptibility are available to aid in further validation of this marker as a selection tool for breeding programs, field trials are currently underway toward that end. However, the results obtained in this study have identified PIs worthy of such evaluation in the field for S. minor resistance and also identified those PIs with low probability of being a source of resistance, reducing the amount of field work required to test the mini-core collection by 34% for the runner, Valencia, and Spanish market type accessions. This accomplishment is important because it has allowed for improved efficiency in the breeding of peanut for resistance to Sclerotinia blight and has identified new sources of possible disease resistance.
2. Non-destructive Method Developed for Determining Oleic and Linoleic Acid Concentration from Individual Seed: We developed a new technique using capillary electrophoresis to determine the concentration of oleic and linoleic acid in a single peanut seed. This method is important because it does not destroy the peanut seed and allows a peanut breeder to use the regenerated plant for breeding purposes if the seed contains the desired traits. Gas chromatography, the conventional method for determining oil content of seed, requires that the seed be destroyed in the process. This technique will allow rapid screening of early generation breeding lines for those individual seed possessing the desired high oleic acid trait. This accomplishment is important because the method will allow scientists to screen for value-added genetic traits without spending the time and resources to grow the plants in the greenhouse or field first and will avoid the extreme costs of sending numerous samples to quality labs for testing.
3. Advanced Breeding Lines Identified for Potential Variety Release: The peanut industry in the Southwestern US is in great need of high oleic, Sclerotinia blight-resistant peanut varieties suitable for commercial production. We have identified advanced breeding lines with these characteristics that are possibly suitable for variety release. Disease and agronomic trait screening of 90 advanced F7 peanut breeding lines was completed during the 2008 growing season. Seventy-six lines were advanced to the F8 generation for similar testing in 2009, after which superior lines of the same genetic background will be bulked for performance trials and the national Uniform Peanut Performance Trials (UPPT) in 2010. This accomplishment will enable us to identify three outstanding peanut breeding lines being considered for release.
4. Defined the post inoculation relative humidity on the infection of peanut with Sclerotinia minor: In this study, peanut plants were kept under different intervals of 100% relative humidity after inoculation with S. minor in order to determine the length of time required to achieve infection. We determined that under relative humidity of 100% it takes 24 hours to initiate successful infection and normal expansion of lesions caused by S. minor. This accomplishment is important because it will allow a more realistic reaction to quantify disease under laboratory and greenhouse conditions for acceleration of progress in our peanut breeding program.
5. Compared inoculation of peanut leaflets with that of stem inoculation to quantify disease reaction of S. minor on Sclerotinia-resistant and susceptible peanut entries: In this study, limited stems or leaflets of peanut plants (from two entries: susceptible Okrun and resistant Tamspan 90) were inoculated with S. minor and evaluated for their reaction based on rate of lesion expansion. Stem inoculation were superior to leaflet inoculation in identifying physiologic resistance in peanut to S. minor based on rate of lesion expansion. This accomplishment is important because it showed the reliability of stem inoculation to predict the presence of physiologic resistance when testing breeding progeny to advance the peanut breeding program.
Chenault, K.D., Maas, A.L., Damicone, J.P., Payton, M.E., Melouk, H.A. 2009. Discovery and characterization of a molecular marker for Sclerotinia minor (Jagger) resistance in peanut. Euphytica. 166:357-365.