Location: Crop Diseases, Pests and Genetics Research
2022 Annual Report
Objectives
The long-term goal of this project is development and introduction of new, high-quality and disease resistant cultivars of Prunus (almond and apricot) and Vitis (table grapes and raisins) that will sustain American agriculture and supply high quality, nutritious fruits and nuts to U.S. consumers and international markets. Further, the project will strive to identify molecular markers linked sufficiently close to fruit quality traits of interest such that marker-assisted selection will be possible in future breeding efforts. Specifically, during the next five years we will focus on the following objectives:
Objective 1: Enhance breeding efficiency for table grape fruit quality, and other priority traits, by identifying associated molecular markers, and through trials, to determine commercial acceptability of advanced table grape selections.
Sub-objective 1A: Develop segregating populations and map fruit quality traits related to flowering time, rachis architecture, and berry size.
Sub-objective 1B: Through trials, determine commercial acceptability of advanced table grape selections.
Sub-objective 1C: Identify sources of resistance and develop molecular markers associated with resistance to Botrytis cinerea.
Objective 2: Develop durable resistances to powdery mildew and Pierce’s disease in table grapes and natural dry-on-vine raisins.
Sub-objective 2A: Develop durable resistance to powdery mildew in table grapes and natural dry-on-vine raisins.
Sub-objective 2B: Identify sources of durable resistance to Pierce’s disease and determine the relative susceptibility of existing commercial cultivars.
Sub-objective 2C: Develop table grape and natural dry-on vine raisin cultivars with durable resistance to Pierce’s disease.
Objective 3: Develop, select, and evaluate new, high quality scions of Prunus, e.g., high yielding self-compatible almond and glabrous-skinned apricot.
Sub-objective 3A: Develop, select, and evaluate new high-yielding self-compatible almonds.
Sub-objective 3B: Develop, select, and evaluate new glabrous-skinned apricots.
Approach
Classical breeding has been used to create segregating populations in Prunus and Vitis where the expression of quantitative traits has been concentrated and newly available characters have been transferred into adapted germplasm. New segregating populations will be created in Vitis to develop molecular markers for fruit quality traits, rachis architecture, resistance to Botrytis, flowering time and berry size. Advanced table grape selections will be compared for production timing and fruit quality after cold storage with existing table grape cultivars through public fruit showings held during each ripening season. Commercially acceptable advanced table grape selections will be introduced as new cultivars through consensus evaluation with the table grape industry.
New powdery mildew resistance sources will be evaluated in established segregating populations, and resistant accessions will be backcrossed with high quality table grapes and natural dry on the vine raisins as molecular markers are being developed. These new resistance sources will be used along with other mapped PM resistance sources in hybridizations designed to stack the resistances for durability. Empirical screening of Vitis germplasm for reaction to Xylella fastidiosa will continue, and existing commercial table grape cultivars will be evaluated for their relative susceptibility to Pierce’s disease. Crosses will be conducted to stack Xf resistances from Vitis arizonica and southeast U.S. Vitis germplasm into hybrids with high product quality. Promising high quality accessions with will be evaluated for survival and productivity in regions with high Pierce’s disease pressure.
In Prunus, hybridizations will be performed to identify and select new high-yielding self-compatible almonds that are California-adapted and have Nonpareil-like kernel characteristics. Apricot populations will be developed through hybridization among glabrous-skinned accessions, and new glabrous apricots will be evaluated for fruit quality and productivity. Newly-available glabrous-skinned apricot accessions from Kyrgyzstan will be propagated when available from plant protective quarantine and used in hybridizations to assist in the glabrous-skinned apricot breeding effort.
Progress Report
Due to infection by a vector-transmitted virus, all of the segregating populations that were developed and established for fruit quality traits including flowering time, berry shape, size, texture, color and bud break had to be destroyed to protect the remaining grapevine germplasm collection in Parlier. Similarly, the grapevine population that segregated for development of resistance to Botrytis was destroyed. Field evaluations were carried out for Powderly mildew (PM) resistance in a grapevine population with Vitis cinerea genetic background. In support of Objective 2, for both table and raisin grapes, hybridizations were carried out to develop lines that have more than one resistance locus and could provide durable resistance to powdery mildew and Pierce’s disease. Female flowered vines with combination of Run1Ren1Ren4 loci have been used as females in backcrosses with high quality table grapes and natural dry-on-vine raisin selections. Pollen from other perfect-flowered Run1Ren1Ren4 vines have been collected and shared with Vitis breeders in Minnesota and New York for use with hybrids having different combinations of powdery mildew resistance genes. Pollen from Run1Ren1Ren4 hybrids was used in hybridizations with powdery mildew resistant selections of Vitis amurensis, in which a new locus for powdery mildew resistance has been identified and named “Ren12”. Relative to increasing durability of resistance to Pierce’s disease, hybridizations were made to combine the two different Pierce’s disease resistance sources. Pierce’s disease-resistant vines with Vitis arizonica background were used as seed parents in hybridizations with Pierce’s disease-resistant vines with a southeast U.S. background.
Accomplishments
1. Grapevines resistant to powdery mildew. Estimates indicate the potential economic benefit of powdery mildew (PM)-resistant varieties of grapes to producers in California can reach $48 million per year because nearly all grape varieties are highly susceptible to PM. ARS scientists in Parlier, California, and Geneva, New York, collaborated to discover genetic regions responsible for two mechanisms of PM resistance in grapevines. Traditional breeding was combined with molecular approaches using DNA markers to introduce the different PM resistance modes into table grapes. Additional grape breeding and selection will augment the impact of this research at the national level where in the case of table grapes PM resistance would enable 90% reduction in pesticide use, which represents approximately $287 production savings per acre per year.
2. Castlecrest apricot variety release. Nearly 75% of the apricots grown in the United States come from California. ARS scientists in Parlier, California, developed a new mid-season apricot variety that is suitable for fresh market production. Evaluations of the new variety in comparison with other varieties were carried out for three years. The fruit of Castlecrest is larger, has higher Brix (sugar content) and finer texture, and is juicer than other varieties, which provides a significant improvement for fresh eating quality.
Review Publications
Karn, A., Zou, C., Brooks, S., Fresnedo-Ramirez, J., Gabler, F., Sun, Q., Ramming, D.W., Naegele, R.P., Ledbetter, C.A., Cadle Davidson, L.E. 2021. Discovery of the REN11 locus from Vitis aestivalis for stable resistance to grapevine powdery mildew in a family segregating for several unstable and tissue-specific quantitative resistance loci. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2021.733899.
Krugner, R., Rogers, E.E., Burbank, L.P., Wallis, C.M., Ledbetter, C.A. 2022. Insights regarding resistance of ‘Nemaguard’ rootstock to the bacterium Xylella fastidiosa. Plant Disease. 106(8):2074-2081. https://doi.org/10.1094/PDIS-01-22-0136-RE.
Ledbetter, C.A. 2021. 'Yorizane': A new self-compatible almond cultivar suitable for California production. HortScience. 56(9):1142-1143. https://doi.org/10.21273/HORTSCI16066-21.
Chen, J., Ledbetter, C.A., O'Leary, M.L. 2022. Complete genome sequence of Curtobacterium sp. strain TXMA1, isolated from a grapevine in Texas, USA. Microbiology Resource Announcements. 11(1). Article e00968-21. https://doi.org/10.1128/mra.00968-21.
