Location: Crop Diseases, Pests and Genetics Research
2020 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
In completion of Sub-objective 1A, 1,000 images of grape clusters were collected from 150 grapevines during the 2019 fruit season. Mature vines were evaluated for flowering time and images were collected to evaluate rachis architecture and berry shape/size. Leaf samples from two segregating populations were collected and genotyped using rhAmpSeq. Genetic maps of both populations have been constructed. Flowering time data (2016-2019) has been formatted both as calendar days and growing degree days for quantitative trait locus (QTL) mapping. QTL mapping has identified significant associations with flowering, veraison, full color time and other fruit quality traits. Additional samples from genetically different vines with known fruit quality traits have been collected to confirm identified QTL markers in unrelated populations. Images from the 2019 season were submitted to ARS in Kearneysville, West Virginia, for image analyses.
In support of Sub-objective 1B, 10 new table grape selections were identified in seedling blocks during the 2019 fruit season that had sufficient fruit quality and production characteristics to warrant placement in the advanced trial block. Propagation materials were collected from the mother vines during dormancy for clonal rooting to be established for future trials.
In support of Sub-objective 1C, a segregating population derived from a V. labrusca x V. vinifera cross is maintained in the field. Five clusters from each vine were inoculated with a mixed spore suspension of B. cinerea isolates for evaluating resistance potential under field conditions.
For Sub-objective 2A, grape progenies derived from Run1Ren1 x Ren4 hybridizations have now been field evaluated for fruit and crop qualities. Weak vines and those lacking powdery mildew resistance have been eliminated from the populations. Vines with all three powdery mildew resistance genes have been identified. Evaluations during bloom periods have allowed identification of numerous reflex-flowered vines including three male sterile Run1Ren1Ren4 hybrids that are currently being used for breeding.
Table grape cultivars were evaluated for relative susceptibility to Pierce’s Disease (Sub-objective 2B). Cuttings from known and novel sources of resistance to Pierce’s disease have been propagated (~ 200 individuals for each genotype) and are in the greenhouse for testing against 20 Xylella fastidiosa isolates. Xylella isolates representing the genetic and geographic distribution diversity of California were activated by inoculating grape seedlings and recovered isolates are being propagated for inoculations.
Hybridizations during the 2020 bloom included crosses between Pierce’s disease tolerant Southeast U.S. grape selections and similarly tolerant selections derived from B43-17 (V. arizonica-based tolerance) in support of Sub-objective 2C. Crosses were made to develop both new table grape and raisin progenies. Crosses involved both seed cultured accessions and combinations requiring embryo culture to rear embryos from stenospermic ovules.
In support of Sub-objective 3A, almond seedlings derived from hybridizations among self-fertile almond accessions have been screened for the presence of the self-compatibility gene. Self-incompatible seedlings were identified and removed prior to field planting the self-compatible seedlings.
In support of Sub-objective 3B, glabrous-skinned apricot germplasm has been evaluated for fruit quality characteristics to identify elite individuals for use in hybridizations. Specifically, larger-fruited accessions and accessions with neutral-flavored (non-acidic) skin were identified for use as parents in planned crosses. The glabrous-skinned accessions have been hybridized with F1 pubescent-skinned heterozygotes (carrying the glabrous gene).
Accomplishments
1. Marginal soils in western San Joaquin Valley may now prove valuable with new ‘FXA’ rootstock. In the southern San Joaquin Valley of California, there are over 1.3 million acres of salt-affected and boron-laden soils that are currently unsuitable for almond production. However, a recent rootstock release by ARS researchers in Parlier, California, may soon render the affected acreage usable for almond culture. The new rootstock, a hybrid of Flordaguard and Israeli alnem named FXA, has performed well in several rootstock trials established on boron-laden or salt-affected soils. Almond scions grown on FXA rootstock accumulate less chloride in vegetative tissues than when alternative rootstocks are used. Similarly, FXA-grown almonds accumulate lower levels of boron in hull tissue as compared with alternative rootstocks when cultured in boron-laden soils. These factors lead to healthier and more productive trees under marginal growing conditions.
Review Publications
Ledbetter, C.A. 2019. ‘Solbrio’ table grape. HortScience. 54(10):1864-1865. https://doi.org/10.21273/HORTSCI14311-19.
Naegele, R.P., Delong, J.A., Saito, S., Xiao, C. 2020. Population genetics and fungicide resistance of Botrytis cinerea on Vitis and Prunus spp. in California. Phytopathology. 110(3):694-702. https://doi.org/10.1094/PHYTO-09-19-0362-R.
Naegele, R.P., Daane, K.M., Cousins, P. 2020. Identification of vitis cultivars, rootstocks and species expressing resistance to a Planococcus mealybug. Insects. 11(2):86. https://doi.org/10.3390/insects11020086.
