Location: Crop Diseases, Pests and Genetics Research
2019 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 support of Sub-objective 1A, six biparental table grape populations were developed. These populations segregate for rachis architecture, flower time, fruit quality and berry size. Populations were transplanted to the field and are being maintained and trained for fruit production and evaluation in year two. For individuals in populations with fruit in year one, initial evaluations were performed for cluster differences. Existing populations (11-3527) are continuing to be evaluated for cluster architecture, berry size/shape, and fruit development.
In support of Sub-objective 1B, 16 advanced table grape selections and six competing cultivars were evaluated in three fruit showings conducted during the harvest season. Interested growers and stakeholders participated in the evaluations to identify those selections worthy of continuing in the effort to identify new cultivars with fewer cultural inputs necessary to produce a commercial crop.
A major source of grapevine resistance to Botrytis cinerea was identified in support of Sub-objective 1C. Hybridizations were made with this source of resistance and the F1 progeny planted into field plots. Evaluations for cluster susceptibility to Botrytis were initiated this spring and will continue throughout fall of 2019. Field inoculations and evaluations will be repeated in subsequent years.
Grape populations segregating for resistance to powdery mildew (PM) are currently being evaluated for fruit quality and harvest timing. In support of Sub-objective 2A, these populations are also being evaluated for susceptibility to PM on leaves, stems, rachis and berries. These populations were derived from PM resistant female-flowered wild Vitis species (amurensis, cinerea, riparia and vulpina) and high quality V. vinifera table grapes Fresno Seedless and Valley Pearl. Vines are evaluated in the field twice a year (one month after berry set and after fruit harvest) to determine PM susceptibility in the various plant organs.
In support of developing Pierce’s disease (PD) resistance, as part of Sub-objective 2B, green and dormant cuttings were collected from known and putative sources of resistance. Sufficient numbers of materials (approximately 300 plants per genotype) are still being generated to assess durability of resistance to multiple isolates of the pathogen. Two biparental crosses were made using a previously unidentified source of Xylella fastidiosa resistance, and seedlings planted into the field. Another grape mapping population that was developed for identifying molecular markers linked to PD tolerance is now being evaluated for fruit/crop quality in support of Sub-objective 2C. Vines found to have fruit quality inferior to parental PD-tolerant vine BD5-117 are being rogued from the population. BD5-117 is an early generation PD-tolerant cultivar developed in the southeast U.S., and distinctly different germplasm than V. arizonica-based hybrids for which a molecular marker linked to PD tolerance has been developed. Through empirical screening, it was determined that vines in this population segregate for bacterial titer and expression of PD symptoms; however, no molecular markers were ever identified that were linked to PD tolerance or susceptibility. Hence, vines exhibiting improved fruit and crop qualities in the BD5-117 population will be used as parental germplasm in future hybridizations with V. arizonica-based PD-tolerant grapes.
Numerous self-fertile almond selections have been identified over the last decade that would be suitable parents in planned hybridizations. The California almond industry desires early-ripening cultivars having high yields of undamaged Nonpareil-shaped kernels. In support of Sub-objective 3A, hybridizations have been performed among self-fertile selections having Nonpareil-like kernels.
Multivariate kernel analyses are being used to identify new selections with Nonpareil-shaped kernels. A research-sized roller/cracker provides data on kernel durability at harvest.
Several glabrous-skinned apricots were collected from Central Asia and imported to the U.S. in the early 1990s. These accessions were hybridized with California-adapted apricots as soon as the young trees became reproductively fit. Glabrous-skinned apricots were not identified in the initial crosses, but when these first-generation trees were crossed amongst themselves, 25% of the offspring produced glabrous-skinned fruit. In support of Sub-objective 3B, trees with glabrous-skinned fruit are being evaluated for fruit quality characteristics and ranked for use as parents. Of particular concern is fruit size and detrimental skin characteristics. The initial glabrous-skinned plant introductions were small-fruited, being only 20–25 gram fresh weight. Compared to the neutral-flavored skin that predominates pubescent-skinned apricots, glabrous-skinned apricots can exhibit both acidic or astringent skin flavors. Current fruit evaluations of the glabrous-skinned accessions will identify the largest-fruited accessions having neutral skin flavor for use in planned crosses.
Accomplishments
1. Solbrio: making table grape growing easier. While consumers may find that table grape varieties are diverse in appearance, flavor and ripening season, growers know that varieties are also diverse in the cultural treatments necessary to produce quality table grapes. ARS researchers in Parlier, California, have released a new early-season table grape named Solbrio that has both exceptional eating quality and reduced cultural input needs. Consumers have shown a strong interest in the new variety because of its large berry size, crisp and crunchy flesh and fully colored berries. Many of the standard cultural practices used to enhance characteristics of other table grape varieties were evaluated on the new variety, but none of the treatments produced a significant change on berry quality or yield. Hence, growers are very pleased with the new variety that is both easy and inexpensive to grow.
Review Publications
Ledbetter, C.A., Lee, S.A. 2018. Diversity of Xylella fastidiosa host suitability among siblings from a non-traditional almond X peach cross. Euphytica. 214:84. https://doi.org/10.1007/s10681-018-2167-6.
Baumgartner, K., Fujiyoshi, P.T., Ledbetter, C.A., Duncan, R., Kluepfel, D.A. 2018. Screening almond rootstocks for sources of resistance to Armillaria root disease. HortScience. 53(1):4-8. https://doi.org/10.21273/HORTSCI12038-17.
Ledbetter, C.A., Lee, S.A. 2019. Maintaining product quality in raisins throughout the introgression of powdery mildew resistance. Journal of Horticultural Science and Biotechnology. 94(4):460-467. https://doi.org/10.1080/14620316.2018.1561213.
