ASHS 2025 Conference

A forum for discussion of potential collaborations with regards to fruit, vegetable, and edible crops – i.e. citrus, breeding, production systems, postharvest, pomology, crop management, viticulture, etc.

Blueberry is a high value fruit crop and there is an increased demand for its cultivation and consumption all over the world. The blueberries are grown in diverse regions with varied weather, soil, biotic and abiotic stress limiting their yield. Interspecific hybridization plays a significant role in broadening the genetic base of blueberries that subsequently helps in developing improved cultivars adapted to specific regions. The genetic diversity for cultivated rabbiteye (RE) (2n=6x=72) blueberries is very narrow. Introgression from wild and other cultivated species would provide beneficial diversity for RE blueberries. Vaccinium elliottii (2n=2x=24) is an early flowering, tall and highly deciduous wild blueberry with small black fruits. Like RE, it is adapted to a wide range of soil types in the southeastern United States. On the other hand, southern highbush is a cultivated tetraploid (2n=4x=48) with high fruit quality but requires intensive soil amendment. To broaden the genetic base of blueberry breeding, interspecific crosses were made between three southern highbush breeding lines and V. elliottii. The success of these crosses was challenging due to strong triploid block from heteroploid crosses. Out of 422 pollination events only 8 triploid seedlings were produced, and their ploidy level was confirmed by flowcytometric analysis. Three seedlings were confirmed to be triploids (3x) however, it is expected that triploids are sterile. Seedlings of the triploids were established in tissue culture and chromosome doubling of these triploid hybrids was carried out to improve their crossing efficiency with cultivated blueberries. The shoots produced through tissue culture were treated with 0.02% colchicine for 48 hours and transferred to woody plant medium (WPM). Synthetic hexaploid hybrids produced from this study will be cross compatible with RE blueberries. These synthetic hexaploid hybrids will bridge the ploidy gap among diploid, tetraploid and hexaploid blueberry species.

Grapevine cold hardiness is a critical trait that determines the geographical distribution and winter survival of cultivars, thus a key consideration for viticulture in regions with severe winter conditions. This study investigated the cold hardiness of 312 Vitis riparia F1 hybrids, using differential thermal analysis (DTA) and survival assessments at 4°C over seven and 28 days. The DTA analyses to identify significant changes in progeny lethal temperature exotherm (LTE) values with a wide range of LTE values from -24.3°C to -12.3°C observed after 28 days of cold acclimation. The mean LTE value improved from -16.8°C after seven days to -18.6°C after 28 days with 80.2% of progenies showing significant decreases in LTE values. Progenies were divided into five groups based on cold hardiness: very cold hardy, cold hardy, moderately hardy, susceptible, and very susceptible. Principal component analysis (PCA) demonstrated clear separation among the five cold hardiness groups, with PC1 accounting for 71% of the variance. This study provides valuable insights into the genetic basis of cold hardiness in V. riparia progenies, offering critical information for grape breeding programs aimed at developing cold-resistant cultivars. The identification of highly cold-hardy progenies and the understanding of their genetic diversity will aid in selecting parent plants for breeding, ultimately improving grape production in cold regions.

Combining muscadine (M. rotundifolia) with bunch grapes (V. vinifera) addresses the challenges faced by each species. Muscadines provide strong disease resistance and adaptability to the southeastern U.S climate but lack key fruit quality traits such as seedlessness and crisp texture. In contrast, V. vinifera, produces high-quality fruit but is highly susceptible to diseases like Pierce’s disease and powdery mildew. Hybridization between the two aims to retain the desirable fruit characteristics of V. vinifera while incorporating the resilience of muscadines. Understanding the genetic basis of key agronomic traits in Vitis × Muscadinia (V × M) hybrids is essential for introgressing valuable traits such as seedlessness, disease resistance, and flavor into breeding populations. This study aimed to develop a high-density genetic linkage map to investigate the inheritance of seedlessness, sterility, and recombination patterns in wide hybrids. The mapping population was derived from a cross between AM-116, a seeded Muscadinia rotundifolia selection, and JB15-43-N0-25, a seedless V × M hybrid. Phenotypic evaluations of traits such as flower sex, winter injury, berry size, berry, color, yield, seedlessness, cluster architecture, and vigor were collected and integrated with genotypic data to create a linkage map and identify quantitative trait loci for these traits. RhAmpSeq genotyping was initially used as the primary method to genotype the population and assess recombination frequency across homeologous and homologous chromosome regions. However, because rhAmpSeq markers were designed using Vitis genomes without incorporating Muscadinia, marker coverage in Muscadinia genomic regions was limited. To overcome this limitation, SkimSeq whole-genome sequencing was later employed to construct a high-density linkage map. Principal Component Analysis (PCA) of genomic data revealed that individuals positioned closer to the V × M parent (JB15-43-N0-25) had a higher proportion of Vitis ancestry and an increased number of homeologous chromosomes. This pattern suggests that structural genomic incompatibilities affecting segregation distortion and recombination rates may result in reduced recombination in homeologous regions in V × M hybrids. These findings provide new insights into the genetic mechanisms governing sterility, recombination, and trait introgression in Vitis × Muscadinia hybrids, contributing to the development of breeding strategies aimed at integrating desirable traits across subgenera. Understanding these factors is crucial for overcoming genetic barriers in interspecific breeding and optimizing hybrid vigor for commercial muscadine grape production.

Eastern filbert blight (EFB) disease caused by the fungal pathogen Anisogramma anomala (Peck) E. Müller is a major threat to Oregon’s hazelnut (Corylus avellana) industry. Although cultivars with ‘Gasaway’ resistance have sustained the Oregon hazelnut industry, ‘Jefferson’ and ‘McDonald’ trees protected by a dominant allele for resistance from ‘Gasaway’ have shown small cankers when under high disease pressure for several years. In late 2023, a few orchards in the northern Willamette Valley reported infections on ‘Gasaway’ protected trees with highly aggressive cankers and fully developed stromata, indicating the ability to reproduce on resistant varieties and signaling the breakdown of ‘Gasaway’ resistance in Oregon. The OSU hazelnut breeding program has conducted extensive EFB screening efforts on curated germplasm, and more than 30 sources of EFB resistance have been assigned to a linkage group (LG) using SSR loci and disease score correlation analyses. Fine mapping of the LG7 resistance region using the ‘Ratoli’ and OSU 1166.123 backgrounds with simple sequence repeat (SSR) and PCR allele competitive extension (PACE) SNP genotyping assays narrowed the resistance region. However, gene expression profiling is desired to further elucidate which candidate genes overlap between fine-mapping efforts and gene expression to refine marker development efforts for LG7 resistance. The objective of this study was to identify differentially expressed genes (DEGs) in six hazelnut genotypes (‘Ennis’, OSU 1026.073, OSU 1166.123, OSU 541.147, ‘Ratoli’, and ‘Sacajawea’) at different stages of EFB infection. Layered and grafted trees with ‘McDonald’ rootstocks were grown in 3L pots at the OSU West Greenhouses in Corvallis, OR in spring 2023. Inoculations were conducted in an outdoor container pad and new shoot tips with at least two true leaves were spray inoculated with a spore suspension (1x106 spores·mL-1) until run-off. Apical shoots (2-3 internodes) from four biological replicates were collected at 0, 12, 24, 48, 72, 96, 168, and 240 hours post-inoculation, with the addition of two biological mocks sprayed with deionized water. Total RNA was extracted and sequenced using a BRB-seq library prep for the Illumina NovaSeq X Plus platform. Detailed analyses of differentially expressed genes between susceptible and resistant genotypes during the initial stages of EFB infection will be discussed. The results of this study will provide valuable insights into the molecular resistance of hazelnut to EFB, aid marker development for breeding EFB-resistant cultivars, and facilitate the pyramiding of R-genes in a single clonal selection for more durable resistance.

Peach fungal gummosis (PFG), caused by fungal pathogens in the Botryosphaeriaceae family, including Botryosphaeria dothidea, Lasiodiplodia theobromae, and Diplodia seriata. This disease is a major threat to Prunus production worldwide. The initial symptoms often remain latent until trees are exposed to stress, at which point it can lead to substantial yield losses and even tree mortality. Given the lack of curative fungicide options and the difficulty of managing prolonged latent infections, genetic resistance has become a central focus for integrated PFG management strategies. A key breakthrough in this effort was the identification of Botd8, a major resistance locus derived from the almond cultivar ‘Tardy Nonpareil.’ Mapped to linkage group 8, Botd8 confers dominant resistance to B. dothidea and co-segregates with the red leaf locus (Gr). Its resistance effect was validated in multiple environments and under different inoculation methods, including field, trellis, and wound inoculation. Further validation using detached stem and leaf assays confirmed that genotypes homozygous or heterozygous carrying Botd8 exhibited significantly reduced lesion development following inoculations with B. dothidea and D. seriata. However, Botd8 was not effective against L. theobromae, underscoring the need for additional resistance sources. The strong correlation between detached stem assay results and field ratings also demonstrated the utility of these rapid, non-destructive screening tools in breeding programs. To expand resistance options beyond almond-derived sources, a recent study focused on the low-chill peach landrace ‘Okinawa’. A segregating F1 population from ‘Okinawa’ × (‘Flordaguard’ × PI91459) was phenotyped for disease severity and genotyped via Genotyping-by-Sequencing. QTL analysis revealed a novel resistance locus on linkage group 6, distinct from Botd8. This QTL consistently explained 29–41% of the phenotypic variation across two years, suggesting a polygenic basis for resistance in peach. Notably, the Botd8 haplotype was common in high-chill cultivars but absent in low-chill germplasm, positioning ‘Okinawa’ as a valuable resistance source for subtropical breeding efforts. Collectively, these studies establish a comprehensive framework for developing PFG-resistant peach cultivars

As a native species in the southeastern U.S., rabbiteye blueberry (Vaccinium virgatum Aiton) is known for its resilience to environmental stresses and adaptability to various soil types. Rabbiteye was the dominant species grown in the southern U.S. before the rise of southern highbush blueberries (V. corymbosum L. interspecific hybrids). Compared to southern highbush, rabbiteye cultivars are often perceived as seedy and have thick skin. Limited genetic diversity, compounded by repeated use of elite parents and decreased breeding efforts, poses challenges for rabbiteye improvement. To facilitate more efficient rabbiteye breeding in the future, this study aims to assess the genetic diversity of rabbiteye cultivars through pedigree analysis and evaluate the impact of interspecific hybridization on modern cultivars. The pedigree data of 180 rabbiteye blueberry accessions were analyzed to calculate inbreeding coefficients, coefficients of coancestry and the genetic contribution of founders. Five founders ‘Myers’, ‘Black Giant’, ‘Ethel’, ‘Clara’, and ‘W4’, collectively account for a minimum of 73% of the genetic composition of rabbiteye cultivars. The level of interspecific hybridization has been limited in rabbiteye breeding, with V. corymbosum contributing 3% of the genetic composition of rabbiteye cultivars and 1% from other species such as V. constablaei and V. darrowii. The narrow genetic base and repeated use of common parents led to an increase in inbreeding coefficients, from an average of less than 0.0002 before 1980 to 0.014 in the 2000s. In the future, it would be valuable to increase the use of exotic materials from other species or within the rabbiteye germplasm to broaden the genetic base while introducing valuable alleles. Further evaluation of population structure with genomic information is needed to validate pedigree information and provide a more accurate assessment of the genetic diversity and population structure.

The development of Vitis × Muscadinia (V x M) wide hybrids, combining the disease resistance of muscadines (M. rotundifolia) with the berry quality attributes of V. vinifera grapes has be en a longstanding ambition of breeders. Stenospermocarpic seedless muscadine hybrids and wine grapes carrying powdery and downy mildew resistance introgressed from muscadines have been developed using classical techniques without the assistance of molecular tools through sustained effort over a century. However, progress has been impeded by differences in chromosome number (Vitis=38 chromosomes and Muscadinia=40 chromosomes) and genetic distance between the subgenera. A major objective of the new Vitis x Muscadinia Specialty Crop Research Initiative project is to develop and deploy new genotyping platforms and chromosome painting visualization tools to help grape and muscadine breeders track introgressed chromosomal regions in wide hybrids and accelerate trait discovery and cultivar development in new V x M hybrids. In order to develop a chromosome-painting visualization tool for V x M hybrids, we first conducted a large-scale population diversity analysis using historical data from the rhAmpSeq Vitis core panel. Structure analysis was performed to identify individuals belonging to Muscadinia and five additional subpopulations within subgenus Vitis. Approximately 33,000 marker haplotypes were identified as unique to Muscadinia and 671,000 haplotypes were identified as present only in Vitis. This haplotype information was used to create automated chromosome painting visualizations of introgressed segments in historical and modern V x M hybrids. These visualizations allow us to identify which Vitis-type hybrids carry muscadine introgressions in known resistance loci on chromosomes 12 (Run1/Rpv1), 18 (Run2/Rpv2), and 14 (Ren5) and which Muscadinia-type hybrids have introgressions associated with stenospermocarpy and other fruit-quality alleles from Vitis. This information will assist grape and muscadine breeders in planning crosses and discovering new disease resistance and fruit quality alleles. In the coming months, we intend to extend this strategy to a new low-cost DArTag genotyping panel that has been designed to amplify polymorphic markers in both subgenera. Breeders will then be able to rapidly assess which chromosomal regions and major-effect fruit quality and disease resistance alleles are present in a homozygous or heterozygous state in all new V × M hybrids.

Citrus leaves exhibit remarkable morphological diversity, yet the genetic underpinnings of this variation remain poorly understood. In this study, we investigated the genetic architecture controlling citrus leaf shape by performing genome-wide association studies (GWAS) on over 3,000 digitized leaves collected from 150 citrus varieties across two years (2021 and 2022). Using a 50K SNP Axiom array, we identified consistent genomic regions on chromosomes 2, 3, 5, and 7 that were significantly associated with leaf length, width, and area. Linkage disequilibrium patterns within these regions revealed variability, and gene ontology enrichment indicated that genes involved in cell division, photosynthesis, and transcriptional regulation are overrepresented. Our results highlight the strong and heritable basis of leaf morphology in citrus and suggest that regulatory mechanisms play a major role in shaping these traits. This work provides key insights into the genetic control of leaf form and has potential implications for citrus breeding and functional genomics.