Apple Breeding Programs Research Articles

Scab Resistance in some Apples Genotypes from the UASVM Collection Cluj-Napoca

Apple is one of the most consumed fruits in the world and as well one of the most important crops in temperate areas. Apple scab, caused by Venturia inaequalis is damaging diseases that affects apple species and causes up to 70% yield losses depending of cultivar’s resistance. In order to control this disease, costly chemical pesticides are necessary, which increase the pollution of environment by their toxicity. This study is focused on checking the behaviour of certain apple varieties from the pomological collection at UASVM Cluj-Napoca against apple scab infection. Nine apple cultivars ('Fuji Kiku', 'Red Cap', 'Lena', 'Katja', 'Gala Mitchgla', 'Fiesta', 'Yellow', 'Pinova' and 'Reinette du Canada'), were analysed in terms of behaviour to scab attack on leaves and fruits. Visual observations were completed by molecular analyses for the potential presence of the Vf gene in some cases. Combining results from visual observation and molecular assays, the 'Lena' and 'Pinova' cultivars were noted as resistant to scab in our field trial conditions. This indicates a good opportunity to bring them into apple breeding programs for resistance to apple scab.

A comparison of new and existing rootstocks to reduce canker of apple trees caused by Neonectria ditissima (Nectriaceae, Hypocreales)

The grafting of apple rootstocks on to scions confers benefits including reduced tree size/dwarfing for trellis based growing systems, increased tolerance to physiological stress, and pest and disease management. The current study investigated the effect of rootstock selection on canker and tree death using eight common rootstocks M9 337, M9 337 with Golden Delicious (GD) interstock, M9 EMLA, MM106, M116, M26, Geneva® G11 and G41, in addition to six advanced selections from the NIAB East Malling apple breeding programme EMR-001—EMR-006, all grafted with Gala scions. One of the rootstocks, M9 377 was also grafted with a GD interstock. Two locations in England were selected, the first at East Malling, Kent, the second, at Newent, Gloucestershire. Several variables were analysed including cumulative numbers of dead trees per rootstock from 2017 to 2020, number of rootstock ‘A type’ cankers, number of scion mainstem ‘B type’ cankers, and number of peripheral ‘C + D + E’ branch cankers at the Kent and Gloucestershire locations in the fourth and final assessment year of 2020. Kendall’s rank correlation was used to test if trunk circumference (a measure of tree vigour) and canker were statistically dependant. Results showed that in Kent, there were significant differences between rootstocks for scion B cankers and peripheral C + D + E cankers. There were no significant differences found between rootstocks for rootstock A cankers at Kent, or any of the three canker types in Gloucestershire. There were up to 31.25% dead trees in Kent (EMR-004), and 30% in Gloucestershire (M9 337 with GD interstock, M26), but there were no significant differences in number of dead trees due to rootstock type in either Kent or Gloucestershire. The Kendall’s rank correlation analysis indicated there was almost no dependence of trunk circumference on canker. The three rootstocks with the overall lowest susceptibility to canker were M116, EMR-006, and EMR-004. The industry standard rootstocks M9 EMLA and M9 337 were ranked 12 and 14, and with EMR-001 were the three worst performing rootstocks. The Geneva® rootstocks G11 was ranked 6, and G41 was ranked 11. An interplay of factors are likely involved in the development of canker including location and environmental effects—rainfall, relative humidity, temperature, wind, soil type, topography, aspect, and other seasonal infection processes including pathogen inoculum load, and rainsplash of inoculum between trees and within individual trees, the genetics and physiology of each scion/rootstock combination, water and nutritional status of trees, hormonal and molecular signalling, and orchard management including pruning and removal of cankers.

Malus Species with Diverse Bloom Times Exhibit Variable Rates of Floral Development

In response to challenges caused by climate change, apple (Malus ×domestica) breeding programs must quickly develop more resilient cultivars. One strategy is to breed for various bloom times. Members of the genus Malus, including domesticated apple, wild species, and hybrids, exhibit striking variations in the bloom date. Although bloom time is strongly influenced by chilling requirements, other aspects of floral development in Malus and their contributions to bloom time are less known. The purpose of this study was to investigate potential connections between predormancy flower development and final bloom time in Malus species. We performed a phenological analysis of flower development in wild and domesticated apple with extreme differences in bloom time over the course of one developmental season. We tracked histological changes in the floral apex of representatives of three early-blooming Malus genotypes (M. ×domestica ‘Anna’ PI 280400, M. orthocarpa PI 589392, M. sylvestris PI 633824) and three late-blooming genotypes (M. angustifolia PI 589763, M. angustifolia PI 613880, M. ×domestica ‘Koningszuur’ PI 188517). Our study documented their floral meristem progression and organ development and expanded on current staging systems for apple flower development to describe the changes observed. The developmental trajectories of each genotype did not group according to bloom category, and we observed variations in the floral development stage at the time of dormancy onset.

Approaches to implement genomic selection in three Swiss apple breeding programs

Approaches to implement genomic selection in three Swiss apple breeding programs

Deducing genotypes for loci of interest from SNP array data via haplotype sharing, demonstrated for apple and cherry.

Breeders, collection curators, and other germplasm users require genetic information, both genome-wide and locus-specific, to effectively manage their genetically diverse plant material. SNP arrays have become the preferred platform to provide genome-wide genetic profiles for elite germplasm and could also provide locus-specific genotypic information. However, genotypic information for loci of interest such as those within PCR-based DNA fingerprinting panels and trait-predictive DNA tests is not readily extracted from SNP array data, thus creating a disconnect between historic and new data sets. This study aimed to establish a method for deducing genotypes at loci of interest from their associated SNP haplotypes, demonstrated for two fruit crops and three locus types: quantitative trait loci Ma and Ma3 for acidity in apple, apple fingerprinting microsatellite marker GD12, and Mendelian trait locus Rf for sweet cherry fruit color. Using phased data from an apple 8K SNP array and sweet cherry 6K SNP array, unique haplotypes spanning each target locus were associated with alleles of important breeding parents. These haplotypes were compared via identity-by-descent (IBD) or identity-by-state (IBS) to haplotypes present in germplasm important to U.S. apple and cherry breeding programs to deduce target locus alleles in this germplasm. While IBD segments were confidently tracked through pedigrees, confidence in allele identity among IBS segments used a shared length threshold. At least one allele per locus was deduced for 64-93% of the 181 individuals. Successful validation compared deduced Rf and GD12 genotypes with reported and newly obtained genotypes. Our approach can efficiently merge and expand genotypic data sets, deducing missing data and identifying errors, and is appropriate for any crop with SNP array data and historic genotypic data sets, especially where linkage disequilibrium is high. Locus-specific genotypic information extracted from genome-wide SNP data is expected to enhance confidence in management of genetic resources.

Transcriptome Analysis Identifies Genes Associated with Chlorogenic Acid Biosynthesis during Apple Fruit Development

As a predominant phenolic compound in apple fruits, chlorogenic acid (CGA) benefits human health due to its various antioxidant properties. However, little has been known regarding the molecular mechanism underlying the CGA accumulation in apple fruits. In this study, we measured the CGA content and relative enzymes’ activities during fruit development in two different flesh-colored cultivars ‘Huashuo’ and ‘Red Love’. The CGA content in both cultivars decreased sharply from 30 days after full bloom (DAFB) to 60 DAFB. Notably, the CGA content in fruit flesh was relatively higher than that in the peel. Further, the activities of C3H and HCT enzymes downstream of the CGA biosynthesis showed the similar changing trend as CGA content. Based on the transcriptome data of ‘Huashuo’ fruit at 30 DAFB and 60 DAFB, 23 differentially expressed CGA synthesis-related genes were screened. Gene expression analyses further showed that MdHCT1/2/4/5/6 and MdC3H1/2/3 were positively correlated to the variation of CGA content in two cultivars. These findings establish a theoretical foundation for further mechanism study on CGA biosynthesis and provide guidance for nutrient improvement in apple breeding programs.

Elucidation and characterization of QTLs for Russet formation on apple fruit in ‘Honeycrisp’-derived breeding germplasm

Russet formation in apples (Malus domestica Borkh.) is a superficial skin disorder that detracts from fruit appearance and therefore is a breeding target in many apple scion breeding programs. Previous studies have reported quantitative trait loci (QTLs) for russet formation, but these results have been limited to a narrow genetic background, which reduces breeding utility. In this study, 14 breeding parents were represented in 13-full sib families, nine of which were ‘Honeycrisp’-derived while the other four were derived from ‘Minneiska’, an offspring of ‘Honeycrisp.’ Two previously reported QTLs, on linkage group (LG) 2 and LG6, were confirmed and characterized for the first time in ‘Honeycrisp’-derived germplasm. Five other putative QTLs were also detected. The QTLs characterized in this study could be targeted for DNA test development to be used in marker-assisted selection. Non-additive interactions were observed at and between the LG2 and LG6 QTLs. Future work should include validation of QTL and haplotype effects in other germplasm, development of trait-predictive DNA tests for use in apple breeding programs, and exploration of genome-wide prediction models for russet formation.

Evidence of an additional centre of apple domestication in Iran, with contributions from the Caucasian crab apple Malus orientalis Uglitzk. to the cultivated apple gene pool.

Divergence processes in crop-wild fruit tree complexes in pivotal regions for plant domestication such as the Caucasus and Iran remain little studied. We investigated anthropogenic and natural divergence processes in apples in these regions using 26 microsatellite markers amplified in 550 wild and cultivated samples. We found two genetically distinct cultivated populations in Iran that are differentiated from Malus domestica, the standard cultivated apple worldwide. Coalescent-based inferences showed that these two cultivated populations originated from specific domestication events of Malus orientalis in Iran. We found evidence of substantial wild-crop and crop-crop gene flow in the Caucasus and Iran, as has been described in apple in Europe. In addition, we identified seven genetically differentiated populations of wild apple (M. orientalis), not introgressed by the cultivated apple. Niche modelling combined with genetic diversity estimates indicated that these wild populations likely resulted from range changes during past glaciations. This study identifies Iran as a key region in the domestication of apple and M. orientalis as an additional contributor to the cultivated apple gene pool. Domestication of the apple tree therefore involved multiple origins of domestication in different geographic locations and substantial crop-wild hybridization, as found in other fruit trees. This study also highlights the impact of climate change on the natural divergence of a wild fruit tree and provides a starting point for apple conservation and breeding programmes in the Caucasus and Iran.

Current Achievements of the Latvian Apple Breeding Programme

Abstract The commercial apple growing industry in Latvia has a lack of climate adapted modern cultivars. The Latvian apple breeding programme is targeted at cultivars adapted to the cool climate of Baltic countries and Scandinavia, with fruit quality and storage suitable for commercial production. Since 1989, scab resistance (Rvi6, Rvi5) donors are included in crosses. Several cultivars already show commercial promise — ‘Dace’, ‘Gita’, ’Monta’ (Rvi6) and the polygenic resistant ‘Laila’ and ‘Pure Ametist’. However, in 2020, the Rvi6 resistance was overcome in Latvia. Donors of polygenic scab tolerance may prove a better alternative. The most successful donors have been ‘Iedzēnu’, ‘Eksotika’, ‘Alesya’, and ‘Bohemia’. Marker assisted gene pyramiding was started, including markers linked to the resistance genes Rvi5 and Rvi6. Scab resistant cultivars ‘Edite’, ‘Felicita’, ‘Ligita’, and ‘Paulis’ show low susceptibility to Neonectria ditissima canker. To achieve high fruit quality and good storage, worldwide commercial cultivars are crossed with local adapted cultivars. Such crosses that have been registered include ‘Inta’, ‘Lora’, and ‘Saulesmeita’. Recent crosses for quality include ‘Honeycrisp’, where H-1-07-36 and H-1-07-42 show most promise. Improved biochemical content and flesh structure is one of the main targets. An additional line of breeding is aimed at ornamental and cider apples. The scab resistant Nr.19-97-154 (Cidross) was selected for cider. The red leaf columnar apple ‘Karlens’ and the compact red-leaf crab ‘Auce’ have been registered.

Large-Effect QTLs for Titratable Acidity and Soluble Solids Content Validated in ‘Honeycrisp’-Derived Apple Germplasm

Fruit acidity and sweetness are important fruit quality traits in the apple and are therefore targets in apple breeding programs. Multiple quantitative trait loci (QTLs) associated with titratable acidity (TA) and soluble solids content (SSC) have been previously detected. In this study a pedigree-based QTL analysis approach was used to validate QTLs associated with TA and SSC in a ‘Honeycrisp’-derived germplasm set. TA and SSC data collected from 2014 to 2018 and curated genome-wide single nucleotide polymorphism (SNP) data were leveraged to validate three TA QTLs on linkage groups (LGs) 1, 8, and 16 and three SSC QTLs on LGs 1, 13, and 16. TA and SSC QTL haplotypes were characterized in six University of Minnesota apple breeding families representing eight breeding parents including ‘Honeycrisp’ and ‘Minneiska’. Six high-TA haplotypes, four low-TA haplotypes, 14 high-SSC haplotypes, and eight low-SSC haplotypes were characterized. The results of this study will enable more informed selection in apple breeding programs.

Identification and characterization of Apple (Malus × domestica Borkh.) bud sports mutations in the apple growing Northwestern Himalayan region

Spontaneous mutations are exceedingly rare, and bud-sport selection is a promising horticultural breeding strategy that adds value to natural selection by enabling plants to adapt better to climate variability. Based on these observations, the research had been carried out in various apple-growing orchards in the Northwestern Himalayan area, particularly the districts of Kinnaur, Lahaul, and Spiti in the state of Himachal Pradesh. The research led to the identification of 12 bud sports originating from cv. Royal Delicious, 9 of which were found in Kinnaur and 3 in Lahaul and Spiti, which differed in fruit color and belonged to distinct groups. Eight of the twelve bud sports were early, solid-colored, and early maturing, whereas the remaining four were late-colored, late-maturing bud sports. The identified bud sports mature at different rates, and harvesting times varied from the first week of September to the fourth week of October. According to biochemical analysis, 'KAH-1′ and 'DDK' had the highest TTS (16.10 °Brix), total sugars (11.90 %), and titratable acidity (0.70 %). Based on the molecular analysis, one EST (E2) and five genomic SSR primers (HI04E04, HI01D06Y, BACSSR58, GD100, and CH01H01) were found to be informative and resulted in significant polymorphism between seven different bud sports and their mother plants. Bud wood from every identified apple bud sport was collected and conserved to produce plants for future investigation. Commercialization of such bud sports will not only raise market values but also lead to a reduction in the use of coloring chemicals for fetching early market, whilst late coloring and late-maturing bud sports will help to extend the season of apple fruit availability. These detected bud sports could represent a valuable gene pool that could be exploited as a source of genetic material in apple breeding programs or introduced as a new cultivar after systematic evaluation and validation through multi-location field trials.

Apple Fruit Size QTLs on Chromosomes 8 and 16 Characterized in ‘Honeycrisp’-Derived Germplasm

Multiple quantitative trait loci (QTLs) associated with fruit size have been reported in apple (Malus domestica Borkh.); however, few have been fully characterized and/or validated. A pedigree-based QTL analysis approach was used to examine the genetic basis of fruit weight in ‘Honeycrisp’-derived germplasm. Fourteen breeding parents were represented by 814 offspring from 13 full-sib families with breeding parents ‘Honeycrisp’ and ‘Minneiska’ being highly represented. Historical fruit weight data and curated genome-wide single nucleotide polymorphism (SNP) data were leveraged to map QTLs to chromosomes (Chrs) 8 and 16, which together accounted for 15% of the phenotypic variation. The Chr 16 QTL colocalized with other important Chr 16 trait loci. ‘Honeycrisp’ inherited two low fruit weight haplotypes at the Chr 8 QTL from progenitors ‘Northern Spy’ and ‘Grimes Golden’. At the Chr 16 QTL, ‘Honeycrisp’ inherited a low fruit weight haplotype from ‘Frostbite’ and a high fruit weight haplotype from ‘Duchess of Oldenburg’. The small-fruited ‘Honeycrisp’ progenitor ‘Frostbite’ had three low fruit weight haplotypes across the two QTLs. Non-additive interactions were observed at and across QTLs. Results will enable more informed parent selection and/or development of trait-predictive DNA tests for use in apple breeding programs.

The East Asian wild apples, Malus baccata (L.) Borkh and Malus hupehensis (Pamp.) Rehder., are additional contributors to the genomes of cultivated European and Chinese varieties.

The domestication process in long-lived plant perennials differs dramatically from that of annuals, with a huge amount of genetic exchange between crop and wild populations. Though apple is a major fruit crop grown worldwide, the contribution of wild apple species to the genetic makeup of the cultivated apple genome remains a topic of intense study. We used population genomics approaches to investigate the contributions of several wild apple species to European and Chinese rootstock and dessert genomes, with a focus on the extent of wild-crop gene flow. Population genetic structure inferences revealed that the East Asian wild apples, Malus baccata (L.) Borkh and M. hupehensis (Pamp.), form a single panmictic group, and that the European dessert and rootstock apples form a specific gene pool whereas the Chinese dessert and rootstock apples were a mixture of three wild gene pools, suggesting different evolutionary histories of European and Chinese apple varieties. Coalescent-based inferences and gene flow estimates indicated that M. baccata - M. hupehensis contributed to the genome of both European and Chinese cultivated apples through wild-to-crop introgressions, and not as an initial contributor as previously supposed. We also confirmed the contribution through wild-to-crop introgressions of Malus sylvestris Mill. to the cultivated apple genome. Apple tree domestication is therefore one example in woody perennials that involved gene flow from several wild species from multiple geographical areas. This study provides an example of a complex protracted process of domestication in long-lived plant perennials, and is a starting point for apple breeding programmes.

Development of a qPCR test to detect and quantify Elsinoë piri in unsprayed and organic apple orchards and assessment of apple cultivar susceptibility to the disease

AbstractRecently, Elsinoë piri has become an important pathogen in unsprayed and organic apple orchards. The increasing demand for organic apples as well as the limited use of fungicides imposed by European legislation has turned this pathogen, which has been known for over a century in Europe, into a resurgent phytosanitary threat. It also represents a new trait to be taken into account in apple breeding programmes. As E. piri is extremely difficult to isolate and causes symptoms that can be confused with those caused by other pathogens, a quantitative PCR (qPCR) test has been developed based on the internal transcribed spacer (ITS) region of the rDNA gene. This test, which displays a low limit of detection, allows the early detection of the fungus in apple leaves. As a quantitative method, it is a promising tool for breeding purposes as it can be used on leaves to assess the level of quantitative resistance of apple cultivars to the disease. The molecular test is also useful to gain knowledge on the epidemiology of this re‐emerging fungal disease. Spore trapping conducted in 2015, 2016 and 2020 using Rotorod devices and the qPCR test showed that airborne inoculum is released at the end of the growing season. Tests were also carried out to demonstrate that the pathogen was detected in buds during winter. A comparison of 18 E. piri isolates based on the sequencing of five genome regions highlighted a high genotypic diversity. All these isolates were detected with the qPCR test developed in this study.

Susceptibility of Cider Apple Accessions to European Canker-Comparison between Evaluations in Field Planted Trees and Rapid Screening Tests.

European canker, caused by Neonectria ditissima Bres., is an economically damaging fungal disease of apple. Breeding new cultivars with a high level of resistance to European canker is the main aim of apple breeding programs. Observations of symptoms in naturally infected trees were carried out in 400 apple accessions in Asturias (north-western Spain). Young and mature field planted trees were assessed under conditions highly conducive for N. ditissima development. The results demonstrated that juvenile trees (4-year-old) barely showed noticeable symptoms whereas a wide variability in the levels of resistance among accession was observed in mature trees (14-year-old). Around 28% of the locally maintained collection resulted to be highly resistant to this disease in the region. Field observations on mature trees were also compared to four rapid screening tests based on artificially induced lesions. Spearman correlation analysis using two resistance parameters revealed that none of the methods resulted in similar rankings of cultivar susceptibility as some accessions that were ranked as resistant for a given test turned out to be susceptible in the field. This study might suggest that whilst conventional resistance phenotyping techniques are time-consuming, the outcomes of this approach still seem the preferred option to assess the response to N. ditissima of apple accessions.

Extended Pedigrees of Apple Cultivars from the University of Minnesota Breeding Program Elucidated Using SNP Array Markers

Apple (Malus ×domestica Borkh.) breeding at the University of Minnesota (UMN) has been ongoing continuously since 1908 when staff originally planted thousands of seedlings from open-pollinated (OP) seeds collected from regional orchards. The first cultivar from the program, ‘Minnehaha’, was introduced in 1920 and several others from these OP seeds followed over the next 3 decades. Controlled crosses were initiated in 1916, and until the time of this publication, 28 cultivars have been introduced. Historical records of parentage, as recorded by staff in notebooks and in 20th-century publications, have been used to inform breeding decisions but might be incorrect as indicated by earlier explorations of parentage using simple sequence repeat (SSR) markers. Our objective was to elucidate parentage and extended pedigrees of all available cultivars introduced from the UMN apple breeding program using evaluations of Mendelian errors and shared haplotype length information based on data from single nucleotide polymorphism (SNP) arrays. Sixteen of the 21 cultivars introduced before ‘Honeycrisp’ (1991) had incorrect or incomplete pedigrees that are now at least partially elucidated. These include the two most important regional cultivars in the 20th century: ‘Haralson’ (parents: ‘Malinda’ and ‘Wealthy’) and ‘Fireside’ (parents: ‘Wealthy’ and ‘Northwest Greening’). ‘Wealthy’, a widely grown cultivar in the United States in the late 19th and early 20th centuries, was a frequent parent of older UMN cultivars. ‘Malinda’ was a less frequent parent than indicated by breeding records. ‘Duchess of Oldenburg’ (synonym ‘Borowitsky’) was revealed as an ancestor of overwhelming importance in the UMN breeding program. It was an ancestor of 27 of the 28 UMN cultivars, including as a parent of two cultivars, and a grandparent of 15 cultivars, including ‘Honeycrisp’.

As It Stands: The Palouse Wild Cider Apple Breeding Program.

Providing hands-on education for the next generation of plant breeders would help maximize effectiveness of future breeding efforts. Such education should include training in introgression of crop wild relative alleles, which can increase genetic diversity while providing cultivar attributes that meet industry and consumer demands in a crop such as cider apple. Incorporation of DNA information in breeding decisions has become more common and is another skill future plant breeders need. The Palouse Wild Cider apple breeding program (PWCabp) was established at Washington State University in early 2014 as a student-run experiential learning opportunity. The objectives of this study were to describe the PWCabp’s approaches, outcomes, and student involvement to date that has relied on a systematic operational structure, utilization of wild relatives, and incorporation of DNA information. Students chose the crop (cider apple) and initial target market and stakeholders (backyard growers and hobbyists of the Palouse region). Twelve target attributes were defined including high phenolics and red flesh. Phase one and two field trials were established. Two promising high-bitterness selections were identified and propagated. By running the PWCabp, more than 20 undergraduate and graduate students gained experience in the decisions and operations of a fruit breeding program. PWCabp activities have produced desirable new germplasm via utilization of highly diverse Malus germplasm and trained new plant breeding professionals via experiential learning.

Evaluating responses of Caucasian apple (Malus orientalis) from Hyrcanian forests to fire blight (Erwinia amylovora) using an in vitro assay

ABSTRACT Fire blight, caused by the necrogenic gram-negative Erwinia amylovora, is one the most destructive bacterial diseases of apple. The wild Caucasian apple (Malus orientalis) is grown throughout Turkey, Armenia, Georgia, Russia and the Hyrcanian forest in the northern part of Iran. So far, no studies on resistance to fire blight in M. orientalis from the Hyrcanian forest have been conducted. We therefore evaluated the response of four different populations of M. orientalis (two individual trees per population) eight individuals of M. orientalis from four different populations in this region for fire blight resistance, using an in vitro shoot-inoculation assay. The fire blight susceptible MM.106 and partly resistant MM.111 rootstocks were included as references. Two one-year old trees were selected from each population, which were then micropropagated and inoculated with the fire blight bacterial strain ‘Ea273ʹ. Necrosis growth along the shoots was measured every 24 h for 10 consecutive days. Selected individuals from Soordar, Asalem and Siahbil populations did not differ in the final necrosis growth from susceptible MM.106. However, the MM.111 and Afratakhteh individuals showed the lowest necrosis growth. The largest final necrosis growth was observed in the Siahbil individuals. The capacity of main scavenging enzymes and antioxidant activity were monitored in the Siahbil and the Afratakhteh individuals. The largest difference in the 2, 2-diphenyl-1-picrylhydrazyl (DPPH) activity was observed at 72 hours post-inoculation (hpi) between resistant Afratakhteh and susceptible Siahbil individuals. The highest activity of catalase (CAT) enzyme occurred in the Afratakhteh individuals after 168 hpi. The peroxidase (POD) activity increased in both Afratakhteh and Siahbil individuals. The two Afratakhteh individuals were more resistant to fire blight compared to Asalem, Siahbil and Soordar individuals. Our results suggested that large differences in fire blight resistance existed in natural populations of M. orientalis. Potential fire blight-resistant individuals should be of interest for future apple-breeding programs.

Trends in Fruit Quality Improvement From 15 Years of Selection in the Apple Breeding Program of Washington State University.

Washington State University's apple breeding program (WABP) was initiated in 1994 to select new apple cultivars with improved eating quality, appearance, and storability that are suitable for production in the main growing regions of the state. Fruit quality is phenotyped using various instrumental measures, such as penetrometers (texture), titrator (acidity), and refractometer (soluble solids concentration; SSC), as well as sensory assessment. The selection regime of WABP occurs in three sequential phases: phase one (P1)—single, unreplicated seedlings at one site, phase two (P2)—replicated selections at three geographically diverse sites, and phase three (P3)—highly replicated elite selections at one to two grower sites. Most of the data collection of WABP occurs in P2. Knowledge of trends/changes associated with advancing selections is essential for understanding the selection criteria and progress of WABP throughout the changing compositions of advancing and culling selections. For each post-harvest trait, P2 data from harvest years 2005 to 2019 were split across sites, and between selections and reference cultivars (e.g., Cripps Pink, Gala, and Honeycrisp). Means of instrumental crispness (Cn) and inner cortex firmness for the advancing selections increased gradually over this period and were significantly higher than those for cultivars. Means of outer cortex firmness measurements were stable for selections but significantly higher than those for cultivars. The average fruit acidity of selections increased marginally over this period and was higher than that of the cultivars. Meanwhile, the average fruit SSCs of selections and cultivars were statistically indistinguishable. These 15-year trends indicate that WABP has been selecting apples with improved eating quality and storability through increased crispness and inner cortex firmness, respectively.

Alleviation of Postharvest Skin Dimpling of MN55 Apple Fruit and a Possible Association with Apple stem pitting virus

MN55 is an apple (Malus ×domestica Borkh.) cultivar recently released by the University of Minnesota apple breeding program, with fruit marketed in the U.S. as Rave®. When stored for 4 months at 0 to 4 °C, MN55 fruit can develop several storage disorders, including skin dimpling. Skin dimpling incidence was greater for fruit harvested 1 week later than those harvested earlier. Dimpling was not alleviated by prestorage treatments of 1-methylcyclopropene or diphenylamine or by holding fruit at room temperature for 1 day before long-term cold storage. However, dimpling incidence was very low when fruit were stored at 6 to 7 °C. Because viruses have been implicated in other fruit dimpling disorders, the presence of viruses in MN55 leaves and fruit was studied. Apple stem pitting virus (ASPV) was detected by microscopy, reverse transcriptase polymerase chain reaction (RT-PCR) methodology, and high throughput sequencing (HTS) in peel of fruit from MN55 trees that exhibited skin dimpling after 4 months of storage at 0 to 1 °C. ASPV was also detected in supermarket-purchased fruit of other cultivars with noticeable skin dimpling. Although ASPV was not conclusively demonstrated to cause skin dimpling in our work, its prevalence indicates that further investigations are warranted to determine the relationship between viruses and skin deformities in stored apples.