The United States Department of Agriculture National Plant Germplasm System Apple Collection: Program and Impact – DRAFT

Introduction

The domesticated apple, Malus domestica, is one of the world’s most valuable fruit crops. Centuries of selection and propagation across Asia, Europe, and North America resulted in an array of apple cultivars rich in quality and cultural significance. Modern apple breeding continues to enhance fruit quality to meet consumer preferences while addressing changing horticultural practices to ensure future production. Genetic diversity is essential for long-term crop improvement and promotes the sustainability of American horticulture.

The United States Department of Agriculture (USDA)-National Plant Germplasm System (NPGS) Apple Collection is maintained by the Plant Genetic Resources Unit (PGRU), co-located with the USDA Northeast Regional Plant Introduction Station on the Cornell AgriTech campus in Geneva, NY. PGRU maintains 6,086 diverse accessions of apple and its wild relatives, including 48 species and hybrids. Accessibility of apple genetic resources is the central focus of the PGRU and drives approaches to acquisition, maintenance, distribution, and research. The vulnerability of apple genetic resources in the United States was addressed by Volk et al. 2015. This is an overview of PGRU’s effort to acquire, maintain, document, and distribute germplasm, as well as highlight the value of these resources to apple research and industry stakeholders.

Acquisition of Malus Germplasm

The USDA NPGS Apple Collection began in 1983 with much of the founding material originating from Cornell University’s apple breeding program. The Purdue University, Rutgers University, University of Illinois (PRI) Disease Resistant Apple Breeding Program also donated introgressed breeding lines. PGRU has continued to expand the collection through plant exchanges and explorations. From 1989 to 1996, PGRU led explorations for wild Malus in Central Asia, of which the collection of Malus sieversii from Kazakhstan was a crowning achievement (Forsline et al. 2003). M. sieversii and M. sylvestris are primary wild progenitors of modern apples and USDA genetic resources were critical in elucidating the domestication history of apple (Velasco et al. 2010).

More recently, PGRU has actively pursued wild North American Malus species, Malus angustifolia, Malus coronaria, and Malus ioensis. Additionally, plant explorations acquired Malus sylvestris from Romania and Malus doumeri from Vietnam. Seeds obtained from exploration and exchanges are grown-out for evaluation and select genotypes are introduced into the collection as permanent accessions. The composition of the USDA NPGS Apple Collection is unique, with nearly 77% (n=4,676) representing wild Malus species. Incorporation of wild species into breeding programs allow for introgression of beneficial traits, such as disease resistance, into modern apples (Brown 2012). Species represented in the USDA NPGS Apple Collection are listed in Table 1. \[\\[0.1in]\]

Table 1. Malus species and hybrids represented in the USDA NPGS Apple Collection. Nomenclature based on Germplasm Resource Information Network (GRIN-Global).

Species	Accessions
Malus adstringens Zabel	2
M. angustifolia (Aiton) Michx.	151
M. ×arnoldiana (Rehder) Sarg. Ex Rehder	2
M. ×asiatica Nakai	20
M. ×astracanica hort. Ex Dum. Cours.	1
M. ×atrosanguinea (hort. Ex Spath) CK Schneid.	2
M. baccata (L.) Borkh.	69
M. brevipes (Rehder) Rehder	2
M. coronaria (L.) Mill.	141
M. ×dawsoniana Rehder	2
M. domestica Borkh.	1410
M. doumeri (Bois) A. Chev.	2
M. florentina (Zuccagni) CK Schneid.	3
M. floribunda Siebold ex Van Houtte	12
M. fusca (Raf.) CK Schneid.	234
M. halliana Koehne	17
M. ×hartwigii Koehne	5
M. honanensis Rehder	4
M. hupehensis (Pamp.) Rehder	164
Malus hybrid	350
M. ioensis (Alph. Wood) Britton	81
M. kansuensis (Batalin) CK Schneid.	40
Malus komarovii	1
M. ×magdeburgensis Hartwig	2
M. mandishurica (Maxim.) Kom. Ex Skvortsov	4
M. ×micromalus Makino	25
M. ×moerlandsii Door.	2
M. ombrophila Hand.-Mazz.	2
M. orientalis Uglitzk.	837
M. orthocarpa Lavallee ex anon.	1
M. ×platycarpa Rehder	7
M. prattii (Hemsl.) CK Schneid.	36
M. prunifolia (Willd.) Borkh.	53
M. pumila Mill.	13
M. ×purpurea (A. Barbier) Rehder	5
M. ×robusta (Carriere) Rehder	13
M. sargentii Rehder	23
M. ×scheideckeri (LH Bailey) Spath ex Zabel	2
M. sieversii (Ledeb.) M. Roem.	1807
M. sikkimensis (Wenz.) Koehne ex CK Schneid.	14
M. ×soulardii (LH Bailey) Britton	3
M. spectabilis (Aiton) Borkh.	10
Malus spontanea (Makino) Makino	9
Malus spp.	90
M. ×sublobata (Dippel) Rehder	4
M. sylvestris (L.) Mill.	84
M. toringo (Siebold) de Vriese	121
M. toringoides (WW Sm.) JB Phipps	95
M. transitoria (Batalin) CK Schneid.	51
M. trilobata (Poir.) CK Schneid.	3
M. tschonoskii (Maxim.) CK Schneid.	3
M. yunnanensis (Franch.) CK Schneid.	14
M. zhaojiaoensis NG Jiang	33
M. ×zumi (Matsum.) Rehder	5

\[\\[0.10in]\] Figure 1 depicts the growth of the collection from 1983 to 2019, with primary acquisition phases occurring in the mid-1980s and through the mid to late-1990s. Note the emphasis of wild Malus species. Prior to 1983, the National Plant Germplasm System held close to 600 Malus accessions which were transferred to PGRU. Most accessions were introduced as clones (Figure 1). The collection is separated into 4,483 field and 1,603 seed accessions. PGRU seed accessions are generally open pollinated seed lots collected from individual wild apple trees.

Figure 1. Number of accessions added to the PGRU Malus Collection annually. M. domestica (blue), M. sieversii (yellow), wild Malus (red). Nested pie chart indicates proportion of clones or seed accessions currently in the NPGS Apple collection.

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Maintenance

Field-based collections are vulnerable to environmental pressures such as disease and weather. To preserve the apple collection, accessions are propagated as two trees on ‘EMLA7’ semi-dwarf rootstock at 12 ft × 20 ft spacing. As trees establish in the field, one tree is removed to reduce crowding. Each year an annual collection inventory records the status of each tree in the field. Trees that are weak are flagged for additional observations and grafting. Tree health can quickly change within a season, resulting in death or inadequate material for propagation. To safeguard against losses, accessions are duplicated through cryopreservation. See Volk et al. 2020 for more details. Currently, 73% (n=2,052) of the clonal apple collection is duplicated in cryopreservation.

The vulnerability of the USDA NPGS Apple Collection was evident during an outbreak of fire blight in 2020. Fire blight is caused by the bacterium Erwinia amylovora, and trees are typically treated with the antibiotic streptomycin during infection periods. After observing mild to severe fire blight symptoms throughout the collection despite chemical treatments, a streptomycin-resistant strain of E. amylovora was identified. Apples vary in their response to fire blight ranging from highly susceptible to disease resistant, and we observed a broad phenotypic response to the 2020 outbreak (Figure 2). By June 2020, fire blight severity ranged from 0 to 85.7% with an average severity of 17.4%. These evaluations were described by Dougherty et al. (2021).

Figure 2. Examples of fire blight severity in the USDA NPGS Apple Collection (Dougherty et al. 2021) ranging from 0-85%. The fire blight severity percentages are shown in bottom right of each panel.\[\\[0.1in]\]

Fire blight is the primary maintenance challenge for the USDA NPGS Apple collection and has been documented throughout our history (Forsline and Aldwinckle 2002; Thapa et al. 2021). PGRU is striving to enhance orchard management practices to safeguard its genetic resources. PGRU expanded its spray program to include more diverse chemicals. However, consistent and effective applications are difficult to achieve in diversity collections where accession development, and subsequently susceptibility, is staggered throughout the growing season. Rootstock mediated resistance can provide additional protection for highly susceptible accessions (Singh et al. 2019).

While pest and disease management are primary concerns, PGRU is exploring ways to save field space and increase replication within the collection. The following are aspects in which a superior rootstock could enhance the USDA NPGS Apple collection:

• Improved tolerance to biotic and abiotic stress.
• Improved grafting success rate and tree survivability during propagation.
• Reduced root suckering to decrease annual maintenance.
• Reduced tree size to accommodate two or more trees per accession, while reducing the space between accessions.

Genebank Distribution of Malus germplasm

Distribution records, including accession information, tissue type, intended use, and recipient affiliation, date back to 1988. Figure 3 highlights the upward trend of both individual requests for apple germplasm and number of samples distributed from 1988 to 2020. The significant decrease in orders and items distributed in 2020 was due to restricted distribution related to fire blight. Over the past five years, PGRU distributed an average of 6,176 items to 380 requestors annually.

PGRU receives an increasing number of requests from unaffiliated individuals (non-research, non-commercial) as depicted in yellow. Cooperators (germplasm donors or recipients) separated by category are tabulated below (Table 2). PGRU has distributed Malus germplasm to 2,618 unique cooperators since 1988. U.S.Individuals comprise the largest group of apple germplasm recipients, followed by USDA-ARS and U.S. state agencies/Universities. The majority of cooperators make fewer than five requests for germplasm (Table 3). Distribution of germplasm to customers and stakeholders accounts for 88% of the distributions of the USDA NPGS Apple Collection (Table 4).

Figure 3. Number of items distributed from the USDA Malus collection from 1988 to 2020 separated by Research (blue) and Unaffiliated (yellow) recipients. Total number of requests indicated above bar.

\[\\[0.25in]\] Table 2. Number of items distributed by cooperator type from 1988 to 2019. ‘Other’ category includes DNA, pollen, roots, and seeds.

Cooperator Type	Scions	Leaves	Fruit	Other	Total
Research
University	16,913	5,376	3,355	1,553	27,197
USDA-ARS	15,290	11,124	1,283	2,818	30,515
US Company	10,885		78	124	11,087
US Non-Profit	6,261	62	89	263	6,675
Foreign Non-profit	6,100	2,297	398	900	9,695
Foreign Genebank	881	234		83	1,198
US Federal Agency	265			13	278
Foreign Company	33			27	60
Unaffiliated
US Individual	35,945		334	2,533	38,812
Foreign Individual	149			764	913
TOTAL	92,722	19,093	5,537	9,078	126,430

\[\\[0.25in]\] Table 3. Cooperator request frequency from 1988 to 2019.

No. Cooperator Requests	Frequency	Percentage of 5,967
1	1,650	28%
2	405	13%
3	192	10%
4	108	7%
5	69	6%
6 to 10	131	16%
11 to 20	49	12%
>20	16	8%

\[\\[0.25in]\] Table 4. Malus germplasm utilization classes from 1988 to 2020.

Request Type	No. Orders	No. Items
Distribution	5,886	115,564
Evaluation	108	6,552
Backup	32	3,118
Information Only	18	2,347
Transfer	11	2,184
Regeneration	15	791
TOTAL	6,070	130,556

\[\\[0.25in]\] PGRU Malus distribution makes a global impact on apple research, as highlighted in Figure 3. PGRU has distributed apple germplasm to 47 countries. Distribution outside of the United States is limited due to foreign regulations for importing plant material, although we continue to work with state, national, and foreign authorities to ensure the regulated distribution of PGRU apple germplasm. Historically, PGRU distribution primarily serves stakeholders within the United States, with New York and Colorado (National Laboratory for Genetic Resources Preservation) being the largest users. This reflects USDA-ARS and collaborators efforts to characterize/evaluate and cryopreserve the collection.

Figure 3. Distribution of Malus germplasm from 1988 to 2021 within the United States (top) and internationally (bottom). Inset table lists international distribution by country. Items represent number of accessions distributed in a request. Color scale represents distribution of items with NA values in grey. U.S is excluded from international requests to simplify color scale. \[\\[0.05in]\] Though PGRU’s emphasis is on wild Malus resources, requests for germplasm is heavily skewed towards M. domestica; M. domestica represents 23% of the USDA NPGS Apple Collection, but accounts for 55% (72,618/130,928) of PGRU’s distribution (Figure 4). Table 5 includes the top requested accessions since 1988, nearly all of which are popular cultivars. PGRU distributes plant material as scions, leaves, fruit, DNA, pollen, and other tissues for research and commercial applications. Scions for propagation are the most common form of distribution, representing 74% of all tissue types distributed. Leaves are the second highest group, which mostly accounts for DNA isolation.

Figure 4. Utilization of the PGRU Malus Collection from 1988 to 2021 by species (top) and tissue type (bottom). M. domestica (green), M. sieversii (yellow), and Malus hybrids (purple) are separated. Additional Malus species are grouped by geographic origin. ‘Other’ tissue category accounts for DNA, roots, and pollen.\[\\[0.05in]\]

Table 5. Top 36 requested accessions from the USDA NPGS Apple Collection from 1988 to 2020. Follow table links to accession pages on GRIN-Global.

Accession	Species	Cultivar	Times.Shipped
PI 588853	M. domestica	‘Cox’s Orange Pippin’	400
PI 588785	M. domestica	‘Esopus Spitzenburg’	349
PI 589654	M. domestica	‘Ashmead’s Kernel’	336
PI 589892	M. domestica	‘Golden Russet’	333
PI 588971	M. domestica	‘Roxbury Russet’	317
PI 589596	M. domestica	‘Calville Blanc’	310
PI 588943	M. domestica	‘Liberty’	309
PI 613818	M. domestica	‘Wickson’	289
PI 588872	M. domestica	‘Northern Spy’	286
PI 588803	M. domestica	‘Chestnut Crab’	283
PI 588791	M. domestica	‘Grimes Golden’	283
PI 589073	M. domestica	‘Dabinett’	268
PI 657037	M. sieversii	KAZ 95 18-05	261
PI 590157	M. domestica	‘Hudson’s Golden Gem’	260
PI 589117	M. domestica	‘Arkansas Black’	251
PI 588842	M. domestica	‘Empire’	246
PI 588861	M. domestica	‘Winter Banana’	231
PI 588799	M. domestica	‘Winesap’	226
PI 590180	M. domestica	‘Blue Pearmain’	221
PI 123960	M. domestica	‘Court Pendu Plat’	218
PI 588980	M. domestica	‘Pink Pearl’	216
PI 589703	M. domestica	‘Kingston Black’	215
PI 589906	M. domestica	‘Sweet Sixteen’	215
PI 588807	M. domestica	‘Wolf River’	215
PI 588870	Malus hybrid	‘Dolgo’	210
PI 589469	M. domestica	‘Haralson’	206
PI 589894	M. domestica	‘Keepsake’	206
PI 588773	M. domestica	‘Yellow Newtown’	204
PI 588788	M. domestica	‘Wealthy’	203
PI 588844	M. domestica	‘Fuji’	202
PI 589697	M. domestica	‘Stoke Red’	202
PI 300258	M. domestica	‘Belle de Boskoop’	200
PI 588806	M. domestica	‘Chisel Jersey’	199
PI 392303	M. domestica	‘Gala’	198
PI 588880	M. domestica	‘Granny Smith’	197
PI 588837	M. domestica	‘Gravenstein Washington Red’	197

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Documentation

Germplasm documentation includes the acquisition, management, storage of information about genetic resources. The depth, accuracy, and accessibility of this information increases the value of germplasm and facilitates utilization. Documentation and data for the USDA NPGS Apple Collection is primarily maintained on GRIN-Global. This includes passport information, accession narratives, images, descriptor data, and genetic markers. This section highlights efforts to evaluate and document apple genetic resources.

Descriptor Data

Descriptors for the apple collection were assembled and defined by the Apple Crop Germplasm Committee. There are 111 Apple descriptors, divided into 8 categories: Chemical, Cytologic, Disease, Growth, Molecular, Morphological, Phenological, and Production. Nearly half the descriptors are for fruit. Currently, descriptor data is available for 3,763 accessions, representing 62% of the collection, with an average of 36 traits. Close to 2,500 accessions have digital images.

Genetics and Genomics

Molecular markers are essential for exploring the genetic diversity of genebank collections. In apple, SSRs, SNP arrays, and Genotyping-by-Sequencing have been utilized to understand relationships and identify new loci associated with key traits. The USDA NPGS Apple Collection was recently genotyped using Genotyping-by-Sequencing, a method that combines SNP discovery and mapping. This platform is beneficial for the apple collection because it does not rely on previously identified SNPs that limit inclusion of apple wild relatives. A Genome Wide Association (GWA) of 36 descriptors from PGRU historic data sets and 8,000 SNPs, identified a major locus associated with fruit firmness (Migicovski et al. 2016) controlled by NAC18.1 (Migicovsky et al. 2021). The GBS markers were further enhanced, increasing the number of SNPs to 30,000. This set was used to determine clones and first degree relatives within the USDA NPGS Apple Collection. In a study of 1,000 accessions, over half were related through an interconnected web of 1st degree relatives (Migicovsky et al. 2021). This data is valuable in determining collection gaps and setting criteria for inclusion into the USDA NPGS Apple Collection.

Gutierrez et al. 2018 used GWA and linkage mapping of F1 hybrids to identify loci associated with major dihydrochalcones in the USDA NPGS Apple Collection. The dihydrochalcone phloridzin is an important nutritional compound found in apple, however, some wild species replace phloridzin with trilobatin and sieboldin which each have unique nutritional properties. These compounds are controlled by single, unlinked genes and are codominantly expressed. GWA and linkage mapping identified two loci on apple linkage groups 7 and 8 that were tightly linked to trilobatin and sieboldin synthesis. Currently, no commercial cultivars contain sieboldin or trilobatin, which could be a target for breeding selection.

Advances in genomics and transcriptomics has benefited research into apple and its wild relatives, but are not without their challenges. Apple biology, including ancient genome duplication and high heterozygosity, and limited genomic resources for diverse cultivars and wild apples made advances difficult. PGRU and its collaborators have explored the genetic variation of the USDA NPGS Apple Collection and developed new insights into apple domestication. Duan et al. 2017 used genomic resequencing of 117 diverse cultivars and 22 wild species to identify patterns of selection in apple, including support for a two-stage model for increased fruit size during hybridization and domestication. Sun et al. (2020) assembled new phased diploid genomes of cultivar ‘Gala’ and two progenitor species, M. sieversii and M. sylvestris, and developed a pan-genome which identified thousands of new genes, some related to selection and domestication of apple. In this same publication, transcriptome profiles identified a significant portion of introgressed alleles were associated with fruit quality.

Citations and Published Data Sets

Publications featuring the USDA NPGS Apple collection can be documented in GRIN-Global under the crop citation page, including links to accessions utilized. This is a useful resource for replicating studies and enhanced utilization of apple germplasm.

For example, the accessions used in the following citations can be retrieved through the Number of accession(s) cited hyperlinks.

• Dougherty L, Zhu Y, Xu K. 2016. Assessing the allelotypic effect of two aminocyclopropane carboxylic acid synthase-encoding genes MdACS1 and MdACS3a on fruit ethylene production and softening in Malus. Hort. Res. 3:16024 DOI: https://doi.org/10.1038/hortres.2016.24. Note: ISSN 2052-7276 (online) Number of accession(s) cited: 951

• Thapa R, Singh J, Gutierrez B, Arro J, Khan A. 2021. Genome-wide association mapping identifies novel loci underlying fire blight resistance in apple. Pl. Genome e20087 DOI: https://doi.org/10.1002/tpg2.20087. Number of accession(s) cited: 579

Additionally, data associated with publications can be incorporated into GRIN-Global. Data can overlap with existing apple descriptors or new traits can be added to the Apple Descriptor List. All descriptor data are associated with distinct Studies or environments allowing them to be either incorporated into the full data set or retrieved separately. Deposit of data into GRIN-Global can fulfill journal requirements to make published data publicly available.

As an example, explore the Cider.Apple.Classification.Peck.2021 data set associated with the following citation.

• Kumar SK, Wojtyna N, Dougherty L, Xu K, Peck G. 2021. Classifying cider apple germplasm using genetic markers for fruit acidity. J. Amer. Soc. Hort. Sci. DOI: https://doi.org/10.21273/JASHS05056-21. Number of accession(s) cited: 217

Media Highlights

The following are a few media highlights covering the USDA NPGS Apple Collection.

• How About Them Apples? Research Orchards Chart a Fruit’s Future
• An Apple Detective Rediscovered 7 Kinds Of Apples Thought To Be Extinct
• Apples of Uncommon Character: Heirlooms, Modern Classics, and Little-Known Wonders
• Around the World in Rare and Beautiful Apples – Atlas Obscura
• Fire Blight Spreads Northward, Threatening Apple Orchards–NYTimes
• Kazakhstan’s treasure trove of wildly-flavoured apples
• Preserving the Future: the National Collection of Tart Cherry, Grape, and Apple in Geneva, NY
• The Botany of Desire: A Plant’s-Eye View of the World
• The History of the “Forbidden” Fruit
• Was Johnny Appleseed for real?
• A Wild Goose Chase in New York – ‘From Scratch’

Conclusions

Plant genetic resources are essential for the sustainability of American agriculture. Continual development of elite apple cultivars and rootstocks with durable disease resistance and novel qualities involves detailed evaluations of apple germplasm and utilization of genetic and genomic tools (Evans and Peace 2017; Luo et al. 2020). PGRU supports this process by providing plant material for research, breeding, and industry development. To meet stakeholders’ current and future needs, the USDA NPGS Apple collection need to be fortified against pests and pathogens. Additionally, improved evaluation and documentation of apple genetic resources are critical for increasing the value and accessibility of the USDA NGPS Apple Collection.

References

• Brown S (2012) Apple. In: Badenes ML, Byrne DH (eds) Fruit Breeding. Springer US, Boston, MA, pp 329–367

• Dougherty L, Wallis A, Cox K, Zhong G-Y, Gutierrez B (2021) Phenotypic evaluation of fire blight outbreak in the USDA Malus collection. Agronomy 11:144. https://doi.org/10.3390/agronomy11010144

• Duan N, Bai Y, Sun H, Wang N, Ma Y, Li M, et al. (2017) Genome re-sequencing reveals the history of apple and supports a two-stage model for fruit enlargement. Nat Commun 8. https://doi.org/10.1038/s41467-017-00336-7

• Evans K, Peace C (2017) Advances in marker-assisted breeding of apples. In: Evans K (ed) Achieving sustainable cultivation of apples. Burleigh Dodds Science Publishing, pp 165–191

• Forsline PL, Aldwinckle HS (2002) Natural occurrence of fire blight in USDA NPGS Apple Germplasm Collection after 10 years of observation. Acta Hortic 351–357. https://doi.org/10.17660/ActaHortic.2002.590.52

• Forsline PL, Aldwinckle HS, Dickson EE, Luby JJ, Hokanson SC (2003) Collection, maintenance, characterization, and utilization of wild apples of Central Asia. In: Janick J (ed) Horticultural Reviews. pp 1–61

• Gutierrez BL, Arro J, Zhong G-Y, Brown SK (2018) Linkage and association analysis of dihydrochalcones phloridzin, sieboldin, and trilobatin in Malus. Tree Genet Genomes 14:91. https://doi.org/10.1007/s11295-018-1304-7

• Luo F, Norelli JL, Howard NP, Wisniewski M, Flachowsky H, Hanke M-V, et al. (2020) Introgressing blue mold resistance into elite apple germplasm by rapid cycle breeding and foreground and background DNA-informed selection. Tree Genet Genomes 16:28. https://doi.org/10.1007/s11295-020-1419-5

• Migicovsky Z, Gardner KM, Money D, Sawler J, Bloom JS, Moffett P, et al. (2016) Genome to phenome mapping in apple using historical data. Plant Genome 9:1–15. https://doi.org/10.3835/plantgenome2015.11.0113

• Migicovsky Z, Gardner KM, Richards C, Thomas Chao C, Schwaninger HR, Fazio G, et al. (2021a) Genomic consequences of apple improvement. Horticulture Research 8:1–13. https://doi.org/10.1038/s41438-020-00441-7

• Migicovsky Z, Yeats TH, Watts S, Song J, Forney CF, Burgher-MacLellan K, et al. (2021b) Apple Ripening Is Controlled by a NAC Transcription Factor. Front Genet 12:671300. https://doi.org/10.3389/fgene.2021.671300

• Singh J, Fabrizio J, Desnoues E, Silva JP, Busch W, Khan A (2019) Root system traits impact early fire blight susceptibility in apple (Malus × domestica). BMC Plant Biology 19:579. https://doi.org/10.1186/s12870-019-2202-3

• Sun X, Jiao C, Schwaninger H, Chao CT, Ma Y, Duan N, et al. (2020) Phased diploid genome assemblies and pan-genomes provide insights into the genetic history of apple domestication. Nature Genetics 52:1423–1432. https://doi.org/10.1038/s41588-020-00723-9

• Thapa R, Singh J, Gutierrez B, Arro J, Khan A (2021) Genome-wide association mapping identifies novel loci underlying fire blight resistance in apple. The Plant Genome n/a:e20087. https://doi.org/10.1002/tpg2.20087

• Velasco R, Zharkikh A, Affourtit J, Dhingra A, Cestaro A, Kalyanaraman A, et al. (2010) The genome of the domesticated apple (Malus × domestica Borkh.). Nat Genet 42:833–839. https://doi.org/10.1038/ng.654

• Volk GM, Chao CT, Norelli J, Brown SK, Fazio G, Peace C, et al.

  2015. The vulnerability of US apple (Malus) genetic resources. Genetic Resources and Crop Evolution 62:765–794. https://doi.org/10.1007/s10722-014-0194-2