Loading variety overview…
World of CitrusWorld of CitrusX-639 Citrandarin
The Mulberries · YouTube · 2025-01-24
https://www.youtube.com/watch?v=ptwl1aaYYUQ (opens in a new tab)
Summary from transcript: X-639 plants had good thick caliper and only minor, variable tender-tip injury in the field (25:16–26:03).
Accessed
The Mulberries
https://themulberries.org/wp-content/uploads/2026/01/X-639.png (opens in a new tab)
Individual scorecard image for X-639 Citrandarin. The printed preference ratings are sweetness 7/10, sourness 4/10 and bitterness 7.5/10; the linked fruit reference preserves these original ratings alongside catalog numeric scores. Sweetness is stored as printed; sourness and bitterness use 10 minus the printed score. Any Brix or pH printed on the card is a separate reading, not derived from ratings. Sample method and harvest date are not stated unless noted in the reference.
Accessed
Greg McCollum and Kim D. Bowman · HortScience 52(4):541–546 · 2017-04-01
https://doi.org/10.21273/HORTSCI11435-16 (opens in a new tab)
Peer-reviewed trial of Ray Ruby grapefruit scions on seven rootstocks in a commercial Indian River County, Florida orchard, with 2012–14 harvests. This record is cited only for the trial’s total-solids comparison for X639 as a rootstock; it does not describe fruit borne by X639 itself.
Accessed
The Mulberries · YouTube · 2025-12-23
https://www.youtube.com/watch?v=pU3TR4tLUlM (opens in a new tab)
Summary from transcript: The presenter reports several X-639 rootstock fruits weighing 54.9, 41, 46, and 41 g, calls their mean about 45 g, describes a navel-orange-rind and vegetal/planty smell, and reports 12 °Brix at 20:47–20:54. These are presenter-reported observations; the sample count for the aroma/Brix observations is not stated. X-639 is compared with US-1282 in the video. The 45 g fruit observations may overlap the existing Mulberries scorecard sample; do not count them as an independent scored or measured fruit sample without evidence resolving sample identity. English auto-generated captions.
Accessed
UF/IFAS Statewide Citrus Research and Extension
https://citrusresearch.ifas.ufl.edu/cultivars/rootstocks/x-639/ (opens in a new tab)
Current UF/IFAS rootstock profile. The visible page is marked last modified August 5, 2026; it shows no original publication date or credited author. General Information describes X-639’s South African source, 1950s development, Cleopatra × Poncirus hybrid identity, Florida availability in 1994, qualified Florida disease/salinity performance, grower selection/propagation ranking, relative tree size, and spacing recommendation. These are profile statements, not a replicated trial or numeric ratings.
Accessed
USDA APHIS IDtools, Citrus ID · 2011-10
https://idtools.org/citrus_id/index.cfm?packageID=1179&entityID=8912 (opens in a new tab)
Citrus ID Edition 2, October 2011. The X639 entry lists synonym “639” and cites multiple literature treatments of its taxon/cross; its morphology description is a generalized identification-key account. No entry author is displayed. Do not treat the differing cited cross formulations or broad fruit/taste key categories as resolution of precise pedigree or as scored cultivar-wide fruit data.
Accessed
Jude Grosser, Manjul Dutt, and Fred Gmitter · Citrus Industry Magazine · 2022-07-18
The web page displays Josh McGill as its byline and credits Jude Grosser, Manjul Dutt, and Fred Gmitter as article authors. In the “XXX-639 MUTANT” section, the authors report that one vigorous X-639 rootstock in HLB-positive Valencia trees was recovered after marker and flow-cytometry comparisons with standard X-639; XXX-639 is their name for that recovered mutant rootstock. The article’s nearly 50 fruit were borne on the unusual tree, not reported as XXX-639 scion fruit. It describes early propagation and testing, not a replicated trial or general HLB-resistance claim.
Accessed
Ute Albrecht · Citrus Industry Magazine · 2021-11-30
https://citrusindustry.net/2021/11/30/which-rootstock-to-select/ (opens in a new tab)
UF/IFAS extension guidance for Florida rootstock choice. The article describes X-639 as vigorous, producing large and healthy canopies, and notes that vigor can delay early productivity while trees establish vegetative growth. The possible link between vigor and tolerance is tentative; the article cautions that rootstock tolerance may diminish with a susceptible scion and that no rootstock fully prevents HLB damage.
Accessed
Citrus Australia
The page 5 “Lemon Variety Rootstock Compatibility Chart” lists “Nelspruit hybrid 639” among rootstock options. This is a chart entry, not an individual compatibility trial; page 6 gives general rootstock characteristics.
Accessed
mkono · Tropical Fruit Forum · 2024-02-20
https://www.tropicalfruitforum.com/index.php?topic=53795.0 (opens in a new tab)
Five-page thread started by mkono in Pennsylvania on 20 February 2024, with updates through September 2025. Reports a proposed F2 trial from X-639 seed, mixed and uncertain seedling identities, a reported station low of −10.8°F, widespread initial damage, and later root/stem resprouting in the author's cohort. These are grower reports about seedlings, not verified X-639 clones or a cultivar hardiness threshold; the mixed cohort is not attributable to X-639.
Accessed
Peter Chaires · Growing Produce · 2012-09-01
https://www.growingproduce.com/citrus/citrus-rootstock-selection-on-the-grow/ (opens in a new tab)
The “Top Rootstocks in Florida” table footnote identifies the values as propagations in the past year and attributes them to the DPI Bureau of Citrus Budwood Registration. X-639 is listed at 7.18% of total propagations (79.3% cumulative through that row). Historical propagation figures do not establish acreage, plantings, performance, or current market share.
Accessed
IFAPA
LIFE18 CCA/ES/001109, Action C.1 deliverable; cover gives a due date of 31 July 2020. Page 4 lists X-639 as a selected parent plant and describes it as tolerant to HLB and salinity/root asphyxia, with intermediate Phytophthora behavior. The report frames these as selection characteristics for planned testing, not completed trial results; preserve the source’s inconsistent cross notation without changing the catalog pedigree.
Accessed
J. P. Syvertsen and Wije Bandaranayake · Proceedings of the Florida State Horticultural Society · 2012
https://www.cabidigitallibrary.org/doi/pdf/10.5555/20183052437 (opens in a new tab)
Proceedings of the Florida State Horticultural Society 125:56–60 (2012). The paper compares two-year-old Hamlin orange trees on Cleopatra mandarin, US-897, and X-639 in a greenhouse under 0, 30, or 60 mM NaCl irrigation for four months. Outcomes concern Hamlin scions and root/leaf responses on the tested rootstocks, not fruit borne by X-639.
Accessed
The Mulberries · YouTube
https://www.youtube.com/watch?v=vQjNgKy3wx4 (opens in a new tab)
Summary from transcript: The speaker describes a second-year X-639 tree flowering late the previous season and blooming again, with two other X-639 trees in the field also flowering (0:16–0:31). The speaker says the tree flowers on new growth and old growth also blooms (0:35–0:42). These are grower observations; the clip provides no controlled precocity or cold-hardiness result. English auto-generated captions.
Accessed
Xiuhua Zhang, Magda Derival, Ute Albrecht, and Yiannis Ampatzidis · Agronomy · 2019-07-02
https://doi.org/10.3390/agronomy9070354 (opens in a new tab)
The supplied ResearchGate Figure 6 page links to this paper. Section 2.3 describes one five-year-old HLB-affected Hamlin sweet orange tree on X-639 at the SWFREC grove; it was scanned with 1600 MHz ground-penetrating radar before excavation and direct root measurement. Section 3.8 reports results from that single tree. The methods call it Experiment IX while Figure 10 says Experiment X. These are observations about one grafted tree and GPR performance, not fruit or general X-639 rootstock performance.
Accessed
jibro · Citrus Growers Forum · 2020-08-04
https://citrusgrowersv2.proboards.com/thread/584/cold-hardy-rootstock (opens in a new tab)
Post #4144 (4 August 2020) gives a generalized unfavorable fruit-taste opinion about X-639 and African shaddock × Rubidoux. The author says these are commonly used rootstocks but personally uses Poncirus and Citrumelo. No tasting sample, site, scion distinction, or trial method is provided; this is attributed forum opinion, not measured fruit evidence.
Accessed
Indira Paudel, Asher Bar-Tal, Eran Raveh, Nirit Bernstein, and Shabtai Cohen · Scientia Horticulturae · 2019-06-01
https://www.sciencedirect.com/science/article/abs/pii/S0304423819301530 (opens in a new tab)
Paudel et al., Scientia Horticulturae 251 (2019), 9–19, DOI 10.1016/j.scienta.2019.02.071. The abstract describes one-year-old unworked rootstocks, while the methods describe 240 uniform four-month-old seedlings per genotype; this age wording difference is unresolved. X639 and Volkameriana seedlings were tested under water-quality and root-zone-aeration treatments for four months, followed by two months of recovery. Results concern unworked seedlings, not grafted trees or fruit.
Accessed
Citrus Research International (Pty) Ltd
The PDF’s X639 citrandarin profile is marked “Updated: July 2016”; the document carries a 2018 copyright. It reports tolerance to Citrus tristeza virus (CTV) but high susceptibility to citrus viroids (CVd), and attributes the historical Cleopatra mandarin × Poncirus trifoliata cross to Dr Hojby at CSFRI (now ARC-ITSC) in the early 1950s. Tree, fruit, yield, and disease observations concern rootstock/scion combinations; the document supplies no accession-specific trial design or quantified outcomes.
Accessed
Dr. Jude Grosser · SWFREC · 2025-03-04
https://www.youtube.com/watch?v=H_0RhIVHpKw (opens in a new tab)
Summary from transcript: In this auto-captioned SWFREC seminar, Dr. Jude Grosser describes a putative mutant of commercial X-639 rootstock, which the slide at 44:14 labels “x639 mutant STR-4-1.” He says leftover trees from a nutrition experiment were infected Valencia on five rootstocks and replanted as resets (44:06–44:54). One Valencia scion on X-639 rootstock then grew unusually vigorously and carried about 50 Valencia fruit while other X-639 trees did not show the same behavior (45:03–45:21). He describes recovering that tree and says flow cytometry suggested possible missing DNA while 13 SSR markers matched X-639 (45:23–46:44); sequencing produced a list of mutations and the plant looked much like standard X-639 (46:54–47:10). This is an early report about one candidate, not a replicated trial or validated X-639 performance result. The slide and speaker do not name XXX-639.
Accessed
University of California, Riverside — Citrus Variety Collection
https://citrusvariety.ucr.edu/crc3957 (opens in a new tab)
The CRC 3957 profile identifies the accession as PI 300256 / VI 469 and reports budwood imported from South Africa in 1985. This documents the accession receipt, separate from the profile’s reported early-1950s development history.
Accessed