2019 ASHS Annual Conference

The disease huanglongbing (HLB) has devastated the Florida citrus industry, and HLB-tolerant planting material will likely be key to sustaining its continued existence. Potentially useful tolerance to infection by the HLB pathogen (hereafter “HLB-tolerance”) has been identified in some mandarin hybrids, and in some hybrids with the highly tolerant Poncirus trifoliata in their pedigree. The USDA/ARS citrus breeding program has been making crosses with Poncirus for over a hundred years, primarily to introgress cold hardiness and disease resistance into conventional citrus. Currently, many of the most widely grown citrus rootstocks are Poncirus hybrids. However, scion cultivars derived from Poncirus have been previously released only as breeding parents, as early generation hybrids have a very unpleasant flavor. Scion breeders have continued to work with Poncirus, some of these hybrids fortuitously display remarkable HLB-tolerance, and one of these hybrids has just been released as US SunDragon. US SunDragon is derived from a cross made by José Chaparro in 1999 of two complex hybrid selections, one of which has two Poncirus grandparents, resulting in a pedigree that is 1/8 Poncirus trifoliata. Seedling trees were planted at the USDA St. Lucie County FL farm in 2000 and US SunDragon was selected in 2011. US SunDragon has been planted at many sites, including replicated trials, and has shown remarkable HLB tolerance at all sites. The fruit is mature from Oct-Jan in Florida, reaching 11% soluble solids content (SSC) by Oct. During the period of optimal maturity, SSC ranges from 11-15% and titratable acidity from 0.4-0.8%. Color score is low at 33-34. If harvested early, some people note a slight off-taste, which diminishes as fruit mature. US SunDragon has been well-received in consumer taste panels, with fruit and juice most similar to sweet orange. Fruits have a pyriform shape, a distinctive pebbly skin, and green color is usually retained at the stem end of the fruit. US SunDragon is a valuable monoembryonic breeding parent, and has good potential for niche uses by local markets, gift-fruit-shippers, and home orchardists. Dooryard citrus trees have become uncommon in Florida as trees have died or become unproductive due to HLB. We anticipate that US SunDragon will be productive and healthy in a dooryard environment with a relatively low level of care, even in areas where HLB is endemic. Recently US SunDragon has received considerable interest as a potential contributor to commercial juice blends as production of sweet orange declines.