By Manjul Dutt, Sheetal Ramekar and Gary England
Developing new citrus cultivars, whether through conventional breeding or genetic engineering methods like transgenics or CRISPR is potentially the most viable long-term strategy to combat huanglongbing (HLB). Identifying potential survivor trees — citrus trees that exhibit disease tolerance in areas with endemic HLB — can also serve as a useful strategy for discovering promising new cultivars. This strategy isn’t new and has been explored since Candidatus Liberibacter asiaticus (CLas) was detected in Florida. Survivor trees may potentially carry HLB-tolerance genetics. Identifying such trees can provide valuable genetic resources for breeding programs aimed at developing HLB-tolerant citrus varieties.
In Lake County, Florida, a grove of Hamlin-type citrus trees displaying significant tolerance to HLB was observed. The putative HLB-tolerant trees in this grove showed higher CLas titer levels, ranging from 28 to 37. These tolerant trees also exhibited noticeably higher chlorophyll and carotenoid content compared to declining trees in the same grove. Interestingly, while all the trees in this grove were supposed to be Hamlin cultivars, they displayed variations in seed counts. Some trees had seed counts ranging from 8 to 11, which align more closely with the characteristics of Pineapple or Parson Brown cultivars. To investigate their origin, whole genome DNA sequencing was conducted on these cultivars alongside reference cultivars such as Hamlin and Parson Brown, obtained from the Florida Department of Agriculture and Consumer Services Division of Plant Industry (DPI).
DNA sequencing of the escape trees from the Buckhill block revealed that most of these better-performing escape trees were likely either Parson Brown types or Hamlin clones. These early-season sweet orange trees, planted around 1987, have consistently shown good production, despite being infected with HLB. Among them, two promising lines have been identified and included in the Citrus Research and Field Trial (CRAFT) Foundation application period for Cycle Six.
The first is the R20T30 tree, which is a Parson Brown-type with 8 to 11 seeds per fruit and the second is the R21T36 tree which is a Hamlin-type cultivar with three to five seeds per fruit (Figure 1). Both cultivars have been observed for several years and have met juice-quality standards, demonstrating excellent yield and juice characteristics (Table 1). Certified budwood of these two selections is freely available with the DPI Citrus Budwood Program.
Manjul Dutt is an assistant professor, and Sheetal Ramekar is a postdoctoral research associate — both at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred. Gary England is a UF/IFAS emeritus Extension agent.
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