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Sudip Kunwar, Jude Grosser, Fred G. Gmitter Jr., William S. Castle, and Ute Albrecht

Most of the commercially important citrus scion cultivars are susceptible to Huanglongbing (HLB), which is the most devastating disease the citrus industry has ever faced. Because the rootstock can influence the performance of the scion in various ways, including disease and pest tolerance, use of superior rootstocks can assist citrus growers with minimizing the negative effects of HLB. The objective of this study was to assess rootstock effects on the horticultural performance and early production potential of ‘Hamlin’ sweet orange (Citrus sinensis) trees in commercial field settings under HLB-endemic conditions. Two field trials were conducted in different locations in Central and Southeast Florida. The trials were established in 2015 and included 32 diverse diploid and tetraploid rootstock cultivars and advanced selections. One trial was performed in Highlands County, FL, on a poorly drained flatwoods-type site. Another trial was performed in Polk County, FL, on a well-drained sandy Central Florida Ridge site. Horticultural traits including tree height, canopy volume, trunk diameter, canopy health, leaf nutrient content, yield, and fruit quality were assessed during the 2018–19 and 2019–20 production years. Significant differences were found among trees on different rootstocks for most of the measured traits, particularly tree vigor and productivity, but rootstock effects also varied by location. Rootstocks that induced large tree sizes, such as the diploid mandarin × trifoliate orange hybrids ‘X-639’, ‘C-54’, ‘C-57’, and ‘C-146’, also induced higher yield, but with lower yield efficiency. Most of the tetraploid rootstocks significantly reduced tree size, among which ‘Changsha+Benton’, ‘Green-3’, ‘Amb+Czo’, ‘UFR-3’, and ‘UFR-5’ induced high yield efficiency. Therefore, these rootstocks have the potential to be used in high-density plantings. However, trees on some of these small size-inducing rootstocks had a higher mortality rate and were more vulnerable to tropical force winds. This study provides important information for the selection of rootstocks with the greatest production potential in an HLB-endemic environment, especially during the early years of production.