2018 ASHS Annual Conference

The disease huanglongbing (HLB) continues to devastate the Florida citrus industry, has become widespread in Texas, and infected tree discovery is accelerating in California dooryard citrus. While highly resistant or even immune trees are the long-term sustainable solution, HLB-tolerant planting material will likely be crucial in the near term. In Florida, where ubiquitous infection provides ample disease exposure, some mandarin hybrid cultivars display potentially useful tolerance. At the USDA/ARS Ft. Pierce, FL farm, there are several replicated trials of cultivars and advanced selections exposed to endemic HLB that are providing further insights into potential tolerance. In a trial that is in its fifth year in the field, accessions in the least diseased statistical categories include some with predominately mandarin pedigree, one that is predominately pummelo and several with Poncirus in their pedigrees. The susceptible sweet orange ‘Valencia’ remains in the best categories for all traits except that its growth rate declined markedly in the last year. Many of the cultivars and selections displaying tolerance include ‘Clementine’ in their pedigree. A large trial of diverse ‘Clementine’-derived material is being phenotyped and genotyped to identify markers associated with HLB-tolerance in a collaboration between UC Riverside, USDA/ARS and the U of Florida, and statistical separation is now apparent for HLB-tolerance parameters after 2.5 years. A large trial of diverse Poncirus hybrids focusing primarily on citranges (sweet orange x Poncirus), another collaboration but led by U of Florida, has been in the field for 6.5 years and may also provide markers for HLB-tolerance, as might an adjoining trial of Poncirus and its hybrids, led by UC Riverside. Verification that HLB-tolerance in growth parameters translates into economic-tolerance through adequate crops of high quality fruit with require several additional years of trial data. A new collaborative NIFA grant will capitalize on these plantings and explore juice quality parameters of HLB-tolerant accessions. Single replacement cultivars or complementary cultivars in blends may provide a solution to inadequate supplies of sweet orange and grapefruit juice in FL.