Huanglongbing (HLB) is a devastating bacterial disease in Florida and has been spreading in citrus-growing regions in Texas, Georgia, and California. HLB threatens the entire US fresh and processed citrus industries. To date, all commercial sweet orange (Citrus sinensis L. Osbeck) cultivars are susceptible, though some are more tolerant than others. The citrus industry has an estimated economic impact nationwide of over $13 billion. The currently applied strategies for mitigating this disease are costly and unsustainable. The development and commercialization of HLB tolerant sweet orange scions is needed for sustainable and profitable citriculture in the US. Citrus Under Protected Screen (CUPS) is a method used in Florida to exclude the Asian Citrus Psyllid (Diaphorina citri Kuwayama, or ACP), which is a phloem-feeding hemipteran insect that is the vector for Candidatus Liberibacter asiaticus CLas the causal agent believed to be responsible for HLB disease in citrus. ‘Hamlin’ sweet orange is the most important early sweet orange for processing in modern history. ‘Hamlin’ has a budline called ‘Hamlin 1-4-1’, which is the predominant budline in the State of Florida. ‘Hamlin 1-4-1’ has high susceptibility to HLB, which causes symptoms such as blotchy mottle, small fruit size, high titratable acidity (TA), low total soluble solids (TSS), and severe fruit drop. ‘Hamlin N13-32’ is a new budline of ‘Hamlin’ sweet orange that appears to have enhanced tolerance to HLB in terms of tree health. ‘Hamlin 1-4-1’ and ‘Hamlin N13-32’ trees were followed for two years in multiple environments with and without treatments of oxytetracycline. Tree size, rootstock, fruit number, fruit size, TSS, TA, TSS:TA ratio, and pounds-solids data were collected to determine the effects of environment (CUPS versus outside grove), antibiotic treatment effects, and budline effects on production. There were some significant differences among groups for fruit quality, but more research is needed to determine effects of oxytetracycline, budline, rootstock and environment on ‘Hamlin’ sweet orange production.
