## 4 Conclusions  
HLB decreased grapefruit quality, with smaller and greener fruit and a lower SSC and SSC/TA ratio. However, the responses of VOCs to HLB varied depending on the source (juice or peel oil) and harvest season. Nootkatone, the most important volatile component for grapefruit flavor, significantly decreased in juice and peel oil regardless of the harvest year. HLB disease resulted in a decrease of abundant monoterenes, including D-limonene, \(\beta\)-myrcene, and \(\alpha\)-pinene in the peel oil; opposite trends were found in the juice samples. On the other hand, the oxidative/dehydrogenated terpen were increased by HLB in peel oil but decreased in the juice sample. The long chain aldehydes, decanal, nonanal, and octanal also increased in HLB peel oil but decreased in the juice, resulting in likely lower flavor quality in the juice. Our results indicated that the volatile profiles of peel oil and juice had different responses to HLB than those in sweet orange fruit. This may affect the decision of fruit processors to favor processing for peel oil or juice depending on the fruit source received at the processing plant.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/foods12040713/s1](https://www.mdpi.com/article/10.3390/foods12040713/s1)]([https://www.mdpi.com/article/10.3390/foods12040713/s1](https://www.mdpi.com/article/10.3390/foods12040713/s1)), Figure S1: GC-MS chromatograms of peel oil samples extracted from HLB+ (top) and HLB\(-\) (bottom) 'Ray Ruby' grapefruit; Figure S2: GC-MS chromatograms of juice extracted from HLB+ (top) and HLB\(-\) (bottom) 'Ray Ruby' grapefruit.  
Figure 7: Summary of metabolic pathways leading to monoterenes, sesquiterpenes, and nootkatones [49; 52; 53; 55]. Bar graphs show changes (%) caused by HLB affection in peel oil and juice in 2020 and 2021. MEP, methylerythnitol phosphate pathway; MVA, mevalonic acid pathway; IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl pyrophosphate; FPP, famesyl pyrophosphate; Cstps1, valencene synthase.  
**Author Contributions:** Conceptualization, M.A.d.C., R.S.F., J.B.; methodology, M.A.d.C., A.P., R.S.F., J.B.; investigation, M.A.d.C., A.P., R.S.F., J.B.; writing--original draft preparation, M.A.d.C., J.B.; writing--review and editing, M.A.d.C., A.P., R.S.F., R.P.L.J., J.B. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work is supported by the USDA National Institute of Food and Agriculture, USDA-NIFA-CDRE project 2018-70016-27387 and Capes (Coordination and Improvement of Higher Level or Education Personnel) by providing a scholarship for the first author, [PERSON], grant number 88881.361826/2019-01.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article.  
**Acknowledgments:** We thank [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] for their technical assistance in the field. We also thank Sungro Horticulture ([PERSON] and [PERSON]), ICL Specialty Fertilizers ([PERSON]), and Harrell's Fertilizers ([PERSON] and [PERSON]) for product donations.  
**Conflicts of Interest:** The authors declare no conflict of interest.