## 1 Introduction  
Much effort has been given recently to evaluating the disease tolerance of new citrus varieties and scion/rootstock combinations to the citrus disease known as Huanglongbing (HLB) [1; 2; 3; 4; 5]. Citrus rootstocks are generally selected to improve fruit yield and quality [7; 8; 9]; help control soil-borne pests and diseases such as _Phytophth_ root rot [10], nematodes [11], and root weevils [12]; defend against abiotic factors [13; 14]; or for soil compatibility [8]. Rarely have rootstocks been compared for their contribution to the leaf volatile profile [15; 16]. As the leaves are the landing, feeding, and reproduction sites for _Diaphorina citri_, the vector of HLB, determining whether rootstock choice has any influence on leaf volatile organic compounds (VCCs) that could be exploited to repel or deter _D. citri_ could be beneficial. Huanglongbing (HLB), or citrus greening disease, was first discovered in southern China in 1919 [17], and has spread to many citrus growing regions including Brazil and the U.S. [18; 19]. In Florida, HLB has been responsible for a decline of 71% in citrus production for the 2016-2017 season compared to 2000-2001 levels [20]. The putative pathogen of HLBis a Gram-positive, phloem-limited \(\alpha\)-proteobacteria known as _'Candidatus_ Liberibacter asiaticus', which is transmitted by the Asian citrus psyllid, _Diaghorina citri_ Kuwayama [Hemiptera: Liviidae] [19; 21]. _D. citri_ transmits the bacterium from tree to tree while it feeds on the phloem sap of infected citrus trees, and currently, no effective cure is available. The diseased trees produce a low yield of unevenly ripened, lopsided, and bitter fruit, followed by early tree death [22; 23]. The fruit from sweet orange trees such as 'Valencia' and 'Hamlin' are processed into orange juice, and make up the majority of the Florida citrus acreage [24]. Unfortunately, these widely planted varieties are particularly susceptible to HLB disease, with many Florida groves reaching 100% infection [23]. The choice of rootstock can play a role in many horticultural traits such as tree size, fruit yield, and fruit quality [25; 26; 27; 28], as well as tolerance to salinity [29; 30]. A recent survey of 'Hamlin' on 32 rootstocks in two locations with different soil types showed that the rootstock choice affected the severity of HLB foliar disease symptoms and sturdiness under storm-force winds [31]. However, [PERSON] [32] propagated 'Valencia' sweet oranges on 11 rootstocks in a greenhouse setting, and found that only a few parameters such as leaf surface area and root susceptibility to '_Ca._ L. asiaticus' were influenced by rootstock. No significant differences were found in the bacterial titer in the leaves after graft-inoculation with '_Ca._ L. asiaticus', and both the leaf size and number were reduced in all of the seedlings after infection regardless of rootstock [32]. Vector control has become the primary weapon for combatting HLB disease, mostly through chemical means. Biological control with the parasitic wasp, _Tamarixia radiata_, has also gained some success [33]. Enhanced citrus nutrition programs to maintain tree productivity have met with mixed results [34], and the development of molecular tools such as interference RNA (RNAi) against _D. citri_ are promising, but face practical hurdles outside of the laboratory [35; 36]. Finally, many growers have removed heavily diseased trees hoping to replace them with disease-free tolerant varieties, but truly tolerant citrus varieties are still lacking. In 2009, the University of Florida IFAS/CREC cultivar improvement team released many mandarin hybrids including the 'Sugar Belle' mandarin hybrid, previously known as LB8-9 ['Clementine' mandarin (_Citrus reticulata_) \(\times\) 'Mineola' tangelo (_Citrus \(\times\) Tingelo_), 'Duncan' graprefruit (_Citrus paradise_) \(\times\) 'Dancy' tangerine (_Citrus reticulata_)].