## 1 Introduction
Recently published data on the field trials of these new releases suggested that the growth and yield were vigorous in Sugar Belle mandarin trees even in the presence of HLB symptoms, which indicated that it could be tolerant to HLB disease [37]. In greenhouse and semi-field experiments, we found that the Sugar Belle mandarin was high in many secondary metabolites including phenolic compounds, which could act as antibacterial agents, as well as sugar alcohols, which could protect it from stress during pathogen attack [38; 39; 40]. We found that Sugar Belle was high in several leaf volatiles such as thymol, \(\beta\)-elemene, and \(\beta\)-caryophyllene, which are known for their anti-microbial activity. These results were obtained from Sugar Belle scions on UF-15 and 'Carrizo' rootstocks, respectively [38; 39], but these were not comparative studies. Taken together, however, these data indicate that citrus leaf volatiles could play a major role in tolerance against the '_Ca._ L. asiaticus' pathogen. Among the many roles of leaf volatiles are those of attracting pollinators, repelling herbivores, or attracting their natural predators. Induction of terpenids as a plant defense following insect herbivory has been widely reported in both annual and perennial crop plants including corn [41], citrus [42], peach [43], and pear [44]. Psyllid-infested pear trees produced more limonene, \(\alpha\)-farnesene, and \(\delta\)-cadinene after infestation, as well as green leaf volatiles such as (Z)-3-hexen-1-yl acetate and (Z)-3-hexen-1-ol [44]. Peach trees that were infested with the green peach aphid released higher levels of \(\beta\)-ocimene, \(\alpha\)- and \(\beta\)-farnesene, and \(E\)-nerolidol [43]. Many VOCs also stimulate the volatile defenses of nearby plants (defensive priming) or act as kiromones for attracting the natural predators or parasitoids of the target pest, demonstrating complex inter-trophic relationships between plants and their pests [41]. For example, sour orange trees that were infested with the two-spotted spider mite, _Tetranychus urticae_, showed increased levels of herbivore-induced volatiles in cluding \(\alpha\)-pinene and 2-ethyl-1-hexanol, and also induced neighboring un-infested 'Cleopatra' mandarin trees to produce similar defensive volatile compounds [45]. In rice, the herbivore-induced linalool and \(\beta\)-caryophyllene attracted predators and parasitoids of leafhoppers [46], while in citrus, d-limonene and \(\beta\)-ocimene attracted the parasitoid of _Thuumatotibia leucotreta_, a pest of citrus in Africa [47]. In our earlier work comparing the leaf volatile organic compounds (VOCs) of 14 citrus varieties [16], we found that citrus cultivars that were considered relatively tolerant to HLB such as Carrizo citrange and _Citrus latipes_ contained higher amounts of volatiles than the susceptible cultivars, especially those known for their antimicrobial activities, such as sesquiterpenes and aldehydes. Herein, we hypothesize that rootstock influences the VOC biosynthesis in scion leaves leading to an alteration of the VOCs profile. We expect that this alteration might be more pronounced after the infestation with \(D\). _citri_. To test this hypothesis, we investigated the leaf VOCs of Sugar Belle that was grafted onto three locally available and popular rootstocks, C-35, US-897, and sour orange, with and without infestations of \(D\). _citri_, the vector of HLB.