Field Performance of ‘Valencia’ Sweet Orange Trees Grafted onto Pummelo Interstocks and Swingle Citrumelo Rootstocks under Huanglongbing (HLB) Endemic Conditions

Interstocks have been used in fruit tree cultivation to regulate size and improve production quality. In this study, several Huanglongbing (HLB)-tolerant open-pollinated pummelo interstock candidates were evaluated as interstocks between the Swingle rootstock ‘Valencia’ scion, with serving control interstock. After 5 years field, most trees did not exhibit visual HLB symptoms, although infected HLB, CaLas Ct values leaves of different treatments ranged 25.88 27.82. Although foliar chlorophyll content among was highly significant (p-value = 0.0313), starch significantly 0.0018). grafted onto 5-1-99-3 HBJL-4 (both open pollinated seedlings Hirado Buntan pummelo) exhibited highest total phenolic compound (TPC) levels (46.44 46.36 mg gallic acid g−1 FW). Transcripts CsPR1 CsPR2, two pathogenesis-related (PR) genes, upregulated seedling selections red shaddock pummelo, Liang Ping Yau compared Swingle. All influenced growth rate improved canopy volume field without any 0.0085). The 5-4-99-7 (red 8-1-99-1B (Liang had all treatments. We propose, based on our current results, that HLB-tolerant citrus accessions, when judiciously interstocks, may enhance plant defense provide increased tolerance susceptible scions.