Response of Citrus spp. germplasm from Puerto Rico grafted on two rootstocks to early infection of huanglongbing

Abstract

Huanglongbing (HLB), associated with the proteobacterium ‘Candidatus Liberibacter asiaticus’(CLa), is a destructive disease in citrus. Resistance/tolerance has been identified in some citrus species, wild relatives, and transgenic citrus genotypes. The objectives of this study were to: (1) evaluate the response of the citrus germplasm collection from Puerto Rico (namely ‘Rico’) to HLB, (2) determine the most appropriate rootstock-genotype combination, and (3) quantify the presence of the bacterium in the plant tissue by quantitative polymerase chain reaction (qPCR). Seventeen genotypes were grafted onto ‘HRS 812’ and ‘Swingle’ citrumelo rootstocks. The plants were inoculated with HLB infected buds and evaluated at 12 months after inoculation in a greenhouse in Adjuntas, Puerto Rico. The genotypes ‘Lares’ and ‘Padilla’ had significantly higher incidence (>65.0%) and severity (3.2–4.3) compared to ‘Mexican Lime’ and ‘Tahiti Lime’ that had lower scores (<18.0%, 1.1–1.3) using both rootstocks. In contrast, the genotypes ‘Fronton’, ‘PR-Sel-1’, and ‘Rico 7–376’ grafted on HRS 812; and ‘Pietri 334’ grafted on Swingle citrumelo had lower HLB incidence (<10%) and severity (<1.5). Significantly higher mean cycle threshold (Ct) values were only observed for the genotypes Fronton (37.0), Tahiti Lime (36.5), and Pietri 334 (36.2) compared to Padilla (34.7 and 32.7 when HRS 812 and Swingle citrumelo was used as rootstock, respectively). Citrus genotypes having lower HLB scores were identified, however, their reaction varied according to the rootstock used.