Response of citrus germplasm seedlings to Candidatus Liberibacter Asiaticus infection under controlled greenhouse conditions

Authors: BISI, RAYANE - UNIVERSITY OF FLORIDA; ALBRECHT, UTE - UNIVERSITY OF FLORIDA; Bowman, Kim

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2023
Publication Date: 1/31/2024
Citation: Bisi, R., Albrecht, U., Bowman, K.D. 2024. Response of citrus germplasm seedlings to Candidatus Liberibacter Asiaticus infection under controlled greenhouse conditions. HortScience. 59(3), 278-287. https://doi.org/10.21273/HORTSCI17514-23.
DOI: https://doi.org/10.21273/HORTSCI17514-23

Interpretive Summary: Huanglongbing (HLB) is a devastating disease of sweet orange, grapefruit, mandarin, and other citrus crops. The disease is endemic in Florida and Texas, and threatens to spread in California. Using a Candidatus Liberibacter asiaticus (CLas) tolerant citrus scion and/or rootstock at planting helps to reduce costs of production in an HLB-endemic environment and maximize financial returns. To better understand scion and rootstock response to CLas infection, four experiments were carried out under controlled greenhouse environmental conditions. In these experiments we studied the scion and rootstock response to CLas infection among specific clones, related species, and hybrids, to guide choices by citrus growers on cultivars to use for new plantings, and to support future citrus breeding efforts. Responses to CLas infection differed greatly among the germplasm evaluated, with trifoliate orange and its hybrids demonstrating strong tolerance to the disease, while many of the commercial fruiting types, such as sweet orange and grapefruit, exhibiting strong sensitivity and severe reactions to infection. The results of the experiments help to identify the most promising cultivars to grow in the HLB-endemic environment now, and provide new knowledge that can lead to the development of more resistant cultivars for the future.

Technical Abstract: Huanglongbing disease (HLB) is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), and affects all known citrus species and relatives, with many commercial cultivars being greatly damaged. In endemic regions, such as Florida, HLB causes a significant reduction in fruit crops and increased cost of production. Development of new tolerant or resistant citrus cultivars is considered one promising strategy to overcome this destructive disease. Testing cultivar tolerance to HLB in field conditions is difficult, due to the erratic spread of CLas, scion and rootstock interaction, and the strong influence of many biotic and abiotic factors on tree response to the disease. This study aimed to 1) define graft-inoculation methods that can be used to evaluate citrus germplasm for tolerance or resistance to CLas under greenhouse conditions, and 2) employ these methods to evaluate the effect of CLas infection on several different citrus species and hybrids thought to have different levels of tolerance to the disease. Young potted seedlings from 12 different selections of citrus germplasm were graft-inoculated with CLas or mock-inoculated. Plants were monitored periodically during 18 months for canopy growth, HLB and nutritional leaf symptoms, canopy color, leaf chlorophyl content index, and leaf CLas population using quantitative polymerase chain reaction. Roots were also assessed at 18 months after inoculation for CLas population and biomass distribution. Infected seedlings from susceptible cultivars started to show significant HLB symptoms one year after inoculation. There were significant differences between healthy and infected plants for most variables analyzed. The most susceptible cultivars showed more severe HLB symptoms and tolerant cultivars had mild or no symptoms. Some plants of all cultivars were successfully infected by CLas, but overall, the CLas transmission rate using graft-inoculation was low and inconsistent. This study provides new knowledge on HLB-tolerance of different citrus germplasm, supporting the development of new cultivars with improved HLB tolerance or resistance.