ARS | Publication request: Tolerance of the trifoliate citrus hybrid US-897 (Citrus reticulata Blanco x Poncirus trifoliata L. Raf.) to Huanglongbing

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 27, 2010
Publication Date: January 1, 2011
Citation: Albrecht, U., Bowman, K.D. 2011. Tolerance of the trifoliate citrus hybrid US-897 (Citrus reticulata Blanco x Poncirus trifoliata L. Raf.) to Huanglongbing. HortScience. 46(1):16-22.

Interpretive Summary: Huanglongbing (HLB) is a destructive disease of citrus in most citrus-producing countries. The disease is caused by bacteria, which are restricted to the phloem, and affects all known citrus species and citrus relatives with little known resistance. Infected trees typically show chlorotic- or blotchy mottled leaves, produce fewer and abnormal looking fruit and decline rapidly. Field observations in other countries showed that trifoliate orange and some of its hybrids lack distinct disease symptoms despite infection with the bacterium. US-897 is a hybrid of trifoliate orange and mandarin. This study investigated whether field-grown, naturally infected US-897 trees and greenhouse-grown, artificially infected US-897 seedlings display tolerance or resistance to HLB. It was shown that most naturally infected US-897 trees did not exhibit any distinct disease symptoms. Analysis of fruit and seed from infected trees did not detect any growth reduction or otherwise negative impact on development. Like the field trees, most artificially infected greenhouse-grown US-897 seedlings did not develop disease symptoms typical for HLB. In addition, stem growth was only moderately reduced or unaffected. On the contrary, mandarin seedlings which were studied for comparison developed severely chlorotic leaves and grew stunted. The superior performance of US-897 plants in greenhouse- and field locations suggests tolerance of this genotype to HLB. Identifying the mechanism of tolerance or resistance in trifoliate orange might yield a strategy to combat this destructive disease of citrus.

Technical Abstract: Huanglongbing (HLB) is a destructive disease of citrus in most citrus-producing countries worldwide. The disease, presumably caused by phloem-limited bacteria of the genus Candidatus Liberibacter, affects all known citrus species and citrus relatives with little known resistance. Typical disease symptoms are the production of abnormal looking fruit and chlorosis or blotchy mottle of the leaves, followed at advanced stages by tree decline and death. Trifoliate orange (P. trifoliata L. Raf.) and some of its hybrids reportedly lack distinct disease symptoms despite infection with the pathogen. US-897 is a hybrid of trifoliate orange and ‘Cleopatra’ mandarin (C. reticulata Blanco), the latter being highly susceptible to HLB. This study investigated whether field-grown, naturally infected trees and greenhouse-grown, graft-inoculated seedlings of this genotype display tolerance or resistance to HLB. It was shown that naturally infected US-897 trees exhibited no distinct disease symptoms commonly associated with HLB, except for the occurrence of few mottled leaves in a small percentage of trees. Analysis of fruit and seed from infected trees did not detect any growth reduction or otherwise negative impact on development. Graft-inoculated US-897 seedlings became PCR-positive for the pathogen, but exhibited a superior performance compared with ‘Cleopatra’ mandarin seedlings, which displayed severe disease symptoms soon after inoculation. Despite infection, most US-897 seedlings did not develop leaf symptoms typical for HLB. Foliar symptoms observed in a small number of plants at later stages of the disease were faint and difficult to discern. Contrary to ‘Cleopatra’ seedlings, growth in stem diameter was only moderately reduced or unaffected in infected US-897 seedlings. The superior performance of US-897 plants in greenhouse- and field locations suggests tolerance of this genotype to Ca. L. asiaticus.