Title: Candidatus Liberibacter asiaticus and Huanglongbing effects on citrus seeds and seedlings Authors
Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 19, 2009
Publication Date: December 1, 2009
Citation: Albrecht, U., Bowman, K.D. 2009. Candidatus Liberibacter asiaticus and Huanglongbing effects on citrus seeds and seedlings. HortScience. 44:1967:1973. Interpretive Summary: Huanglongbing (HLB) is a devastating disease of citrus and threatens the citrus industry worldwide. HLB is caused by a bacterium and can be transmitted through insect vector or grafting with diseased budwood. This study investigates whether the bacterium is transmissible from fruiting tree to seedling through seed, and how HLB infection affects seed quality and seedling development. Infection of trees and fruit with the bacterium significantly reduced seed weight, seed germination and seedling height, but seedlings did not develop symptoms typical of HLB. Molecular analysis initially identified less than one percent of all seedlings positive for the pathogen, but re-analysis after additional growth resulted in negative results. The bacterium that causes HLB may be translocated into external parts of the embryo during seed development, but this infection typically disappears during early growth and is unlikely to result in seedlings that have HLB disease symptoms.
Technical Abstract: Huanglongbing (HLB) is a devastating disease of citrus and threatens the citrus industry worldwide. The suspected causal agent of the disease is a phloem-limited bacterium of the genus Candidatus Liberibacter, transmitted through insect vector or grafting with diseased budwood. Currently, most seed source trees for citrus rootstock propagation are located outdoors and unprotected from disease transmission. In addition, fruit from HLB-affected scion varieties in Florida containing seeds, enter the commercial trade and move into other citrus growing areas. The objective of this study was to determine how Ca. L. asiaticus infection affects seed quality and seedling development, and whether the disease appears in seedlings grown from infected fruit. Two experiments were conducted, involving thousands of seedlings produced from seeds from infected rootstock seed source trees and ‘Valencia’ sweet orange trees, respectively. Infection of trees and fruit with Ca. L. asiaticus significantly reduced seed weight, seed germination and seedling height. Seedlings did not develop symptoms typical of HLB throughout the experiment. PCR analysis initially identified two of 686 rootstock seedlings and three of 431 sweet orange seedlings positive for the pathogen when they were very young. Resampling and PCR analysis of these five seedlings at older ages consistently indicated they were negative for the pathogen and none of these plants ever developed symptoms of HLB. It is suggested that Ca. L. asiaticus may have been translocated into some part of the embryo during seed development, but that it was not present in cells or tissue which permitted replication or disease development as the seedling grew.