Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/16/2010
Publication Date: 9/15/2010
Citation: Hartung, J.S., Halbert, S., Pelz-Stelinski, K., Brlansky, R.H., Chen, C., Gmitter, F. 2010. Lack of Evidence of Transmission of 'Candidatus' Liberibacter Asiaticus Through Citrus Seed Taken From Affected Fruit. Plant Disease. 94(10):1200-1205. Interpretive Summary: Citrus huanglongbing disease is the greatest threat to the world citrus industry today. The cornerstone of disease control programs is the production of healthy seedlings for transplantation into orange grove, but it is not known whether or not the pathogen can infect seed and be transmitted to seedlings. If this were the case, the disease control program would be compromised. This research was undertaken to determine whether or not the pathogen can be transmitted through seed. Fruit affected by citrus huanglongbing were collected in Florida and seed obtained from them were grown in screened greenhouses in both Florida and Maryland. Symptoms of huanglongbing were not observed in any of the 700 seedlings included in this study, even after 3 years of growth. Nor were any of the laboratory tests positive for the pathogen. Thus no evidence of transmission of the citrus huanglongbing pathogen was obtained in this study and we conclude that the pathogen is not transmitted through seed. This research is of significance to the world-wide citrus industry.
Technical Abstract: Citrus huanglongbing disease, putatively caused by the associated bacterium Ca. Liberibacter asiaticus is the greatest threat to the world citrus industry today. The bacterium is spread locally and regionally by the citrus psyllid, Diaphorina citri Kuwayama, and can also be disseminated by grafting with contaminated budwood. Affected fruit produce aborted seed, suggesting that seed can become contaminated with the pathogen, and there is a previous preliminary report of transmission of the pathogen through true seed. This research was undertaken to determine whether or not Ca. Liberibacter asiaticus can be transmitted through seed. Seed from symptomatic fruit was confirmed to be contaminated by Ca. Liberibacter asiaticus by qPCR. Two groups of approximately 350 seedlings each, of various citrus species were grown from seed removed from fruit on trees that were symptomatic for huanglongbing and confirmed to be infected with the bacterium by PCR-testing. These seedlings were tested over a period of either 7-10 months or up to three years. No symptoms of huanglongbing, such as blotchy leaf mottle, spreading chlorosis and dieback of branches were observed in these seedlings and none of these 700 seedlings tested positive for the presence of the pathogen even after repeated and sensitive testing by q-PCR. Some sour orange seedlings did have quite pronounced and atypical growth habits. These atypical growth habits were limited to seedlings that arose from zygotic embryos as determined by EST-SSR analyses. Thus no evidence of transmission of Ca. Liberibacter asiaticus was obtained, and the earlier report of transmission of the pathogen through seed was not confirmed.