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Sultan Qaboos University (2017)

Characterization of huanglongbing disease associated with acid lime (Citrus aurantifolia Swingle) in Oman

Al-Fahdi, Ahmed Rashid Marhoun

Titre : Characterization of huanglongbing disease associated with acid lime (Citrus aurantifolia Swingle) in Oman

Auteur : Al-Fahdi, Ahmed Rashid Marhoun

Université de soutenance : Sultan Qaboos University

Grade : Master of Science (MS) in Crop Protection 2017

Résumé
Huanglongbing (HLB) or citrus greening disease is considered one of the most destructive diseases associated with the Citrus genus. It is caused by three common species of Candidatus Liberibacter bacterium : Candidatus Liberibacter asiaticus, Candidatus Liberibacter africanus and Candidatus Liberibacter americanus. This study was conducted to characterize the casual agents as well as the occurrence and distribution of citrus greening associated with acid lime (Citrus aurantifolia Swingle) in farms and nurseries in Oman. For the first time, a survey of different geographical areas in Oman has shown that the HLB disease is present in some northern and southern farms. Among the three causal agent of HLB, phylogenetic analysis of the 16SrRNA gene of 38 isolates indicated an association of Ca. L. asiaticus (Las) with the citrus greening disease of acid lime in Oman. The disease symptoms in some areas were observed year around, but more apparent from October through March. HLB affects the entire canopy of acid lime trees including leaves, fruits, and twigs. HLB symptoms are characterized by yellow veins, blotchy mottled patterns on leaves, small green fruits, lopsided fruits, mixed color and half green fruits, yellow shoots, twig dieback, and tree decline. The disease incidence was 71.8% in Sohar and Wkaan, 40% in Al Jabal Al Akhdar and 62% in Salalah. The phylogenetic analysis in this study showed that Oman isolates are clustered with isolates from Iran, India, USA, China, and other countries. Surveys from commercial nurseries detected no seedlings infected with citrus greening, but this does not eliminate the nurseries’ role in transmitting the disease. This study reports for the first time the presence of citrus greening associated with acid lime in Oman. The application of extension programs to educate farmers and the application of strict quarantine measures is recommended to avoid further introductions of new diseases into Oman or the spread of HLB to other locations. Further studies focusing on the HLB disease possibly infecting other citrus species in the country is required.

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