Publications

4453

Abstract Whole‐genome sequencing of “ Candidatus Liberibacter asiaticus” (Las) indicated some polymorphic gene regions enabling the molecular characterization of this bacterium. Although the population diversity of Las in China has been previously studied, no reports have used a combination of a prophage region and short tandem repeat (STR) loci for phylogenetic relationship characterization. In this study, we investigated the genetic diversity and structure of 667 Las strains from nine provinces in southern China using multiple genetic loci including three type‐specific prophage loci, two STR loci, and a miniature inverted‐repeat transposable element (MITE) region. The results indicated that the genetic diversity varied according to the gene loci used. The prophage regions, including the MITEs, revealed significant genetic differences of Las populations in Yunnan and Guizhou from other provinces, while the STR loci only indicated a difference of Las population in Guizhou from those of other provinces. In particular, the Las population shown to be diverse in Jiangxi at the CLIBASIA_01215_STR locus was not diverse when measured using the other loci. Considering all loci, the Las populations from Yunnan and Guizhou were different, those from Guangdong and Guangxi were complex, while the Las population in Jiangxi was comparatively simple. Las populations from five neighbouring provinces, including Guangdong, Guangxi, Hunan, Fujian, and Jiangxi were grouped in a big cluster. The Las population structure changed with age according to the prophage type. This study evaluated the method used for studying the molecular diversity of Las populations and provided more detailed information on the geographical origin and epidemic trend of Chinese Las populations.

Abstract“Huanglongbing” (HLB) is one of the most devastating diseases of citrus orchards worldwide. Samples from 183 citrus plants of different cultivars and rootstock/cultivar combinations, showing HLB symptoms in three Caribbean countries (Cuba, Jamaica, and Guadeloupe-France), were collected to verify the possible co-infection of ‘Candidatus Phytoplasma’ and ‘Candidatus Liberibacter’ species. The 64% of the samples resulted positive to the ‘Ca. L. asiaticus’ and the 27% to diverse ‘Ca. Phytoplasma’-related species, moreover about the 14% of the samples infected with ‘Ca. Liberibacter’ were also found positive to phytoplasmas, indicating the presence of mixed infection especially in the orchards located in Cuba. Moreover, in one of the samples from Jamaica mixed phytoplasma infection was detected. Moreover the detection of only phytoplasmas in 11 symptomatic citrus samples collected from Cuba and Guadeloupe without ‘Ca. Liberibacter’ detection, confirmed that the symptomatology cannot be the sole criterium to discriminate between the presence of the two pathogens, and molecular detection is necessary to identify single or mixed infections. Diaphorina citri insects collected from Cuba and Guadeloupe resulted infected with ‘Ca. L. asiaticus’ confirming its active role in the dissemination of the pathogen. Only one insect of the Cicadidae family, collected in Guadeloupe, was found positive for phytoplasma presence. Considering that the phytoplasmas belonging to some ‘Candidatus species’ were detected in the three countries in different citrus varieties, a relevant role as phytoplasma reservoir can be attribute to citrus orchards.

Citrus, mainly mandarin (Citrus reticulata Blanco), is an economically important fruit crop in Bhutan. Despite having favorable agroclimatic conditions for citrus cultivation, the early decline of fruit-bearing orchards coupled with low crop productivity is a major concern among citrus growers. During a recent survey, an association of ‘Candidatus Liberibacter asiaticus’ (citrus greening) and citrus tristeza virus (CTV), either singly or as mixed infections in declined citrus trees, was recorded in all four major citrus-growing districts (Tsirang, Dagana, Zhemgang, and Sarpang). Using PCR-based diagnosis, a higher incidence of citrus greening (27.45%) and tristeza (70.58%) was observed in symptomatic field samples. Detection and characterization of ‘Ca. L. asiaticus’ was performed based on the 16S ribosomal DNA, prophage gene, 50S ribosomal rplA-rplJ gene, and tandem repeats of the CLIBASIA_01645 locus. Similarly, the coat protein, p23, and p18 genes were used as genetic markers for the detection and characterization of Bhutanese CTV. The ‘Ca. L. asiaticus’ isolates from Bhutan segregated into classes II and III based on the CLIBASIA_01645 locus, analogous to Indian isolates from the northeast region and Term-A based on the CLIBASIA_05610 locus. CTV isolates of Bhutan were observed as closely related to the VT strain, which is considered to be the most devastating. To the best of our knowledge, this is the first study on molecular characterization of ‘Ca. L. asiaticus’ and CTV isolates and their association with citrus decline in Bhutan.