Mandarin (Citrus reticulata) is the most important cash crop in Bhutan and plantations total approximately 1.8 million trees (Ministry of Agriculture, Thimphu, Bhutan, 2000). Most trees are a local mandarin variety. Seedlings trees are produced by local farmers or supplied by Druk Seed Nursery. Mandarin seedlings have also been introduced from India. In the mid-1990s, mandarin trees growing in Punakha Valley and Wangdue districts began showing symptoms of decline that included sparse yellow foliage and shoot die-back. After initial surveys in 2000, huanglongbing (HLB) was suspected as the cause of declining trees based on symptomatology and presence of the psyllid vector Diaphorina citri, but no confirmatory tests were carried out. In August 2002, we surveyed eight locations in the valley from Rimchu (North) to Kamichu (South). HLB-like leaf mottle symptoms were observed on declining mandarin trees at all locations at altitudes ranging from 700 to 1,450 m. Orchards around Punakha (1,350m) in the center of the valley were more severely affected. Symptoms were also observed on Mexican lime (Citrus aurantifolia), citron (Citrus medica), and on tangelo trees (Minneola, Seminola, and Iyo) introduced originally as certified HLB-free budwoods from Corsica, France and grafted onto Rangpur lime at the Wangdue Research Center (1,300m). Leaves were collected from symptomatic trees and three declining mandarin trees without characteristic leaf mottle symptoms. Two specific polymerase chain reaction (PCR) tests for the detection of HLB Liberibacter species (1,2) were carried out on 16 DNA samples extracted from leaf mid-veins of 10 mandarins, two Mexican limes, three tangelos, and one citron tree. “Candidatus Liberibacter asiaticus” was readily detected by both PCR assays in all but two samples (one mandarin with noncharacteristic symptoms and citron) and all sampled orchards. The presence in the Wangdue Research Center of liberibacter infected trees, propagated from certified HLB-free budwoods, suggests that natural spread of the HLB by D. citri is occuring, as the psyllid had been identified previously in the Punakha area by Bhutanese Entomologists. It is likely that the disease was originally introduced as infected planting material although its source has not been determined. References: (1) A. Hocquellet et al. Mol. Cell. Probes 13:373, 1999. (2) S. Jagoueix et al. Mol. Cell.Probes 10:43,1996.
ABSTRACT
Marginal chlorosis is a new disease of strawberry in which the uncultured phloem-restricted proteobacterium “
Candidatus
Phlomobacter fragariae” is involved. In order to identify the insect(s) vector(s) of this bacterium, homopteran insects have been captured. Because a PCR test based on the 16S rRNA gene (rDNA) applied to these insects was unable to discriminate between “P. fragariae” and other insect-associated proteobacteria, isolation of “P. fragariae” genes other than 16S rDNA was undertaken. Using comparative randomly amplified polymorphic DNAs, an amplicon was specifically amplified from “P. fragariae”-infected strawberry plants. It encodes part of a “P. fragariae” open reading frame sharing appreciable homology with the
spoT
gene from other proteobacteria. A
spoT
-based PCR test combined with restriction fragment length polymorphisms was developed and was able to distinguish “P. fragariae” from other insect bacteria. None of the many leafhoppers and psyllids captured during several years in and around infected strawberry fields was found to carry “P. fragariae.” Interestingly however, the “P. fragariae”
spoT
sequence could be easily detected in whiteflies proliferating on “P. fragariae”-infected strawberry plants under confined greenhouse conditions but not on control whiteflies, indicating that these insects can become infected with the bacterium.