Severe stunting of Vanilla tahitensis in French Polynesia caused by Cucumber mosaic virus (CMV), and the detection of the virus in V. fragrans in Reunion Island

Vanilla (Vanilla tahitensis and V. fragrans) is cultivated in French Polynesia and Reunion Island for its highly priced beans. Severely stunted V. tahitensis plants with conspicuous stem and leaf deformation were observed (Fig. 1) during a survey of vanilla in French Polynesia. Cymbidium mosaic virus, Odontoglossum ringspot virus, potyviruses or rhabdo-like particles, which have been previously detected from vanilla (Wisler et al., 1987; Pearson et al., 1993), were not detected in such plants by electron microscopy or ELISA. However, a virus sap-transmitted from affected plants induced severe systemic leaf mosaic and distortion in Nicotiana benthamiana and N. clevelandii. Double-stranded RNAs extracted from such plants had an electrophoretic profile typical of Cucumber mosaic virus (CMV).

Infection by CMV was confirmed by DAS-ELISA in 23.5% of 179 samples of V. tahitensis from French Polynesia. RT-PCR of total RNA extracts from six samples, using primers specific to the coat protein gene (Wylie et al., 1993), amplified a fragment of the expected size (c. 500 bp). The sequences of these isolates showed 90-98% nucleotide homology to each other, and 89-96% and 80-81% homology to strains of CMV subgroups I and II, respectively.

CMV was also detected by RT-PCR in 5.7% of 105 V. fragrans samples from Reunion Island. The leaves of the CMV-infected plants were slightly elongated, but did not have the severe symptoms shown by V. tahitensis vines in French Polynesia. The presence of CMV in all PCR-positive samples was confirmed by ELISA. Sequences of the Reunion Island isolates shared 99-100% nucleotide homology with each other, 91-95% homology with French Polynesia isolates, and 91-93% and 79-80% homology to strains of CMV subgroups I and II, respectively.

This is the first report of CMV in vanilla. It has not yet been determined whether the differences in symptoms in CMV-infected vanilla in French Polynesia and Reunion are due to virus strain variation or differential tolerance of the vanilla species. However, the wide natural host range of CMV (Douine et al., 1979) indicates that alternative hosts of the virus are probably present in and around vanilla plots in both countries. In view of the severity of the disease, it is recommended that sources of infection adjacent to the crop should be sought and removed.