Population dynamics of Padi avenae (PAV), Macrosiphum avenae (MAV), and Rhopalosiphum padi (RPV) virus serotypes of Barley yellow dwarf virus (BYDV) and of their main aphid vectors were studied in winter barley (Hordeum vulgare) fields for three successive years in western France. An epidemiological model of the spread of viruses in the field was developed based on vector populations as forcing variables and the population dynamics of each virus serotype. This model accurately simulated the kinetics of the epidemic for PAV serotypes, which are the most common ones. For RPV and to some extent for MAV, the results were less satisfactory. The occurrence and spread of PAV and MAV serotypes in the field was clearly and easily related to that of their main vector species. Conversely, the spread of RPV serotypes showed no consistent relationships with the dynamics of their vectors. Incidence of PAV in 1989 to 1990 and 1990 to 1991 showed a bimodal distribution, with maximums in fall (December) and spring (May) that were linked to fall infestations by R. padi and spring infestations by three (R. padi, Sitobion avenae, and Metopolophium dirhodum) or two (S. avenae and M. dirhodum) aphid species. In 1991 to 1992, the PAV infection curve was monomodal and mainly due to a primary spread of the virus by very large populations of alate R. padi. MAV incidence was low in fall and winter and reached a maximum in spring 1990 and 1991 related to the occurrence of S. avenae and M. dirhodum. RPV incidence was low every year, despite the abundance of its vector, R. padi. Mixed infections were more frequent than expected by chance and were assumed to be partly related to heterologous encapsidation. The occurrence of each serotype is discussed in relation to the time of crop infection and possible damage.

Summary During the years 1989–1992 cereal aphids were caught alive in a low level (1.5 m high) suction trap operated in Le Rheu (Brittany, France) and tested for BYDV transmission. In most cases comparisons with data collected simultaneously by a 12.2 m suction trap operating in the same site resulted in good relationships between weekly catches at both heights. Results from transmission tests showed that: (i) the two main BYDV vectors were Rhopalosiphum padi and Metopolophium dirhodum during the years of experiment; (ii) PAV and MAV were the commonest viruses and RPV was relatively scarce; (iii) during spring M. dirhodum appeared to be the most important MAV vector and nearly as good a PAV vector as R. padi; (iv) during autumn R. padi was the only vector of the three viruses with mixed transmission allowing it to transmit also MAV probably by heteroencapsidation. To give an indication of the risk of infection, infectivity indices were calculated by multiplying the numbers of aphids caught by the 12.2 m suction trap by the proportion that were infective. These infectivity indices agreed with field records of primary infections.

Nucleotide and amino acid sequences of the coat protein (CP) of 12 isolates of Cymbidium mosaic virus from Vanilla fragrans in Reunion Island (CyMV‐R) were compared with each other and with those of previously described Asian strains. Alignment revealed that CyMV‐R isolates were highly homologous, suggesting that one strain is prevalent in Reunion. This strain also showed high homology with the Korean CyMV‐K2 and Singapore CyMV‐S2 strains, but nucleotide additions resulted in the carboxy‐terminal ends of the CP sequences differing from those of the Korean CyMV‐K1 and Singapore CyMV‐K1 strains.

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 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 enzyme-linked immunosorbent assay (ELISA). However, a virus, saptransmitted 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 (<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.Cucumber mosaic virus 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.