The spread of CMV was monitored in several shade houses planted with virus-free cuttings and conducted in various locations in the Leeward Islands (Richard et al., 2009). The results showed that in the absence of insect-proofing and in favorable locations CMV can spread very quickly, with 50% of vines infected only two years after planting. In contrast, with insect proofing or in another location, a very low incidence of CMV was recorded. These results demonstrated the role of aphids in spreading the disease and also the importance of the environment possibly through the presence or absence of CMV reservoir among weeds.
C. diffusa (common name climbing or spreading dayflower, ma’apape in Tahitian or herbe d’eau in French, Figure 7.5) was shown to play a predominant role in CMV epidemics in FP (Richard et al., 2009): (1) C. diffusa was the only species that exhibited simultaneously high frequencies of CMV infection and aphid (A. gossypii) infestation, (2) the occurrence of CMV in vanilla vines was highly correlated with the proximity of C. diffusa, (3) the incidence of CMV in the plots surveyed was significantly correlated with the occurrence of infected C. diffusa within the plot but not with the other CMV-infected species, and (4) the ability of A. gossypii and A. craccivora to transmit CMV from C. diffusa to young plants of V. tahitensis was demonstrated in laboratory tests. All these data, along with the high sequence similarities between vanilla and C. diffusa CMV isolates (Farreyrol et al., 2009) strongly support the view that C. diffusa is a major active reservoir of CMV and contributes greatly to the spread of this virus in vanilla plots. This epidemiological scenario resembles that described for CMV in banana plantations (Eiras et al., 2004; Lockard, 2000; Magnaye and Valmayor, 1995).
FIGURE 7.5 C. diffusa Burm. f., the main aphid and virus reservoir involved in CMV epidemics in FP: (a) virus-infected plants; (b) flower; and (c) aphid-infested leaf.
Control
In view of the very severe symptoms induced on leaves and flowers and its potentially high incidence in the field, CMV is a major threat to vanilla plantations and should be a priority for risk assessment and control when implementing intensive cultivation systems.
In FP, an efficient control strategy was set up in 2003 relying on (1) a supply of virus-free cuttings, (2) roofing the shade houses with insect-proof netting, (3) roguing the diseased plants as soon as they are detected, (4) decontaminating tools, and (5) avoiding weeds (particularly C. diffusa) inside and at the proximity of the plantation. A postimplementation survey in 2007 showed that the viral impact (CMV and other viruses) was drastically reduced in the new plantations that followed the recommendations (Richard et al., 2009).
Epidemiological data indicate that CMV prevalence may vary greatly from one area to another, notably in relation to the presence of virus reservoirs. Therefore, control strategies have to be adapted according to each specific cultivation context.
Other Viruses
Besides the major viruses already detailed other viruses with minor economic importance such as Odontoglossum ringspot virus (ORSV) or barely characterized such as a Rhabdolike-virus and a Clostero-like viruses (Bhat et al., 2004; Pearson et al., 1993) have also been reported in vanilla.
Odontoglossum Ringspot Virus (Orsv)
ORSV is a member of the Tobamovirus genus of which the type member is Tobacco mosaic virus (Jensen and Gold, 1951). The rod-shaped particles of ORSV (300 × 18 nm) encapsulate a single-stranded positive RNA molecule of approximately 6.6 kb coding for five proteins (Ryu et al., 1995). ORSV’s natural host range is limited to orchids in which it may cause severe symptoms such as mosaic, ringspots, necrosis on leaves and flowers (Albouy and Devergne, 1998; Gibbs et al., 2000). As a tobamovirus, ORSV has a high stability in sap (temperature inactivation of 90°C and a dilution endpoint of 10−5–10−6) and is readily transmitted by mechanical means. It is frequently found in cultivated ornamental species worldwide (Freitas et al., 1999; Khentry et al., 2006; Zettler et al., 1990).
On vanilla, however, the virus has a very limited incidence and prevalence. It has been serologically detected in few vanilla vines in Tonga, Fiji, Cook Islands, Niue, FP, and the Reunion Island (Farreyrol et al., 2001; Pearson and Pone, 1988; Pearson et al., 1993; Wisler et al., 1987), but no symptoms were consistently associated with the virus. However, in plant quarantine in the Reunion Island, we recently observed a V. pompona vine originating from a botanical garden that exhibited mosaic on young leaves (Figure 7.6). This plant tested positive for ORSV and negative for CymMV, potyviruses, and CMV, by ELISA and RT-PCR.
FIGURE 7.6 Mosaic on leaf of V. pompona infected by ORSV.
Since it is relatively infrequent in vanilla, ORSV has so far been of little concern in production plots. However, due to its possible pathogenicity and absence of a natural vector it is worth testing the planting material for this virus in order to avoid its propagation. As with CymMV, a number of innovative methods have been developed to detect ORSV.
Unidentified Rhabdo-Like Virus
During a survey conducted in Pacific Islands (Pearson et al., 1993), unusual symptoms on leaves, consisting of necrotic spots were observed in vanilla plots (Figure 7.7a) in Vanuatu and Fiji. Leaf dip preparations from symptomatic leaves revealed the presence of enveloped bacilliform particles suggesting a possible Rhabdovirus.
FIGURE 7.7 (See color insert following page 136.) (a) Enations and necrosis on V. planifo-lia leaves infected with (b) enveloped virus-like particles visible under electron microscope.
The Rhabdoviridae family (order Mononegavirales) contains viruses infecting animals and plants, which are transmitted by arthropods and may multiply in the vector (Fauquet et al., 2005). The plant infecting rhabdoviruses are assigned to two genera (Cytorhabdovirus and Nucleorhabdovirus) but a number is still unassigned, including the putative rhabdovirus Orchid fleck virus (OFV), which has become more prevalent in orchids over the last decade (Kitajima et al., 2001; Kondo et al., 2006). OFV has been tentatively classified in a new Dichorhabdovirus genus because its genome, although showing sequence similarities with nucleorhabdoviruses, is bipartite and the particles are not enveloped (Kondo et al., 2009). This last feature contrasts with the putative rhabdovirus particles seen in vanilla which are clearly enveloped (Figure 7.7b). In addition, repeated attempts to amplify OFV sequences from vanilla symptomatic leaves using specific primers (Blanchefield et al., 2001) have failed (Mongredien, 2002). Since the statement from 1993, no further damage has been reported in vanilla that is reminiscent of this still uncharacterized putative rhabdovirus, and this virus should be considered as anecdotal in vanilla plots.