Thus, apart from the universal recommendation of planting virus-free material, the control strategy needs to be adapted to each local circumstance. As a consequence, it is advisable to assess potyvirus risk prior to any development program in a new area or when changing the cultivation system. In addition, molecular tools that enable rapid identification of potyviruses help provide insight into the appropriate control strategy to use in case of unexpected potyvirus outbreaks.
Cucumber Mosaic Virus (Cmv)
CMV is a widespread virus infecting a very broad range of plants. It was identified in vanilla in the early 2000s, associated with severe distortions of vines in FP (Farreyrol et al., 2001). The virus was later found in vanilla in Reunion Island and in India (Madhubala et al., 2005).
Virus and Genetic Diversity
CMV is the type member of the Cucumovirus genus in the family Bromoviridae. It has a tripartite positive-sense RNA genome encoding five proteins (two polymerase components on RNA1 and RNA2, the 2b protein involved in the breaking of plant defense and virus movement in the plant, on RNA2, and a 30 K protein and the CP on RNA3). Each RNA is encapsulated in a distinct polyhedral particle (28 nm diameter) hence the infection of a plant requires the inoculation of the three types of virions. Some isolates also encapsulate a subgenomic RNA or a satellite (sat) RNA. The biology and ecology of this virus have been extensively reviewed by Palukaitis et al. (1992) and Gallitelli (2000).
The CMV strains are divided into two serologically distinct subgroups diverging in about 25% of their nucleotides and also differing in biological traits. The thermo-tolerant strains of subgroup 1 are further divided into two clades (1A and 1B) on the basis of genomic sequences of their RNAs (Roossinck et al., 1999). The subgroup 1A emerged from the subgroup 1B and (until recently) the 1A strains were only found in Asia. The thermosensitive strains of subgroup 2 are restricted to cool climate areas. Three mechanisms have been associated with the evolution of CMV: RNA reassortments between the RNA components, recombination in the noncoding regions and mutations in the coding regions. Several amino acid positions on the CP were shown to be positively selected in relation to virus transmission (Moury, 2004).
To our knowledge, 65 CMV–CP sequences have been obtained for vanilla isolates, originating from FP (58), Reunion Island (6), and India (1) (Farreyrol, 2005; Madhubala et al., 2005; Mongredien, 2002), 53 of which are deposited in Genbank. The majority of the sequences were most closely related to subgroup 1B isolates and clustered in a few clades (Farreyrol et al., 2009). Trees generated from 3′ UTR of RNA3 (Farreyrol, 2005) and ORF2b of RNA2 (Mongredien, 2002) sequences were congruent with CP analysis. Interestingly, an isolate belonging to group 2 (NZ100, AY861389) was mechanically inoculated in vanilla plants, which induced leaf mosaic and deformation (Farreyrol et al., 2009), suggesting that a wide range of CMV isolates can infect vanilla. Attempts to detect CMV sat-RNA associated to vanilla isolates of CMV from FP by RT-PCR using degenerate primers (Escriu et al., 2000; Varveri and Boutsika, 1999) have failed (M. Grisoni, unpubl. data).
Phylogenetic analysis of CMV isolates collected from vanilla plus several associated crops or weeds revealed a clustering according to the geographic origin rather than to the host plant. For instance in the Leeward Islands (FP), the CMV isolates from the islands of Raiatea and Huahine were in distinct clusters, while high similarities were observed between isolates from vanilla and from Commelina diffusa (Farreyrol et al., 2009). These findings supported the hypothesis of transmission of the virus from one plant to another, particularly from C. diffusa to V. tahitensis. Similarly, the CP gene of CMV from vanilla sequenced in Kerala (India) showed the highest identities (99%) with that of another Indian isolate infecting black pepper in Karnataka (Madhubala et al., 2005).
Symptoms and Diagnosis
A variety of symptoms has been associated with CMV on its numerous hosts; most common are mosaics and stunting, but symptoms can be as severe as complete systemic necrosis. Some strains are symptomless on certain hosts or at elevated temperature (subgroup two strains) and symptom expression may also vary over time due to temporary remission of the virus infection or be attenuated or aggravated by the coinfection with a sat-RNA.
In vanilla, the CMV isolates described in FP induced severe leaf and shoot deformation and the infected plants are often sterile, either because the flowers do not fully develop or because they are abnormally formed and contain no viable pollen. On V. tahitensis vines mechanically inoculated with a field isolate of CMV the first foliar symptoms appeared two months after inoculation with an embossing of the young leaves, which usually had a slender and asymmetrical (comma-like) shape (Figure 7.3a). The vines subsequently developed an abnormal phyllotaxy and irregular growth of the apices and flowers (Figure 7.3b and c), typical of the disease. Similar leaf symptoms were described in India and the Reunion Island on V. planifolia. In a few instances, shoot deformations on CMV infected vines have been observed (Farreyrol, 2005) resembling the abnormal tubular leaves described by Jacob de Cordemoy (1899) (Figure 7.4), which are possibly historic evidence of remote infections by CMV in the Reunion Island.
FIGURE 7.3 Symptoms caused by CMV infection in V. tahitensis: (a) young leaves embossed and deformed (healthy leaf on the right); (b) proliferation on infected young shoot; and (c) abnormal flower (right) on CMV-infected vine.
FIGURE 7.4 (Left) Abnormal tubular leaf a, observed on V. planifolia in 1899 by M.H. Jacob de Cordemoy (Jacob de Cordemoy, 1899); f, normal leaf; p, peduncle. (Right) Tubular leaf symptom observed during a survey in FP in 2005.
CMV symptoms are spectacular on vanilla and rather specific and can be good indicators of virus infection. However, these symptoms can be confused, in the early stages, with potyvirus infection, and in the later stages, with physiological disorders. In addition, temporary symptom remission may occur. Therefore, tests based on serology (Devergne et al., 1981; Hu et al., 1995; Maeda and Inouye, 1991; Yu et al., 2005) or nucleic acid amplification (Hu et al., 1995; Wylie et al., 1993; Yu et al., 2005) are recommended to diagnose the disease, or to select virus-free cuttings.
Epidemiology
CMV is a plant virus having the largest known host range of more than 1000 species belonging to more than 30 families (Douine et al., 1979). The virus is transmitted by more than 80 aphid species in a nonpersistent manner (Gallitelli, 2000). In several hosts, CMV is also transmitted at high frequency through the seeds (Palukaitis et al., 1992). Several studies have pointed out the key role of the co-occurrence of reservoir plants (that overwinter the primary inoculum) and aphids (that ensure secondary spread of the virus) in the epidemics of CMV in different crops and countries (Hobbs et al., 2000; Kiranmai et al., 1998; Lavina et al., 1996; McKirdy, 1994; Rist and Lorbeer, 1991; Skoric et al., 2000).
In vanilla plots of the Society Islands (FP), high prevalence of CMV was recorded in the early 2000s, with one out of three plots infected, and an incidence frequently exceeding 25% of vines (Farreyrol et al., 2009). Much lower virus prevalence was observed in the Reunion Island with only 2 infected plots out of 25 with sporadic occurrence of the virus (Farreyrol, 2005) as well as in India where mosaic viruses were observed in <5% of vines (Bhat et al., 2004; Madhubala et al., 2005). In Marquisas and Australes (FP), Comoros archipelagos, and Madagascar no CMV was found in vanilla despite intensive surveys (Grisoni, 2003; Grisoni, 2009; Grisoni and Abdoul-Karime, 2007; Leclercq-Lequillec and Nany, 2000).