NA, not available; SH, intensive under shade house; SI, semi-intensive; UF, extensive under forest.
Mosaic and leaf deformation on vanilla shoots were reported in Puerto Rico as early as 1948 (Childers and Cibes, 1948) and might represent the first record of poty-viruses in vanilla. However, the first confirmed outbreak of potyviruses in vanilla was reported in FP in 1986 (Wisler et al., 1987). It coincided with the implementation of a vanilla development program promoting the intensification of the cultivation practices, which entailed environmental changes in the vanilla agro-system (Anonymous, 1984). Likewise for the later reports of potyvirus infection of vanilla plots where analogous intensification programs were implemented such as in Tonga (Pearson and Pone, 1988), Vanuatu (Pearson et al., 1993), Reunion Island (Benezet et al., 2000), India (Bhat et al., 2004), Madagascar (Grisoni et al., 2006), Mauritius (Rassaby, 2003), and Samoa (Grisoni et al., 2006). Conversely no or only exceptional potyvirus infection has been recorded so far in the under-forest vanilla plots (Grisoni et al., 1997; Leclercq-Le Quillec and Nany, 2000).
FIGURE 7.8 Simplification of Vanilla agrosystem consecutive to intensification: (a) Underforest counting more than 30 adventive species (shade and support trees, weeds, and associated crops); (b) Semi-intensive system (with 10–20 adventive species); and (c) intensive system under shade house (vanilla plus sometimes a few weeds).
We hypothesize that the multiplicity of potyviruses infecting vanilla (seven species recorded) results from the diversity of the epidemiological circumstances where vanilla is grown and from the conditions in intensive cultivation that enhance aphid transmission of viruses present in weeds or associated crops. The incidence of the potyviruses and CMV in the vanilla plots varies greatly from one plot to another, although the heaviest infections (more than 50% infected vines two years after planting) were recorded in shade houses that are characterized by a very low plant diversity compared to the under-forest cultivation (Figure 7.8). It is therefore probable that other opportunistic viruses will infect vanilla as its culture expands into new environments. This highlights the need for rapid, specific, and widely applicable virus identification tools to understand the outbreaks affecting vanilla plantations and implement adapted control strategies.
Conclusions and Perspectives
Virus diseases have become a major concern for vanilla production over the last few decades probably as a consequence of the diversification of the cultivation sites and intensification of vanilla growing systems. In the absence of curative means, prophylaxis is the only way to avoid viral diseases. The data accumulated on the viruses affecting vanilla throughout the world, particularly CymMV, potyviruses, and CMV enabled the introduction of control measures that proved sufficiently effective to preserve and expand a profitable vanilla industry.
However, viruses have the ability of evolving extremely fast and the emergence of new pathogenic isolates in vanilla plots are likely to occur, particularly in the context of quick environmental mutations induced by global change (Canto et al., 2009; Garrett et al., 2006). Huge progress has been accomplished recently in the comprehension of plant and virus biology and interactions, leading to powerful biotechnolo-gies for engineering virus-resistant varieties. The regeneration of vanilla plants from protoplasts that was recently achieved (Minoo et al., 2008) makes such biotechnolo-gies accessible for the production of transgenic vanilla, expressing high levels of resistance to RNA viruses. However, this strategy has yet to be implemented, possibly because virus pressure has been maintained at a tolerable level by conventional methods. In addition, consumers consider vanilla as a natural product and genetic bioengineering is not compatible with that image.
High-performance tools for nucleic acid detection and identification, such as portable molecular tests, polyvalent PCR, microarrays, next generation DNA sequencing, will also contribute to improve virus management in the future. In particular they will assist in better understanding of vanilla virus epidemiology and subsequently in designing agro-systems by reducing virus impact on crop.
Globally, these technologies will help produce planting material of the highest health and genetic status for the vanilla industry. It is undoubtedly on this elite material, properly multiplied and cultivated with adequate prophylactic measures, that the future of a profitable and healthy vanilla industry relies.
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