FIGURE 20.5 Colletotrichum bean rot.
The disease is generally observed in vanilla plantations during the postmonsoon period of September–December. It starts as a brown patch on the petiole and lower portion of the unfolding youngest leaf. The funnel-like leaf holds rainwater and might predispose to infection. Later infection spreads downward to the tender portion of the shoot causing shoot tip rot. F. oxysporum isolated from the affected shoots was found pathogenic. Association of Colletotrichum gloesporide was also observed. Spraying the foliage with 0.2% carbendazim was found effective (Thomas et al., 2002, 2003).
Others
Rhizoctonia solani and Mucor racemosus have also been recorded as pathogens of vanilla (Bhai and Dhanesh, 2008), but their relevance need to be investigated further.
The effectiveness of biocontrol agents for the management of soil-borne plant pathogens has been well established in recent times. Its importance was clearly brought in the disease management of crop spices (Sarma, 2006a, 2006b). Reduction of pathogen inoculum in vanilla plantations by applying Trichoderma viride, Trichoderma harzianum, and Pseudomonas fluorescens was reported earlier (Thomas et al., 2002). Both Fusarium and Phytophthora overlap in the field and spatial segregation of these pathogens in a plantation under a set of ecological conditions is not possible. It is imperative and logical to identify biocontrol agents that are suppressive to both these pathogens of vanilla. Studies carried out with different isolates of P. fluore-scens and Bacillus sp. both in in vitro and in vivo conditions showed the bioefficacy of certain bacterial isolates to suppress P. meadii and F. oxysporum in vitro. They also provided growth promotion in vanilla. The suppression of fusarial wilt in large-scale field establishment with local strains of T. harzianum in Mauritius has been established (Y.R. Sarma, unpubl. data).
Viral Diseases
Importance of vanilla viruses at global level received considerable attention (Wisler et al., 1987; Pearson and Pone, 1988; Pearson, 1990; Benezet et al., 2000; Grisoni et al., 2004). Identification of virus problems of vanilla in India is recent. It was only during 2003 that incidence of viral disease was recognized in the states of Kerala and Karnataka (Bhai et al., 2003; Sudharshan et al., 2003). Systematic investigation on vanilla virus diseases started recently at IISR, Calicut, on identification, characterization, and molecular diagnostics (Bhat et al., 2004; Bhadramurthy, 2008).
Electron microscopic studies revealed the presence of three types of flexuous particles resembling Potexvirus, Potyvirus, and Closterovirus and an isometric particle (Bhat et al., 2004). Investigation on virus incidence in vanilla plots carried out recently revealed about 3–10% incidence in Karnataka and 0.13–5% in Kerala (Bhadramurthy, 2008). These studies revealed that cucumber mosaic virus (CMV, Cucumovirus), cymbidium mosaic virus (CymMV, Potexvirus), bean common mosaic virus (BCMV, Potyvirus), and bean yellow mosaic virus (BYMV, Potyvirus) are infecting vanilla in India and mixed infection have been observed. The first authentic investigation on vanilla CMV strains in India was undertaken by Madhubala et al. (2005). The affected vines appear with distorted foliage with small and leathery leaves (Figure 20.6). In artificial inoculation, the members of Chenopodiaceae, Cucurbitaceae, and Fabaciaceae were found infective with CMV strains of vanilla. Isometric virus particles with 28 μm diameter were seen based on the detailed electronic microscopic investigation. Further studies confirmed that vanilla CMV strains belong to subgroup I of CMV. Mild chlorotic mottling and streaks on leaves are the characteristic symptoms of CymMV (Figure 20.7). The investigation revealed that Indian CymMV isolates are closely related to vanilla CymMV isolates from French Polynesia (Bhat et al., 2004). Studies on the yield losses and the epidemiology are lacking and need to be intensified to develop effective disease management strategies.
FIGURE 20.6 Stem necrosis caused by cucumber mosaic virus.
FIGURE 20.7 Leaf mosaic caused by Cymbidium mosaic virus.
Vanilla vines may be symptomless carriers of viruses. The present status of multiplication and new planting would continue as long as yield reduction is not observed. From the practical view point one can go for replanting when the virus infection exhibits declining phase in the yield. It is desirable to build up information on the yield reduction in relation to virus infection to ascertain and identify the exact declining phase with respect to yield. This would guide the management strategy to know when to resort to replanting.
Since these are all sap transmissible viruses, farm operations such as pruning, looping, or pollinating have to be regulated to avoid undue virus transmission leading to disease spread.
Elimination of viruses from the planting material through meristem tip culture, production, and distribution of healthy planting material would be a priority for sustainable vanilla production in future.
Pests and Their Management
In India, pest problems are generally minor on vanilla; however, several insects are recorded to damage vines, shoot tip, flower buds, or roots, by Hemipteran bugs, Lepidopteran caterpillars, and Coleopteran weevils.
Hemipteran Pests
Halyomorpha picus (Pentatomidae) (Figure 20.8) causes serious damage by sucking the sap from the shoot tip and inflorescence. Subsequently, such affected areas become necrotic and rot. Nymphs and adults suck the sap from the peduncle and flower buds. Pin-prick-like punctures at the site of feeding and subsequent necrosis and rotting are the typical symptoms of bug feeding. The affected vegetative buds drop within 3–5 days and the affected inflorescence become rotten. Incidence of the pest is higher during the inflorescence initiation period, that is, January–February.
FIGURE 20.8 Vanilla bug H. picus (a) shoot tip necrosis, (b) nymph, and (c) adult.
The bug is reported to occur in Karnataka, particularly where vanilla is inter-cropped with arecanut plantations. It is also reported from Koothattukulam in Kottayam district of Kerala.
Female bugs lay spherical eggs in clusters on the lower surface of leaf. The eggs are white when laid and turn cream within 3–4 days. Nymphs hatch in 5–6 days. First-instar nymphs are gregarious and remain on top of the egg shells and do not feed. The second- to fifth-instar nymphs are black and actively feed on the shoot tip.