Handbook of Vanilla Science and Technology

Vanilla is also affected by viral diseases, mainly in the plantations of French Polynesia and India, where they represent a serious problem. In Mexico, there are no scientific reports of damage by viruses (Hernández-Hernández 2008). However, Soto-Arenas (2006) reported the presence of some symptoms of virus in Veracruz, which may be limiting the vanilla production. The damage caused by viruses can be difficult to distinguish, since some plants do not exhibit clear symptoms or are asymptomatic. The viruses most common in vanilla, according to Pearson et al. (1991), are described in Sections 2.8.1 to 2.8.4.

2.8.1 Cymbidium Mosaic Virus (CYMV)

The plants infected with the virus are generally asymptomatic, but occasionally they exhibit mild chlorosis in the leaves of V. planifolia and V. tahitensis. The virus is transmitted through the sap and dispersed through propagation material. It is not known if it is transmitted by a vector. The virus was first reported in the vanilla producing region of the South Pacific (French Polynesia). It has since been found in vanilla plots in many countries, such as Madagascar, Reunion Island, and India (Grisoni et al. 2010).

2.8.2 Vanilla Mosaic Virus (VMV)

The virus causes distortion of the leaf and mosaic lesions in V. planifolia, V. pompona, and V. tahitensis (Figure 2.7). It is transmitted by the sap and is spread by using infected cuttings in the establishment of the crop. Tests of transmission have shown that this virus can be transmitted by aphids (Myzus persicae). This virus occurs mainly in the islands of French Polynesia, where V. tahitensis is the cultivated species.

Fig. 2.7 Typical symptoms of distortion and mosaic lesions on leaves of V. tahitensis infected by vanilla mosaic virus.

2.8.3 Vanilla Necrosis Potyvirus (VNPV)

Plants infected with VNPV exhibit distorted young leaves with chlorotic spots and necrotic lesions in leaves and mature stems, eventually resulting in defoliation and death of the plant. It is transmitted by the sap and dispersed by propagation material. It has been reported in V. planifolia cultivated in Tonga, Fiji and Vanuatu.

2.8.4 Odontoglossum Ringspot Virus (ORSV)

Plants infected by ORSV are generally asymptomatic, although sometimes small spots on the leaves of V. planifolia and V. tahitensis are observed. The virus is reported in the producing region of the South Pacific. The virus is transmitted through the sap and dispersed through propagation material. The transmitting vector is not known.

2.8.5 Prevention of viral diseases

It is important to use healthy certified cuttings, control the insect vectors (aphids), and eliminate weeds and other crops around the plantation that can be reservoirs for the virus, for example, Commelina difusa, Cucurbita maxima, Physalis angulata, Momordica charantia, watermelon, and pumpkin (Wong et al. 2003; Anandaraj et al. 2005). Plants with virus symptoms must be removed from the plantation and burned. The movement of infected cuttings from one region to another must be avoided.

2.9 DAMAGE BY ADVERSE CLIMATIC FACTORS

In addition to the direct damage caused by diseases, vanilla culture is affected by environmental conditions, which can significantly affect the efficiency of vanilla production.

2.9.1 Natural pruning of the apical buds

2.9.1.1 Description

During the winter, when temperatures of around 7°C extend for more than 1 hour, the terminal shoots are burned. They initially exhibit a light brown color, and later with the humidity of rains or dew they began to rot and finally dry out, becoming a dark color (Figure 2.8).

Fig. 2.8 Apical bud showing damage from cold (natural pruning).

2.9.1.2 Damage

A plant without apical buds no longer grows and therefore it must develop a new bud. For small young plants less than 2 years old, “natural pruning” is not recommended because it delays plant growth. However, in mature plants, the “pruning” serves as an indicator that the plant underwent stress by the cold and that it is going to bloom.

2.9.1.3 Control

In small plants, the damage can be minimized by maintaining high moisture levels in the ground and in the mulch, as well as with natural or artificial shade of around 50% during winter time.

2.10 DAMAGE FROM SUNBURN

2.10.1 Description

Initially, a yellowing in the leaves is observed and later some leaves dry completely (Figure 2.9).

Fig. 2.9 Yellowing and sunburn of leaves in a vanilla plantation with deficient shade.