Vanilla Viruses

credit: Dr. Mike Pearson, Univeristy of Auckland, New Zealand

Vanilla is the second most expensive spice on the world market and a valuable source of foreign exchange for several countries. Of the four major types of vanilla, it is the Bourbon vanilla grown on Madagascar, Comoros and Réunion that is the most sought after. However, diseases caused by viruses are an increasingly common problem. Scientists at the University of Auckland in New Zealand are trying to identify and characterise the different viruses involved, advising growers on how to control the diseases and prevent them from spreading.

Dr. Mike Pearson began his work with vanilla diseases by identifying several new viruses from Tonga, Fiji and the Cook Islands. Then, in 1997, he was invited to Réunion to investigate increasingly serious problems being encountered with diseases in vanilla grown intensively in shade-houses.

Originally from Mexico and Central America, the vanilla vine - an orchid species - was introduced to Réunion in 1819. However, because its natural insect pollinator was absent, the flowers never set seed and, without seedpods, no vanilla spice could be produced. It wasn't until 1841, when a slave discovered a method of hand-pollinating vanilla flowers that cultivation spread around the island. Réunion is now the world's sixth largest exporter of vanilla, producing about 20 tonnes each year.

Major export in decline

"Vanilla is historically very important to Réunion and is almost part of its national identity," observes Dr. Pearson. However, in recent years, production has been declining. Part of the problem is that the crop is labour intensive; not only do vanilla plants require hand-pollination, they must be pruned regularly in order to reduce vegetative growth and promote flowering. In turn, these operations make it very easy for mechanically transmitted viruses to spread through a plantation. The fact that the commercial life of a vanilla plant can be 15 years or more also means that there is plenty of opportunity for infections to be maintained.

With the help of Karin Farreyrol, a student from Réunion, at least six different viruses have now been isolated from vanilla in Réunion and French Polynesia. "We have characterised several viruses not previously known from vanilla," says Dr. Pearson, "the ultimate aim being to provide diagnostic tests to assist in the detection and control of these viruses."

The latest to be isolated from vanilla is cucumber mosaic virus, which can infect a wide range of host species - over 700 at the last count. In some, including cucumber, it causes serious damage, whilst in others it shows no symptoms. In a survey of French Polynesian plantations, 23% of vanilla plants that showed virus-like symptoms contained the cucumber mosaic virus. These plants (Vanilla tahitensis) were severely stunted and distorted and any flowers that developed were sterile. On Réunion, however, only 7% of Bourbon vanilla plants (V. planifolia; syn. V. fragrans) showing infection contained the cucumber mosaic virus, and these had only mild symptoms - nothing more than an elongation of the leaves. "It is not yet known whether the differences in symptoms are due to a variation in the strains of virus present in French Polynesia and Réunion, or to differential tolerance of the two vanilla species," says Karin Farreyrol. Even so, given the wide host range of cucumber mosaic virus, it is likely that alternative hosts, which can provide a reservoir of infection, are present around most vanilla plantations. And growers are being recommended to seek out and remove any potential sources of infection adjacent to their crops.

The same surveys of symptomatic vanilla plants also showed that 44% of the samples from French Polynesia and 32% from Réunion were infected with various strains of potyviruses. One, Cymbidium mosaic virus (CymMV) was found in both locations, whereas Odontoglossum ringspot virus (ORSV), originally isolated from another orchid species, was found only on Réunion. During the survey, the New Zealand team also detected a previously unknown rhabdo (or 'bullet-shaped') virus in their vanilla leaf samples.

Whilst several of these viruses are known to be transmitted by aphids, there is no evidence of this happening in the field, says Dr. Pearson. However, they can all be spread easily by propagating from infected material. The main recommendation to vanilla growers, therefore, is only to plant virus-free cuttings. "The best way to do this is to go through tissue-culture so that virus-tested material can be rapidly multiplied," advises Dr. Pearson, who was involved in setting up such a project on the Cook Islands. On Réunion, cuttings are now being tested for the presence of different viruses so that only healthy material reaches the nurseries. However, given the long productive life of the vanilla plant, it will be some time before the beneficial effects of using clean planting material result in better returns to the island's growers.

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