Protecting the world's chocolate

The 500 unique flavours released in a bite of chocolate are a treat enjoyed regularly by millions of people worldwide. Chocolate's unique taste is derived from the beans of the cacao tree, an important crop in South and Central America, the Caribbean, and the West African countries of Cote d'Ivoire and Ghana. Pests and diseases result in losses to the cacao crop of 30-40 per cent each year, approximately half a million tons. While insects and viruses account for some of the loss, the majority of this damage is due to fungal attack.

Two major fungal species - witches' broom and frosty pod - have been advancing steadily through cacao plantations in the Americas and the Caribbean for over a decade. In an attempt to stop the spread and prevent transmission to Africa, an international team of cacao researchers is both attempting to establish quarantine zones and creating new fungal-resistant varieties.

credit: World Cocoa Foundation

While plant geneticists and breeders have so far had some success in creating strains of cacao resistant to the witches' broom fungus, they have made little progress against frosty pod. To identify resistance or tolerance to this and other fungal diseases, Dr. Raymond Schnell, research geneticist at the US Department of Agriculture's Subtropical Horticulture Research Station in Miami, Florida, and head of the world-wide effort to preserve cacao, has been identifying molecular markers in cacao. "Mapping resistance genes is the first step to understanding their modes of action and interaction, both with the pathogen and with each other," says Schnell.

Fungal threats

Different fungal diseases impact on cacao bean pods in various ways; witches' broom (Crinipellis perniciosa) shrivels them, whilst frosty pod (Moniliophthora roreri) leaves them swollen and white. However, the end result is the same: no cocoa beans to harvest. There is currently no effective control against fungal disease.

Of the two fungi, frosty pod is the hardier species, with spores that can travel further in hot, dry conditions. When the disease arrived in Peru in 1991, it wiped out 75 per cent of cacao production in less than two years. It is continuing to invade new territories at alarming speeds and is now believed to have reached Brazil, where the World Cocoa Foundation estimates it could reduce production in the region by 20 per cent in the next decade. To prevent the northern spread of frosty pod, attempts were made to establish quarantine zones in Central America, but to no avail. It has now appeared in southern Mexico.

Schnell's research project began in 1999, and the first crosses to investigate resistance were carried out a year later. Parent plants with resistance to frosty pod are crossed with susceptible plants to create families of 200-300 offspring, which are then examined for those with resistant genes. This is accomplished by looking for genetic markers associated with alleles that produce disease immunity. "Marker-assisted selection increases the efficiency of selection," Schnell explains. "This technique allows us to throw out all the individuals that don't have the markers we are looking for." This saves a great deal of cost and time in the next step of research, field testing of the trees, an endeavour that requires extensive space and data collection over a five-year period. Using marker-assisted selection means that only those trees that have a very good chance of resisting fungal diseases are planted.

Schnell estimates that, by 2010, 8-10 varieties of cacao resistant to frosty pod will be ready to send to the international Intermediate Cocoa Quarantine Facility at the University of Reading in the UK. The trees will be under quarantine for two years before being planted in demonstration plots. After a further four years, by around 2016, farmers should have the new, resistant material to plant in their fields. Although the year seems far off, Schnell stresses that the delivery of resistant trees will be much quicker than could be achieved without marker-assisted selection.

Keeping African cacao safe

In the absence of other solutions, these fungal diseases will continue to spread in the Americas until resistant material becomes available. But, with the right procedures in place, Africa should continue to be free. "The quarantine systems are secure, and diseases like frosty pod and witches' broom are unlikely to make it through," Schnell says. The accidental transfer of spores from airline travel also poses very little risk. Smuggling of plants, however, is one possible way that frosty pod or witches' broom could spread. This is potentially disastrous because, as Schnell notes, "It would take six months or a year for the disease to establish itself and by then it's too late to do anything."

Contributed by Treena Hein.