Chocolate Eggs under Threat from Witches Broom

This is a very timely article from the New Scientist web site. It was written by Debora Mackenzie. Besides, the title alone makes the article a must read!

Chocolate eggs under growing threat from witches' broom
07 April 2009 by Debora MacKenzie
Magazine issue 2703. Subscribe and get 4 free issues.
IT'S chocolate egg season again, and sales of the pagan and Christian symbols of rebirth are as strong as ever. But the hunt for Easter eggs may truly be on next year, because chocolate trees are in increasing trouble.
Chocolate is made from the fermented, roasted seeds of the cacao tree. The cacao swollen shoot virus (CSSV) can kill the trees, and threatens to slash this year's spring crop by a third in the world's biggest producer, Ivory Coast. Meanwhile a fungus called witches' broom is doing the same in Brazil. Now researchers are racing to sequence the cacao genome and find genes that can resist CSSV.
Cacao trees are native to the Amazon rainforest, but west Africa produces 70 per cent of the world's cocoa, virtually all on tiny, impoverished farms. In recent years, demand for chocolate has mushroomed. The farmers cannot afford expensive fertiliser so they boost production by planting more cacao trees over a greater area. That means cutting down other trees that normally grow between cacao crops, which also replicate their rainforest origins and give them the protective shade they prefer.
"Increasingly cacao is grown almost as a monoculture," says Paul Hadley of the University of Reading, UK. That promotes the spread of disease, as does the trend towards growing the trees in drier regions - water-stressed cacao trees are less able to fight off disease.
In recent years, CSSV has become an increasingly serious problem in Ivory Coast. The virus originated in native African trees, in which it is endemic, and is spread by common mealy bugs, so it can't be avoided. The only defence until now has been to destroy millions of infected cacao trees to create disease firewalls. Yaw Adu-Ampomah and colleagues at the Cacao Research Institute of Ghana (CRIG) have found cacao varieties in Africa that partially resist the killer virus, and they are trying to breed more resistant strains.
But progress is slow. New genetic stock brought over from South America must be quarantined for two years before going to Africa, and experimental crosses take three years to grow before researchers can test for CSSV resistance.
Ray Schnell and colleagues at the US Department of Agriculture lab in Miami, Florida, are trying to speed things up. "We're mapping genes for resistance to CSSV now," he says. "It will all be a lot easier in a few years when we've sequenced the cacao genome."
If they can link particular DNA sequences with CSSV resistance, they hope to use them to make a testing kit so researchers in Africa can screen experimental crosses and plant only resistant seedlings. The CRIG is already using such a test to combat a fungal cacao disease called black pod.