* Disease first appeared in central cocoa belt in 2003
* Researchers and farmers say it has since expanded its reach
* Impact being masked by expanding number of plantations
Swollen shoot disease is pushing deep into Ivory Coast's primary cocoa-growing regions despite government efforts to combat it and could hurt output from the world's top grower over the long term.
In the decade since it struck the centre-west growing regions, the disease has spread to 12 percent of all Ivorian cocoa farms, the government says. For now, experts say the impact on output is being masked by production from new plantations.
"It's progressing without a doubt," said Francois Ruf, an economist with French agricultural research centre CIRAD.
"It's already been devastating around Gagnoa, Guitry, Fresco and outside the Tai national park as well," he said, describing an area that straddles some of Ivory Coast's most productive cocoa farmland in the southwest.
The viral disease, first identified in neighbouring Ghana in the 1930s, causes a drastic reduction in yields in the first season following infection and then typically kills trees within a few years.
When it emerged in the centre-west region, the disease soon killed off 8,600 hectares of cocoa in Sinfra and Bouafle, leading to a 66 percent drop in output there, Ivorian authorities said.
It has since also been found near Issia, in the Daloa region that accounts for a quarter of the Ivorian crop, according to the cocoa marketing board, CCC.
Farmers said the disease was also gaining ground in the east along the border with Ghana, an area known for producing high-quality cocoa.
"We started seeing it two years ago. In the beginning, I didn't know what it was," said Noudoun Silue, who farms eight hectares near the western town of Duekoue. "We had to cut down the sick trees so the others wouldn't be contaminated."
The CCC in 2008 started a project to map the affected zones, which is still unfinished. Its preliminary surveys show the disease has affected 12 percent of cocoa farms across Ivory Coast, the government said.
An Ivorian agronomist, who is involved in the study but asked not to be named, said that around 150,000 hectares of Ivory Coast's 2.5 million hectares of cocoa farms are either threatened or have already been destroyed.
Ivory Coast produces nearly 40 percent of the world's cocoa.
PRODUCTION TO FALL?
Hoping to fend off an epidemic like the one that destroyed nearly 200 million trees in Ghana in the 1940s, Ivory Coast said last year it was seeking 46 billion CFA francs ($92.6 million) from international agencies to help fight the disease.
"It's true that there are risks. Production could fall, but not in a drastic way," said Jeremie Kouassi, head of the national agricultural research centre in Abengourou region.
CIRAD's Ruf said that, for now, extra beans from new plantations have masked the real impact of the disease on Ivorian cocoa output.
Little undeveloped farmland remains in the cocoa belt, however, and the government has pledged to clear farmers out of protected forest reserves, which have been used to plant some of the newest farms.
"If the effects of the creation of new plantations fade, production in Ivory Coast will recede, partly due to swollen shoot," Ruf said.
While researchers are working to develop new varieties of cocoa capable of resisting swollen shoot infection, for now farmers are forced to chop down infected trees to save their plantations and those of neighbours.
But it is a difficult decision to make.
And rather than replant and risk another infection years down the line, many farmers choose to abandon cocoa entirely in favour of less vulnerable crops such as rubber.
"At first I thought someone had cursed me ... Soon I'd lost 20 hectares," said cooperative manager N'Dri Kouao, who farms in Ambeyenou, 12 km north of the eastern border town of Niable.
"The (rural development) agents asked me to replant with new, more resistant varieties. But I'm afraid. I'm going to plant rubber instead," he said. ($1 = 497.0310 CFA francs)