A serious disease is affecting opium poppies in Afghanistan, Antonio Maria Costa, the head of the UN Office on Drugs and Crime (UNODC) has said.
Mr Costa told the BBC that this year’s opium production could be reduced by a quarter, compared to last year’s yield.
He said the disease – a fungus – is thought to have infected about half of the country’s poppy crop. Afghanistan produces 92% of the world’s opium.
Mr Costa said opium prices had gone up by around 50% in the region.
That could have an impact on revenues for insurgent groups like the Taliban which have large stockpiles of opium, he added.
Mr Costa told the BBC that the disease was affecting poppies in the provinces of Helmand and Kandahar, the heartland of opium cultivation and the insurgency in Afghanistan.
He said some local farmers believed that Nato troops were responsible for the outbreak, but he said such fungi occur naturally every few years.
"I don’t see any reasons to believe something of that sort. Opium plants have been affected in Afghanistan on a periodic basis," Mr Costa said.
Four years ago there was a similar infestation.
"As the farmers would be tempted because of their loss of income to join the insurgency, I don’t see any reasons why the coalition would be acting in a way that would be so unpopular and so treacherous for the conduct of the conflict," he said.
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