The fungus attacks the root of the plant and climbs up the stem
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 with last year.
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 told the BBC's Bethany Bell.
The fungus attacks the root of the plant, climbs up the stem and makes the opium capsule wither away.
It was affecting poppies in the provinces of Helmand and Kandahar, the heartland of opium cultivation and the insurgency in Afghanistan, he said.
But farmers in Afghanistan are unsure about what is damaging their crops.
Some believe Nato troops are responsible for the outbreak, but Mr Costa denied that this was the case.
A fungal disease is thought to have infected 50% of the country's poppy crop
"I don't see any reasons to believe something of that sort," he said. "Opium plants have been affected in Afghanistan on a periodic basis."
Farmer Haji Mohammad in Nawzad told the BBC that he had seen a dramatic reduction in the amount of opium he was able to harvest. He described the fungus as an "aerial spray".
He said that last year he harvested 450kg (990lb) of opium - but this year he had so far only been able to harvest 4kg.
"[It]... has affected my wheat cultivation and my chickens and other animals as well," he said.
"The powder sprayed has a white colour and I think it is chemical and if you squeeze it in your hand, water comes out of it."
A number of farmers in southern Afghanistan told the BBC they observed a white substance on their crops. They also reported extensive crop damage and also that livestock had been affected.
Mr Costa said this was an opportunity for the international community to bring in support to try to persuade farmers to turn away from planting opium.
He said the amount of opium produced by one hectare (2.47 acres) had almost doubled to 56kg (in the five years to 2009.
"Nature really played in favour of the opium economy; this year, we see the opposite situation," he added.
Mr Costa said that farmers now grew opium poppies in only five or six Afghan provinces, as opposed to all 34 five years ago.