Weakening prospects for cocoa supplies from the Ivory Coast, the world's biggest producer, have prompted ABN Amro to slice one-third from its forecast for the global production surplus.

The bank attributed a cut to 47,000 tonnes, from 72,000 tonnes, in its 2010-11 cocoa output forecast to "weather uncertainties" in the Ivory Coast, where heavy rains have triggered an outbreak of black pod disease, a fungal infection which renders cocoa beans inedible.

"The reduction on our previous forecast is entirely attributable to the forthcoming Ivory Coast main crop," a report from ABN Amro said.

While the disease can be treated with copper-based fungicides, farmers have said that treatments are difficult and less effective in such rainy weather.

Prices 'not high enough'

Growers' comments also reflect a longer-term reluctance to invest in plantations, amid longstanding political uncertainty and occasional conflicts, which look set to keep a lid on Ivory Coast output despite a jump in prices earlier this month to their highest in 32 years.

Farmers in many cocoa-producing countries, such as Indonesia, also face the option of other crops, such as palm oil or rubber, which may be more profitable.

"Current prices, while providing a significant incentive to improve husbandry generally, may not yet be sufficiently high to ensure a global expansion of the area used for cocoa production," the note said.

Cocoa for September delivery stood £5 lower at £2,281 a tonne in lunchtime trade in London, with its New York peer up $14 at $3,060 a tonne.