Black pod disease is ravaging cocoa plantations in Cameroon's Southwest region, one of the country's top cocoa belts, driven by weeks of heavy rains and a surge in counterfeit agrochemicals, an official and industry figures said on Tuesday.

Brown rot or black pod disease is a fungal disease that attacks cocoa pods and trees and can spread in wet conditions. Cameroon is the world's fifth-largest cocoa producer.

Jackson Ntapi Nkwentang, the Southwest region's agriculture delegate, said the downpours have been very intense since July, creating ideal conditions for fungal infection, especially in the main cocoa-producing hubs of Muyuka, Mbonge and Kumba.

"It is a serious issue," the senior official told Reuters. "We are advising [growers] to intensify spraying."

Many growers are relying on cheap, unverified fungicides smuggled from neighbouring Nigeria and Ghana, said Epie Promise Ngolepie, a cocoa consultant at agri-tech company Help Farmers Cameroon.

"Smallholder farmers don't want to follow expert advice because they think they know better," Ngolepie said, adding that his own sharecropper had tried using the wrong agrochemicals.

"Most chemical dealers are not agronomists; they just want to sell," he told Reuters.

Local authorities say poor farm practices, including late pruning, poor field clearing and irregular spraying with approved fungicides, have compounded the outbreak.

Nkwentang told Reuters that a planned field visit to clamp down on counterfeit inputs on the market in the region has been slowed by a month-long lockdown imposed by armed separatist fighters across the English-speaking regions.

For farmers, the outbreak has slashed yields and left many struggling to repay loans, feed their families and cover the cost of the new school year.

"We don't even know how to survive this season," said Divine Ntam, a farmer near Kumba. "The chemicals we bought aren't effective, and the rains have really messed up things."