The Rome-based Food and Agriculture Organisation (FAO) said the TR4 strain of Panama disease, which has already hit tens of thousands of hectares in Southeast Asia, had been reported in Jordan and Mozambique.
The disease is "posing a serious threat to production and export" of bananas, the fourth most important food crop for the world's least developed countries and a key revenue source for poor farmers, FAO said in a report.
The disease affects the trees but not the bananas themselves and the only solution is to cut down the trees, dig trenches between trees to prevent its spread and impose strict quarantine measures.
Top producers in Latin America, including the world's main producer Ecuador, have so far not been affected but FAO warned there was a "potential" risk.
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But he said the key problem was a "reluctance" in the industry to realise the scale of the problem and its excessive reliance on the Cavendish variety.
"The sooner we have replacements for Cavendish that are resistant to Panama the better but this is going to take years," he said, warning there was a risk of repeating mistakes made during a 1950s epidemic.