BANGKOK - A key rubber tree-growing area in Thailand has been hit by an outbreak of a fungal disease, which could halve the area's output, the country's rubber authority said on Monday.

Thailand is the world's top producer and exporter of the natural rubber, accounting for up to 40% of global rubber supply.

The disease, called Pestalotiopsis, has spread into Thailand after hitting plantations in neighboring Indonesia and Malaysia. The three countries account for around 70% of the world's natural rubber production.

The Rubber Authority of Thailand said the disease, which causes leaves to turn yellow and spotted as it spreads, was recently found in three districts in Narathiwat, a key rubber growing province in southern Thailand.

Krissada Sangsing, director of the agency's rubber research institute, said the disease threatens to cut output in the affected areas by up to 50%. The agency also told Reuters that damage was estimated to be around 100,000 rai (16,000 hectares) as of Sunday.

"We are committed to preventing this disease and containing it from spreading further," Krissada said.

Older rubber trees are more vulnerable to the disease, which causes them to eventually lose 90% of their leaves and affects their ability to produce latex, he said.

Rubber farmers in the south were "very worried" about losing output amid falling rubber prices, said Uthai Sonlucksub, president of the Natural Rubber Council of Thailand.

"It spreads so fast. The trees are all bare and cannot be tapped at all," Uthai told Reuters.

"Farmers in the affected districts are losing their income."

The disease currently affects around 382,000 hectares of rubber plantations in Indonesia, especially parts of Sumatra and Kalimantan, according to the International Rubber Consortium (IRCo).

Indonesia already revised its natural rubber production this year due to the outbreak, expecting output to drop by 15%.

(Reporting by Patpicha Tanakasempipat; Editing by Emelia Sihtole-Matarise) ((Patpicha.Tanakasempipat@thomsonreuters.com; +6620802303 / or mobile +66818317884; Reuters Messaging: patpicha.tanakasempipat.thomsonreuters.com@reuters.net))