The U.S. Army tested biological weapons in Okinawa in the early 1960s, when the prefecture was still under U.S. rule, according to U.S. documents obtained by Kyodo News.

In the tests, conducted at least a dozen times between 1961 and 1962, rice blast fungus was released over paddies to see how it affected production, the documents made available by U.S. authorities showed.

Rice blast disease causes lesions to form on the plant, threatening the crop. The fungus, which is known to occur in 85 countries, is estimated to destroy enough rice to feed 60 million people a year.

The U.S. government has previously disclosed information on chemical and biological warfare tests it held at sea and on land in such places as Puerto Rico, Hawaii and Utah.

The United States is believed to have had China and Southeast Asia in mind in developing such crop-harming agents. The U.S. government decided to end all biological weapons programs in 1969.

An international convention against production and possession of biological weapons came into force in 1975.

The obtained documents mention test sites including Nago and Shuri, both in Okinawa, but it is not known whether the experiments were conducted on U.S. bases there.

In the field tests, the army “used a midget duster to release inoculum alongside fields in Okinawa and Taiwan,” measuring dosages at different distances and the effect on crop production, the documents said.

A separate document said: “Field tests for stem rust of wheat and rice blast disease were begun at several sites in the (U.S.) midwest and south and in Okinawa with partial success in the accumulation of useful data.”

After the war, Okinawa was administered by the United States until 1972.

Separate from the latest findings, it has been reported that the U.S. military stored defoliants on Okinawa during the Vietnam War.

According to past reports by Jon Mitchell published in The Japan Times, construction workers unearthed more than 20 rusty barrels from beneath a soccer field in the city of Okinawa in June.

The land was once part of Kadena Air Base — the Pentagon’s largest installation in the Pacific region — but was returned to civilian use in 1987.

Tests revealed that the barrels contained two ingredients of military defoliants used in the Vietnam War: the herbicide 2,4,5-T and 2,3,7,8,-TCDD dioxin. Levels of the highly toxic TCDD in nearby water measured 280 times the safe limit.

The Pentagon has repeatedly denied storing defoliants — including Agent Orange — on Okinawa. Following the discovery, it distanced itself from the barrels.

A representative said the Defense Department was investigating whether the barrels had been buried after the land’s return in 1987, and a U.S. government-sponsored scientist suggested they may merely have contained kitchen or medical waste.

However, the conclusions of the Japanese and international scientific community were unequivocal: Not only did the barrels disprove Pentagon denials of the presence of military defoliants in Japan, but the polluted land also posed a threat to the health of local residents and required immediate remediation.