It was part of the more wide-ranging chemical and biological weapons program, Project 112 (https://en.wikipedia.org/wiki/Project_112).
There's a huge community of veterans who have served at Okinawa who research this stuff online and on the island in order to help process compensation claims and lawsuits against the US government (e.g. http://vets.yuku.com/forums/74/Agent-Orange-Heart-Disease-Pr...).
So this isn't new, but it is absolutely disgusting.
The US army conducted field experiments of biological weapons,
which could harm rice cropping, in the Japanese island of
Okinawa in the early 1960s, a press report said Sunday.
The same experiments were also conducted on the US mainland
and in Taiwan, Kyodo news agency reported, citing US military
documents it said it had obtained.
The obtained documents mention test sites including Nago
and Shuri, both in Okinawa, but it is not known whether the
experiments were conducted within the premises of US bases
there, Kyodo said.
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.