You will have heard of mad cow disease.

Now "mad soy disease", is troubling farmers and scientists in Brazil, where it causes yield losses of up to 40%, and is expanding out of its stronghold in the north of the country. And, like its bovine namesake, it is incurable.

Indeed, its growing occurrence in Mato Grosso state, which produces nearly 30% of Brazil's soybean crop, has "brought this issue to the forefront", US Department of Agriculture staff said.

"In past years, the anomaly affected soybeans in hot northern growing regions on a sporadic basis, but is now extending to more southern temperate growing regions with increased prevalence overall," they said.

'No treatments' 

The disease is "manifested through an endless vegetative cycle" � meaning that the maturation of infected plants is retarded. Typically, they produce deformed pods with fewer beans in each.

It is in fact the second mad soy disease to be identified, after a previous ailment which caused similar symptoms, was proven to be insect borne, and is now under control.

However, while scientists believe that that mad soy disease 2, or soja louca 2 as it is known in Brazil, is linked to mites encouraged by low-till farming methods, the link has not been proven.

"There are no known effective treatments," the USDA staff said.