Repeated use of the chemical glyphosate, the key ingredient in Roundup herbicide, impacts the root structure of plants, and 15 years of research indicates that the chemical could be causing fungal root disease, said Bob Kremer, a microbiologist with the US Department of Agriculture's Agricultural Research Service.

Roundup is the world's best-selling herbicide and its use has increased as Monsanto, the world's biggest seed company, continues to roll out herbicide-tolerant "Roundup Ready" crops.

Roundup Ready corn, soybeans and other crops are beloved by farmers because farmers can spray the herbicide directly onto their crops to kill surrounding weeds, and Roundup Ready corn and soybeans varieties make up the vast majority of those crops grown in the United States.

But as farmers have increased their use of Roundup Ready crops and Roundup herbicide, problems have started to rise. One of the biggest problems currently is spreading weed resistance to Roundup. But Kremer said the less visible problems below the soil should also be noted and researched more extensively.

Though Kremer said research to date has not shown that glyphosate directly causes fungal diseases that limit crop health and production, but the data suggests that could be the case.

"We're suggesting that that potential certainly exists," Kremer said in a presentation to the annual conference of the Organisation for Competitive Markets, held in Kansas City.

As well, Kremer said that research shows that these genetically altered crops do not yield more than conventional crops, and nutrient deficiencies tied to the root disease problems is likely a limiting factor.

Kremer said farmers should take heed and consider more crop rotations and tighter monitoring of glyphosate usage.

Kremer is among a group of scientists who have been turning up potential problems with glyphosate. Outside researchers have also raised concerns over the years that glyphosate use may be linked to cancer, miscarriages and other health problems in people and livestock.

Monsanto had no immediate comment, but has said in the past that glyphosate binds tightly to most types of soil, is not harmful and does not harm the crops.

The company has said that its research shows glyphosate is safe for humans and the environment.

Neither the USDA nor the Environmental Protection Agency, which is reviewing the registration of glyphosate for its safety and effectiveness, have shown interest in further exploring this area of research, Kremer said.

- Reuters