CHICAGO • Tennessee on Thursday imposed restrictions on the use of dicamba, becoming the fourth state to take action as problems spread over damage the weedkiller causes to crops not genetically modified to withstand it. Missouri on Thursday also announced restrictions, partially rolling back an emergency ban announced last week.

Dicamba is sprayed by farmers on crops genetically modified to resist it but it has drifted, damaging vulnerable soybeans, cotton and other crops across the southern United States. Farmers have fought with neighbors over lost crops and brought lawsuits against dicamba producers.