By Karl Plume CHICAGO (Reuters) - Tennessee on Thursday imposed restrictions on the use of dicamba, a flagship pesticide for Monsanto Co, becoming the fourth state to take action as problems spread over damage the weed killer causes to crops not genetically modified to withstand it. 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. Arkansas banned its use last week and Missouri, which initially halted dicamba spraying, has joined Tennessee with tight restrictions on when and in what weather spraying can be done.