have you checked for bugs both under ground and on the leaves? pull one up and check the roots to see if there is any deformities such as nodules or lack of roots, this would indicate a bug or fungus. both very treatable once you can figure out which it is. If it is grubs eating the roots get some nematodes and apply as directed they work great for killing grubs and such that live under ground with out adding poisons to the soil. Also check to see if they are getting too much water and look for root rot.