Without a picture it is difficult to diagnose your problem.  A couple of questions--have you fertilized them with some complete fertilizer like 8-8-8?  If not, I would try that and water in really well.  Thinning might help create more vigorous plants as you have suggested but is probably not the problem.  Often times we get rust on the leaves here in Louisiana, but I have not seen that anywhere this year.  One final thought, are they receiving sufficient water.  
Sorry to be so non specific, but if you send a picture, maybe we can be of more help.