If it's not lack of water or fertility, and not a disease (the fact that both are affected suggests it is not a pathogen), it sounds like a soil contamination problem of some sort. It's difficult to guess what it could be. Old motor oil, carpet cleaning equipment flushed out on the ground, cement truck washouts (from when the house was constructed)...there are many possibilities. Best thing to do is to take a sample in and get it tested. A local extension service or a good garden center can tell you where to do this locally.