Have you been feeding them? It might be a mineral deficiency, magnesium and or calcium. It's also normal for the older lower leaves to yellow and drop, especially this time of year. Some might just be dropping naturally but there's definitely something not right. I have a feeling it's a deficiency of some type but see what others say. If it is a deficiency there's a product called CalMag Plus that works very well or you can just try Epsom Salts.

Thanks, Karyn.

Hope somebody else will look at my posting and help.

Pat

I agree with Karyn.It looks like a calcium deficiency.
CalMag or simply dolomite limestone from the local feed and seed store.Be sure it's dolomite which is the necessary magnesium to go with the calcium.
One other thing to check is the soil.Be sure it drains well.