Actually Millet, I am agreeing with you that it is manganese and wonder if a misting of the leaves with micro-nutrients during a warm spell might help.

I'm ready to give my grapefruit some epsom salt, but am not sure how much to give. I did a search here and found a post that talked about 1 Tbsp per gallon of water.
Since mine is in real need of Mg should I add more?
BTW, we got down below 32 one time again and I covered it again. It's a bit more yellow, but not much.
Thanks!
Barb