All the others look like this...

It could be aster yellows. Cones that stay green are a very common symptom. Are any of the leaves loosing their chlorophyll? It's hard to tell from the photo, but the cone on the left looks like it might be deformed.

Here is a link that might be useful: Aster Yellows

UGH... Never heard of that before, and wasn't expecting it to be a disease... I think you are right though and thanks for the information. The leaves look fine... the plant is probably about 66% the size of the others...

That's definitely asters yellows... Get rid of it before it can be further transmitted to other echs, rubekia, etc. keep an eye out for the tell tale green petals