Plumeria

What is happening to this plumeria?

The leaves are turning yellow and dropping. I have not had this problem before. What is the cause?

Thank you.

Have you checked for spider mites?

I did and there are none.

Watering is the next suspect, here is a trouble shooting link for yellowing plumeria leaves.
http://www.tropicamango.com/plumeria.html

Thank you!

rjuddharrison- Thanks for the link on Plumerias. Lots of good info there. I have recently acquired several plumeria cuttings, so that info should be very helpful. Since I live in KY, they will need to be potted or lifted in the Fall if planted in-ground. I am a newbie to growing plumerias, so I need all the help I can get!