I don't know, but hope someone here can help you!

Here is a citrus resource image and disease gallery. Hope this helps a bit until someone else can help you.

http://idtools.org/id/citrus

Hope the link works..

If it is scale a fungicide will not help I think. You use a fungicide for fungus problem not an insect problem. Use Neem oil, horticultural oil, alcohol, or mild soap and water or Safer soap spray to wipe it off each leaf and stem. Rotate what you use every 7-10 days to get all the eggs and babies. It could take months to get them all. The oils work by smothering, the soaps and alcohol kill on contact.

Cory

I had something similar (sorry no pics), including the yellowing and dropping leaves and used Neem Oil. It's a little messy to spray so I did that in my driveway rather then the patio. Be sure to drench both the top and bottom of the leaves along with the trunk. Watch for ants on your plant - if you see any then address them as well. Ants will "farm" the scale. Keep your plant out of direct sunlight for a few days after spraying or the leaves will burn. I also used rubbing alcohol on cotton pads to kill and wipe off any remaining scale I found on the leaves and stems. Tedious, but meditative work! The plant has been very healthy all summer and I now regularly inspect the leaves and watch for ants. Good luck with yours!

Great looking tree grin!