What's wrong with my avocado tree?

Just noticed this yesterday. It must not have started but a few days before.. Does anyone have any idea why this is happening? I haven't fed it in a while and i'm wondering if maybe that's the problem. It's only happening on two of the oldest leaves. The plant is otherwise completely heathy and has some really nice new growth actually...
 

I have the exact same thing happening on mine. Just a couple leaves and good new growth. It's been indoors for a few weeks and I suspect the problem is somehow related. I hope you find out the problem!

Mike

Mine has been indoors for a few months now..

Pj, your spotting seems to be limited to the perimeter of the leaf which could be a fert burn.  I will try to post a link I have along with my pics.
 
http://plantdiseasehandbook.tamu.edu/food-crops/fruit-crops/avocado/


 
I think mine is related to a heavy rainfall when I put it out for a few warmer days and caught the rain then brought back in to wet.  The link mentions Cotton Root rot, and because I just used some of the compost dirt that it voluteered in is a possibility.  Also Seedling blight seems to fit as well. 
 
I have a volunteer Mango too!
 
Good luck!
 
Mike

Could it have a fert burn if I haven't fed it in a while?

Wish I could have found the one with pictures, but here.
http://www.hydroponics.net/learn/deficiency_by_element.asp

Sometimes my leaves just shrivel up and die. If I see that on mine, I cut it off. I had 4 trees outside this winter. One was 6 foot tall (Hass variety),and my dog snapped it in to. Stake and all. He loves avocados. Good for his coat.

The other one 5 foot tall, all the leaves fell off. Too cold. Fuerte variety.

First 2 I bought from a local nusery.

These last 2 I planted from seed.



They should both be Hass variety. They survived the winter no problem. I grew the last 2 indoors from seed. Only RO water. I now use 8-4-4 tomato Fertalizer on them once a week with RO water and Cal-Mag. They are both about 6 months old.

Once they start to fruit, 16-16-16 fert. They are just like citrus trees. My grandfather had an avocado orchard. He was the master with these trees. Even grafting them. I was very young when he passed away, but I still remember a few things. Don't overwater these trees. They are very prone to root rot. Once in the ground only deep water them. My one in the ground would get watered every 10 days or so for one 24 hour period. Just a slow trickle. A drip system works good for this as long as it has a high flow. Not this drip...drip.