Check the soil the plant is growing in. This is most likely related to soil nutrients.

How long have you had this plant? If it's more than a few months old, do you put it outside in the summer, or keep it in a well-lit location in the home?

You use the term 'rust spots'. I assume that you mean the color of these spots, correct? (There's a fungus disease called rust, but avocado does not get that.) Could you elaborate on those spots a bit?

Just so you know, avocado often have foliar problems when grown inside the average home.

Kimmsr and rhizo1, thanks for working on this!

I grew this plant from a pit and it's now probably four years old, and about three feet high. Up until a few months ago it has been an excessively healthy plant.

The rust spots are brown dry-leaf areas ranging from pin-prick-sized dots covering the entire leaf to large, thumb-print sized spots around the edges of the leaves (although some are in the middle of the leaves as well). Some of the leaf tips have turned brown as well (but not all!).

I originally assumed that this was rust the disease, but like you said my research said it's not something this plant should get.

Kimmsr, when you say it's related to soil nutrients, do you think the plant is lacking a nutrient, or it is burned? Should I maybe change all of the soil?

I would guess that you have a fungus infection. The bad news- many fungi. The good news is that a copper fungicide spray controls many of them. But from what you write, the plant may never be restored to its beautiful self. The rust spots will not go away so the plant will have to replace all its leaves. This will take time and it will look worse before it gets better. I would suggest that you remove all the unsightly leaves, treat with fungicide and keep in bright indirect light. When it recovers you can put it back on display. You have grown it successfully for four years so I would continue to do the things which have worked for it in the past.

Hi everybody,

Thanks very much for all of the feedback - much appreciated. Here's the follow-up: I was agreeing with the fungus theory (and the overfertilized theory), and then I took two leaves from this plant to my local garden center where I got a chance to show them to our local plant expert (sells houseplants and does a regular column in the newspaper).
He looked at the leaves carefully and said that not only does this plant have scale but it has *spider mites* and he believes this is the crux of the problem. He diagnosed the spider mites by a slightly metallic sheen the leaves had when held up to the light. Also, there was a fair amount of honeydew on the leaves, which I thought was a product of the scale.
He sold me some Bayer's Rose and Plant Insect Spray which I have dutifully sprayed all over the plant. I confess I'm feeling pretty guilty about this since I generally prefer organic means to work through things like this but the plant is in such distress I caved in and bought the spray. I'm going to give it a week and see what happens.
I also intend, once it's had a few days to recover from the spraying, to replace all of the potting soil and thus eliminate the overfertilization variable.
Thanks again for all of your suggestions. If any of you thinks that this solution seems mistaken, please feel free to respond.

cheers,
papercarver

It is not that the solution may be mistaken; it is I who am confused. Aphids are insects (adult has 6 legs etc.). Spidermites are mites (8 legs, webs etc.). Only a systemic insecticide has the potential to control both. I would guess that the Bayer product contains a true non systemic insecticide plus a fungicide and might not be effective against mites. Aphids secrete honeydew and if the 'glazing' effect was visible I would expect to see aphids and/or their shed coats and also some sooty mold fungus. I have seen some heavy infestations of spidermites but cannot recall a single instance of a honeydew/sooty mold association. But Mother Nature has a habit of humbling me.