Hello! I sprouted an avocado a few years ago and have been tending to it carefully. I noticed the leaves were turning rusty and brown in the last week, it's progressing quickly.
My search leads me to believe it's a salt issue. Some results have said to do a thorough flushing the next time I water while a few comments say to use a small amount of vinegar.

However, it could possibly even be water damage so I'm not sure if I should let it dry or flush it.

I'll have to upload pics tomorrow, but I thought I'd get a head start. What are your recommendations?

If there is a white crust on the soil, then that's salt accumulation.

To avoid that in the future, water with distilled water (rainwater), and don't allow water to pool around the roots. I suggest an air-space between the bottom of the pot and any saucer you might be using.