I have been growing in my flat an Avocado plant sprouted from a seed in December. It developed well, up until about a month ago, but then brown dots started forming on the leaves, the oldest leaves first and spreading to the newest. The dots start small on the edges and very quickly spread over the leaf, until the leaf dries out and the plant discards it.
Below I put an example. The first picture was taken last Sunday, when the leaf started showing the spots. The second picture was taken today, three days later, showing the progression of the spots. Usually it takes about two weeks for the leaf to be dropped by the plant.
At first I blamed this on dry air caused by the central heating (In the picture you see a radiator, but that's just where I put the plant now that the radiators are off. During winter it was in a different position.). The plant now is next to a window open most of the day, so the air shouldn't be as dry, but still the spots returned.
A second hypothesis I had was high salinity of the water. Tap water here is very hard and reading other articles it seems that there could be a stagnation of salt. To counter this I replaced most of the earth in the vase, but this still didn't stop the leaves from browning.
The plant itself is now down to three leaves, one of which is the one in the pictures above, and the top gem is not showing signs of development, so I'm running out of ideas to save the plant.
Any hints on what might be causing this and how to prevent it from extending to the rest of the plant?