Garden Pests and Diseases: Avocado tree turning brown, 2 by Txema

Txema wrote:
Dear all,

I have some problems with one of my potted avocado trees. The tree is 1.5 years old and had green leaves and looked healthy. A few months ago, the leaves started to turn brown. Following some advice, I used some gypsum in the soil and saw a slow improvement. However, about a month ago, the leaves turned brown/grey and dried out. Now, the stem is turning brown from the top and it is gradually approaching the roots. Do you have any idea what this could be? And if so, how should I proceed to save the avocado tree?

Thank you in advance.