failing avocado plant

Question

Our large avocado plant (indoor house plant) has begun to fail. The leaves are green but brittle and falling off. We have removed it from its pot, cut back the roots and removed more than half the branches and repotted it (in the same pot) with fresh, previously unused soil.
We suspect root rot, but the leaves are not turning yellow – they stay green but are wilting and/or becoming brittle.
There is new growth that appears healthy but the mature leaves are affected.

Answer 1

This appears to be a fungal infection. This is usually caused by overwatering or the soil remaining too wet for too long. It also appears to have a severe lack of light. These are very light hungry trees. Indoors, they will require about 200 watts of horticultural lighting per ground pattern square foot of tree. This is the minimum to keep the tree healthy and productive.

Fixing the light quality, intensity, and duration will have to take priority before you could expect any recovery from this tree. Once you get the light situated, then you can begin to treat the tree. Fungicides containing potassium phosphonate should help. See more below.

https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/treating-avocados-with-root-rot.htm