Q.why is my pussy willow tree dying
About 3 weeks ago. I noticed my tree’s leaves had a orange/yellow fungus on the underneath and it was covered in ants. Since then most of the leaves have gone very dry, turned brown and are falling off–although I can see tiny new green shoots along the stem.

Certified GKH Gardening Expert
It sounds like you have a pest problem...and not the ants. Most likely your plant is infested with aphids or scale, both of which secrete honeydew. It is the honeydew that the ants are attracted to, so getting rid of the pest in questions will eventually get rid of the ants. There could also be a fungal issue as a result of the insects. I would receommend treating the pussy willow with neem oil, which will take care of any insect pests as well as fungus. Here is more info: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm