Q.What’s Wrong With This Hazelnut Tree?
This hazel is turning yellow and dying but two others close by are very healthy. Does anyone know what’s wrong with it and what I can do to help please?

Certified GKH Gardening Expert
I don't see clear signs of disease, which leads me to believe that the soil in that particular spot is deficient in something. Possibly, Nitrogen.
You can confirm with a soil test.
These articles will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/edible/nut-trees/hazelnut/grow-hazelnut-trees.htm