Hazelnut tree (Barcelona) –

Question

Why are the leaves curling up? Not discolored. No obvious signs of pests or mold. The other compatible hazelnut tree is not curling like that. (I forget the name of the other tree’s variety)

Answer 1

Curling leaves can indicate a pest or disease issue.
These articles will help you pinpoint the issue.

http://www.omafra.gov.on.ca/english/crops/facts/12-009.htm
https://www.gardeningknowhow.com/edible/nut-trees/hazelnut/grow-hazelnut-trees.htm
You may also want to check with your County Extension Office for issues specific to your region.
https://www.gardeningknowhow.com/extension-search/