Hello,

Your customer is right, it is a Mountain Ash tree. Read about it here: https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a376

The growths on the leaves are galls which rarely threaten the health of the tree. Galls are caused by tiny Eriophyid mites that are blown by the wind or carried to trees by birds and insects. The adult females overwinter in cracks in twigs and lay their eggs in the spring. They can lay up to 80 eggs a month which will develop into an adult in one to two weeks. The adults feed by sucking up the contents of plant cells. To gall is an attempt by the tree to wall off the attack by the mite with a barrier of plant tissue. The damage is purely cosmetic so treatment is not recommended.

For further information, you can send pictures and/or a sample to MSU Extension’s Plant and Pest Diagnostic Lab at: https://www.canr.msu.edu/pestid/

A certified arborist can assess the tree also. To find an arborist in your area, go to: www.treesaregood.org, the website of the International Society of Arboriculture.

Thank you for contacting Ask Extension.