Question
I have several deciduous trees in my back yard which have white round nodules on the underside of leaves.  They are fuzzy and hard.  They are fixed to the leaves.  It has been on the leaves for a few years.  I live in the Black Hills, SD.  They might be ASH trees.  Trees were before I moved in to the property.  New Growth on the tree is still appreciated while at the same time seeing the nodules on other leaves.  What are these?

Answer
Hi Ed,

Leaf galls can be caused by many things, but the most likely cause are insects what use the leaves for laying eggs.  If you are unsure of the tree species, check out the following website to see if any of those look familiar.  

http://www.bugsandweeds.co.uk/galls%20p1.html

From your description, it sounds much like the Salix caprea gall _Pontania gallarum_.  

Such galls are not harmful to the trees. If you are concerned about the galls, cut one open to see if there are any larvae inside.