There are convex light green lumps on the leaves of my family's black Walnut - only in one region of the tree. The reverse side of affected areas has like a fuzzy yellow crust. What is this and is it unhealthy for the tree? Is it a fungus? (pictures attached)
Most likely to be Walnut Gall Mite, see here http://homeguides.sfgate.com/walnut-gall-mites-75630.html
Although unsightly, treatment is not entirely necessary because the mite damage does not kill the tree. Bonide Citrus, Fruit and Nut Orchard spray might be effective if the infestation of mites is widespread.