Black Walnut - 100 Most Common North American Trees

Pest information courtesy of USFS Fact Sheets:

Pests: Fall webworm larvae web over branches then feed on leaves inside the nest. Nests can be pruned out of small trees or use sprays of Bacillus thuringiensis.
Tent caterpillars also eat foliage in the spring.
Scales of various types attack walnuts. Most scales can usually be controlled with horticultural oil applied.
The leaves may be eaten by any one of several caterpillars. These can be controlled with sprays once identified.
Mites cause speckling and yellowing of the leaves.

Diseases: Brown leaf spot or anthracnose symptoms are irregular dark brown spots occurring in early summer. Severely infected trees may be defoliated. Rake up and destroy infected, fallen leaves.
Canker diseases cause dieback or death of trees. Infected bark may be discolored, sunken, or have a different appearance than surrounding healthy bark.
Bacterial blight causes small, irregularly-shaped spots on the leaves and leaf stems.
Black spots occur on young nuts and shoots. Nearly ripe nuts have large black spots on the husks. Infected nuts fall prematurely or may have husks, shells, and kernels blackened and ruined.
Powdery mildew causes a white coating on the leaves.
During periods of high temperature and drying winds, walnuts may scorch. Make sure plants have adequate soil moisture.

More on Tree Insects and Disease