This leaf-spot disease, caused by the fungus Mycosphaerella juglandis , attacks black walnut, Juglans nigra , and Persian walnut, J. regia . Thus far, the disease has been found in North Carolina, Georgia, Illinois, and Iowa. It is important in young walnut plantations, where it causes premature defoliation, thus reducing growth and nut production. Affected walnut trees appear healthy with good foliage color until July. Then, from a distance, affected trees begin to look yellowish.
Closer examination reveals leaf scorch, vein-pattern necrosis, and lesion fleck symptoms.
Symptom-bearing leaves become increasingly chlorotic and by mid August may have dropped. Look for killing of portions of leaves, particularly at the tips of the leaflets. Vein pattern Necrosis . Look for necrotic flecking among major leaf veins. Leaf spotting .