No chemical control is listed. Leaf spots caused by several fungi can attack catalpa. A symptom of all is brown spots on the leaves. Leaf spots are rarely serious so no chemical control is suggested.
Powdery mildew causes a white powdery coating on the leaves.
When severe, the leaves yellow and fall off. If needed, sprays of benomyl may be used according to label directions. Catalpa is susceptible to verticillium wilt. Branches die and eventually the entire tree may die. A symptom of verticillium wilt is discoloration of the sapwood but it is hard to find.