The easiest treatment method is to apply an iron chelate to the soil. Use Iron EDDHA. This chelate is more expensive than some others, but is the only one that is reliably effective. Apply 2 to 3 lb of the material per 1000 square feet and water it into the soil. Spraying is also highly effective, but more difficult, particularly on a large tree.
I need help with Chinese Elms that are showing signs of iron deficiency