Chlorosis is a term commonly used to describe a tree suffering from an iron deficiency. Without enough iron, the tree is not able to produce an adequate amount of chlorophyll.
Reduction in chlorophyll decreases the amount of photosynthesis taking place within each leaf. This lack of photosynthesis means less energy is being produced for the tree. It causes the tree to become more susceptible to insects and diseases, which can eventually lead to the death of the entire tree.
An iron deficient tree will have yellow or light green colored leaves with dark green veins. This can be found in one part of the canopy or spread throughout the entire tree.
A liquid solution of chelated iron is administered into the tree’s vascular tissue through the process of macro infusion which lasts for 3 years. This lets the chemical spread throughout the entire tree and allows each leaf to become more productive. This treatment is done in the autumn just as fall coloration is setting in.
Useful links:CO State: Iron Chlorosis in Trees