From the Pests of Ornamental Series
Shade trees are subjected to the devitalizing effects of numerous insects and plant pathogens. Because of the high costs of spraying large shade trees, we do not generally recommend that home owners apply chemical controls for leaf spots, leaf blights, aphids or other foliar problems. The theory is that we can generally live with these pests and parasites if we can maintain the tree's vigor. This can be accomplished by (1) sanitation, e.g., raking and burning the affected leaves, and pruning out dead and affected wood; and (2) fertilization.
Contrary to popular belief, lawn and street trees need fertilization. Nutrients are returned to the forest trees by the rotting of fallen leaves, but we remove these potential nutrients by the raking and burning of fallen leaves and the removal of grass cuttings. Grass under trees also compete with the trees for needed ninerals.
If your shade trees have sparse foliage, leaves paler than normal, dieback of the twigs, drying and loosening of the bark, or abnormally slow growth, then the trouble may be due to lack of soil fertility, or to the devitalizing effects of insect feeding or defoliating diseases.
To fertilize, apply 5-10-5 or 5-10-10 fertilizer (at the rate of 2 pounds per inch diameter of the trunk at breast height) in holes 18 to 24 inches deep and two feet apart. These holes should be in three circles around the tree: One circle at the drip-edge, one midway between the drip-edge and the tree trunk, and one circle halfway between the two. Slant the holes toward the tree trunk. Distribute the fertilizer evenly in the holes and water it in. Refill the holes with topsoil.
The application of fertilizer every two years should keep trees in a vigorous growing condition. The best time to apply the fertilizer is in the early spring or in the late-fall after growth has stopped.