Leaf diseases and insect problems are a common problem on deciduous trees. These diseases may be indicated by blotchy, red blisters on the trees' leaves. Diseases and insect infestations can lead to reduced foliage in a tree as well as ineffectual growth. Many of these problems can be corrected if you can first diagnose the reason for the red blister on a tree's leaf. Causes of red blisters vary from tree to tree.
Contact the county extension office of your local land grant college to see if there have been reported cases of fungus or insects in your county or state that can lead to red blisters. This is the fastest way of identifying likely culprits.
Note the shape of a leaf to determine the type of tree that is affected by the red blister. Red blisters on a maple tree's leaf are caused by a different source than red blisters on pine tree needles or oak leaves.
Look over the leaves of your tree for a red or bronze tint in addition to the red blisters. This is a sign of leaf gall. Leaf galls form around feeding insects as a plant's way of insulating the insect and damage. If the leaf bronzes in addition to blistering and forming galls, the source of the red blisters are a mite called Eriophyid. Eriophyid mites will not harm the tree.
Observe the blister to see if it changes color on an oak leaf. If the blister changes color from red to dull brown, it is caused by Taphrina fungus. Taphrina fungus blisters are circular in shape,and clearly raised. If more than one appear on a leaf, they may merge into a larger blister. This fungus is not harmful to the tree. Allow the tree to shed its leaves in fall. The next year's flush of leaves will be blister free.
Cut into the blister of a pine needle. If the blister is filled with spores, then the red blister is pine needle rust. The rust will not harm larger trees, but may defoliate smaller trees. Control strategies include mowing alternative hosts for the fungus.