Question
Our oak tree that is about 3 years old has yellowing of the leaves and tiny black spots all over them. Some of the leaves are turning brown and drying up especially the ones at the ends of the branches.  There does not appear to be an insect problem.  We have had no problems in prior years. We have a sprinkler system so it gets water on a regular basis and the other 2 maple trees in the yard do not appear to be affected.  We live in the Northwestern US

Answer
Sounds like leaf spot disease or leaf gall insect.

Leaf spot is a common descriptive term applied to a number of diseases affecting the foliage of ornamentals and shade trees. The majority of the leaf spots are caused by a variety of fungal pathogens but some are caused by bacteria.

The primary symptom of a leaf spot disease is spots on foliage. The spots will vary in size and color depending on the plant affected the specific organism involved, and the stage of development. Spots are most often brownish, but may be tan or black. Concentric rings or a dark margin around the spot may be present. Fungal bodies may appear as black dots in the center of the spots. Over time the spots may combine to enlarge to form blotches. Leaves may yellow and drop prematurely.

The organisms that cause leaf spots survive in fallen infected leaves and twigs. Some may remain in dead twigs on the tree. During wet weather, spores are released which may splashed or be windblown onto newly emerging tender leaves where they germinate in the moisture and infect the leaf. Overhead watering late in the day or during the night, heavy dews and close spacing of plants prolong wetting of the leaf surface and provide more opportunities for fungal or bacterial infections.

Live with the disease. Leaf spots are largely an aesthetic problem as few leaf spots seriously damage the host. Also control is seldom achieved after the infection has started.
Remove infected leaves and dead twigs. Raking up and disposing of infected leaves as they drop and pruning out dead twigs can help control the disease by removing spores that can reinfect the new leaves. This will not cure the problem but it can help minimize infections.


Keep foliage dry. Avoid overhead watering. Use soaker hoses or water early in the day so the foliage can dry before night. Watering can also spread the disease by splashing. Prune plants and space plants to allow for good air circulation that promotes rapid drying of foliage.

Fungicides can be used but for the most effective use they will need to be applied at bud break. They will not cause the spots to go away that are already present. On small, newly established or especially valuable specimen oak trees previously damaged by leaf blister, apply a protective fungicide. Apply the fungicide at bud swell.
The following fungicides are labeled for use on oak to control oak leaf blister: Bordeaux mixture, Daconil Ultrex, Daconil Weather Stik 6F, Rose and Shrub Disease Combo, Dithane M-45 80W, Fore 80W. Always follow label recommendations and precautions.