Question
Two of my water oak trees have developed many brown spots on their leaves. This is the first time to happen... is this a disease, and if it is, what can I do about it? Thanks a lot.

Answer
A very common leaf spot disease this year is oak leaf blister. All varieties of oaks can get this disease, although water oak (called pin oak by some folks) seems to be the most susceptible. As the name implies, a blister-like spot is created on the leaves, often causing the leaf to distort. If the infection is severe, some of the leaves may fall off of the tree.

The main thing concerned folks want to know is whether leaf blister will hurt or kill the tree. Leaf spots alone will not kill oaks. Premature defoliation is stressful to trees, but if a tree is otherwise healthy, it will quickly grow back another set of leaves.

Leaf blister infection took place in March and early April. When leaf buds open in the spring, fungal spores present on the bud scales infect the expanding leaves. Infection is most severe in wet, cool spring weather. Because of the narrow window for leaf infection, the fungus seldom becomes severe enough to require treatment. Even in years when early infection is severe, it is normally limited to only the first few leaves that emerge. In areas where spring weather conditions frequently favor disease development, fungicides are applied at early bud swell and repeated in 10 days. Due to the limited injury done to the tree by this fungus, fungicide treatments are normally restricted to only those trees that are highly valuable and visible in the landscape.

Since this does not really cause any health problems to the tree except for the possible early leaf drop and the looks, I would not worry about the spots. The leaves should be raked when the fall and removed from the site --this will reduce the amount of fungi spores that will infect the leaves next spring.