Question
QUESTION: We have two huge pin oaks. Both have raised yellow bumps ranging in size from a few millimeters to the size of a pea on the tops of nearly all the leaves. Otherwise, the leaves look shiny and green. What is this, will it harm the tree, and is there anything we can do about it? Thanks!

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.


IF the tree is small and if you want to fungicides can be sprayed to control the blisters but this is not practical on large trees.

Although, oak leaf blister causes little damage to trees, some homeowners may wish to control the disease because of its unsightly appearance. Since infection of leaves by the blister fungus takes place only in early spring from fungus that overwinters in the bud scales of twigs, disease control steps have to begin before tree dormancy is broken in the spring.

Control of oak leaf blister requires a dormant application of a fungicide any time from January until mid-March. The latter application timing probably gives better oak leaf blister control if the fungicide is sprayed onto trees just before new growth starts. Usually one fungicide application will give satisfactory control. Care should be taken to coat all buds and twigs.

Fungicides such as maneb, captan, tri-basic copper sulfate, or those products containing chlorothalonil such as Bravo or Daconil 2787 will destroy the overwintering fungus and are recommended for oak leaf blister control. Chlorothalonil fungicides are sold under a variety of trade names and are commonly available at most garden supply stores.

I would just rake and destroy the fallen leaves. The fungi is highly related to moisture levels at the time of spore production  and the amount of blisters will vary greatly from year to year.  


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QUESTION: I looked at pictures of oak leaf blister. They do not at all look like what is on my tree. The ones we have are like perfectly round hard zits. Any other guesses? Thanks!

Answer
There are many different gall makers some are fungi related some are insect related. I took stab based on the most common fungi gall maker. IF the spot is on both sides of the leaf than it is normally a fungi related gall--IF the gall is on one side of a leaf (newer galls) than this is usually an insect related gall. In either case they will not kill the tree but can if numerous make the leaves look bad and cause them to drop sooner than normal. I really would not worry about these galls or spots. Treatment means spraying early in the life of the leaf and it would be late now to get any control. As I said before you have an excellent chance of having no problem next year. Here is a web link to gall making insets and some pictures. The exact insect or fungi is not really that important in leaf spots.  I hope this sets your mind to rest that it is not a problem to the health of the tree.  

http://www.extension.umn.edu/distribution/horticulture/DG1009.html