Tall, majestic red oak trees add plenty of beauty to yards in California. Sadly, red oaks are also quite susceptible to disease, so if you're lucky enough to have some of them on your property, it's important that you watch them closely. Here is a detailed look at some of the diseases most likely to affect red oaks along the west coast, along with some tips for dealing with each one.


Anthracnose is a common fungal disease that infects many different garden plants, shrubs, and trees. The good news is that it is not deadly. However, it can weaken an oak tree, making it more prone to other infections, and it can also cause twig dieback that later results in bushy growth, giving your tree a less-attractive appearance.

The earliest sign of anthracnose is green-brown and black patches on the leaves--especially along the leaf margins. These patches are often uneven and oddly shaped. Some leaves may tumble to the ground in the early spring.

It's best to treat anthracnose as soon as it appears. Spray the tree with copper-based fungicides, and clean up any fallen foliage quickly so that the fungi do not breed within it and re-infect the tree.

Oak Leaf Blister

Oak leaf blister is another fungal infection that primarily infects red oak trees. It typically appears after a wet, cool spring, as the fungi that cause the disease breed well in these conditions. The primary symptom of the disease is the appearance of small, blister-like spots on the tree's leaves. These spots start off yellow and then turn brown later in the season.

Oak leaf blister will not kill your oak tree, but it does make your tree less attractive. To get rid of the disease, apply fungicides to the tree early in the spring before the buds appear. (Later-season fungicide treatments are not effective.)

Regular trimming can also help protect your red oak tree from oak leaf blister. Trees that have been properly trimmed allow for more airflow though the branches. As a result, the branches dry out faster and become less appealing to fungi.

Sudden Oak Death

This is the last disease that you want your red oak to develop. Sudden oak death, as the name suggests, causes most trees it affects to die off within a few weeks. The disease has ravaged oak forests throughout California over the past several decades.

Sudden oak death has a unique progression. First, the tree becomes infected with a fungus called P. ramorum. This fungus primarily causes spots to appear on the leaves and small wounds to appear on the twigs.

Once the fungus has weakened the tree, bark beetles start feeding on its bark, aiding in the formation of cankers--or oozing wounds in the trunk. Eventually, individual branches die, and then the whole tree succumbs to the infection. 

The early signs of sudden oak death are often mistaken for those of anthracnose and other more minor tree ailments. As a result, many tree owners do not realize their tree is affected until it is either dead or near death. At this point, the best solution is to have the tree removed, as this will help prevent the fungi from spreading to other nearby trees.

Removing a tree affected by sudden oak death can present a few challenges. Branches that have been weakened by the fungus may fall or crack, creating the potential for injury. It's also important for the wood to be disposed of properly. For these reasons, it's a good idea to hire a professional, like those at Simply Trees, to remove a dead or dying oak.

Read oaks are a thing of beauty, but they are also quite prone to infection. Keep a close eye on your trees, and address any possible disease as early as possible to protect your oak tree and others nearby.