Many trees, both native and ornamental can be affected by anthracnose disease. These fungal diseases cause premature browning, shriveling, and leaf drop. For some trees, such as Modesto Ash, Sycamore, and Chinese Elm, this can be an almost annual event often causing complete defoliation. For other species of trees, including native oaks, the effects are dependent on spring weather and / or, the local micro-climate. Often these symptoms subside or disappear as summer approaches with warmer, dryer conditions.
Weather Conditions: For many species of trees, wet cool springtime conditions have the greatest influence on the severity of infection. In Sonoma County and the San Francisco Bay area in general, late rain from April through June and or dense morning fog during the same period promotes the development of the fungi that cause anthracnose. Even the micro-climate within a neighborhood or property may affect which trees become infected with some trees exhibiting minor or no leaf damage while others become defoliated.
Symptoms: For oaks, the leaf damage caused by anthracnose infections vary from small brown spots on the leaves, to large brown spots, to curled and deformed brown or dead. Severe infections can cause current season twig growth to become infected and die back as well. The most obvious sign of this disease for most people is early leaf drop which sometimes continues through the summer. Complete defoliation is uncommon in my experience but 50%-75% does occur. Because anthracnose is not a problem every year, our native oaks seem to tolerate the disease very well. While mostly a benign disease, many property owners become justifiably concerned when their oaks begin dropping leaves prematurely.
Affected Trees: In the Bay Area of Northern California all our native oaks are susceptible to this disease. Black oak in particular seems to be most affected by anthracnose in part, I believe, because it is the first oak to break bud in the spring, making it more susceptible to disease infection. Valley oak and blue oak are also susceptible if cool damp conditions persist through the spring. Coast live oak may also be affected but their leathery leaves seem less affected than the other species. Trees affected by anthracnose early in the season may produce a second flush of leaves in the summer provided the climatic conditions that foster the disease have passed.
Native California oaks, in general, don’t need any type of treatment to control anthracnose infections because of the infrequency of infection. Generally the disease is cyclic with several years between severe outbreaks which may cause some individuals to be almost completely defoliated. Sanitation is the best treatment for native oaks which focuses on the removal of infected plant parts such as leaves and twigs. Pruning can also help if done properly by removing only small twigs and dead branches. Severe pruning can be detrimental to oaks and can promote other diseases such as powdery mildew by promoting uncontrolled sprouting. Overhead irrigation systems used on other plants or lawns but near oaks can increase the frequency of anthracnose as well.
Sherby Sanborn is a Consulting Arborist in Sonoma Valley, California.
My professional career began as an economic entomologist and then a forester working on the Department of Forestry and Fire Protections (CDF, Cal Fire) Dutch Elm Disease (DED) project.