Over the past couple weeks, I’ve gotten quite a few calls from concerned homeowners inquiring why the leaves on some of their oak trees are turning brown. There are numerous factors that can impact foliage and twigs on oaks, but one of the most common is oak anthracnose.
Symptoms vary with host, weather, and time of infection. Shoot blight is one of the first symptoms seen in spring. Blighting causes leaves and shoots to brown and shrivel. Young leaves become cupped or distorted with necrotic lesions. Large lesions often follow leaf veins or are delimited by leaf veins. Old lesions are papery and gray to white in color.
On the underside of an infected leaf, tiny brown fungal fruiting bodies may be visible on or near major veins. Premature leaf drop is common. Mature leaves are fairly resistant and the symptoms are simple necrotic spots. If infection is severe, branch cankers and twig dieback can occur during winter and early spring. The symptoms usually first appear on lower branches and then spread up the canopy.
Many species of oak are infected, and regionally important species include: white (Quercus alba), northern red (Q. rubra), black (Q. velutina), pin (Q. palustris), chestnut (Q. prinus), scarlet (Q. coccinea) and swamp white (Q. bicolor). White oak is particularly susceptible to the disease and suffers greater damage compared to other oak species. The fungus can also be found on beech (Fagus), chestnut (Castanea) and linden (Tilia).
Most often, oak anthracnose causes only minor damage to landscape oaks; however, after prolonged periods of wet weather early in the growing season, especially on members of the white oak group, damage can be severe.
Photo by Joseph OBrien, USDA Forest Service, Bugwood.org
Spores from the fungus that causes this disease in oaks is spread mostly by wind and running or splashing rain water. Twig dieback may lead to secondary colonization by wood-boring insects and stem cankering fungi.
Disease outbreaks usually subside by mid-summer when conditions become warmer and drier. In autumn, when cooler weather returns, there is often resurgence in disease development as the pathogen exploits senescing foliage. Resting structures then develop, which allows the pathogen to overwinter.
At the same time, it is important to note that spores can travel long distances and anthracnose fungi are widely abundant in both forest and landscape settings. This makes eradication of the fungus unlikely in most situations.
When healthy trees are defoliated early in the season, most have the reserves to produce a second flush of foliage and suffer only minor growth losses; however, when oaks are weakened by other stresses, such as gypsy moth or Armillaria root and butt rot, the effects of oak anthracnose are amplified.
For smaller or newly planted trees, one management strategy is to prune and discard dead stems and branches and thoroughly remove all fallen leaves in autumn and spring as they harbor the fungus and allow inoculum to remain at the site. Maintaining tree vigor through adequate fertilization, supplemental watering (if possible), mulching to help moderate soil temperatures and pruning of dead branches will help oaks to prosper despite the presence of disease.
