Anthracnose lesions on a white oak leaves
By Paula Flynn
Extension Plant Pathologist
Iowa State University
Concerns about oak trees have been common this spring. Careful observers have noticed that some of the leaves on white oak trees appear more brown than green, especially the leaves on the lower branches. Some of these leaves have even curled and fallen to the ground. The question on everyone’s mind is “Is this a serious condition or something superficial?”
Anthracnose has been a widespread disease on white oak trees this season. Spring conditions that are cool and wet favor the development of anthracnose diseases. Although the common name of the disease sounds quite alarming, it is actually a fairly minor problem on established oak trees.
Anthracnose is a fancy term for a leaf spot or leaf blight disease. Caused by a fungus, typical symptoms appear as papery brown spots on leaves, often associated with the leaf veins. These spots can coalesce, causing large areas of browning on the leaves. The leaves appear as if they have been scorched. With time, diseased leaves may curl and drop from the tree.
Anthracnose also may occur on other broadleaf trees, including ash, maple, sycamore and walnut. Anthracnose diseases are caused by a number of different but closely related fungi. Each fungus is specific to the host tree if affects. For example, the fungus that causes anthracnose on oak does not infect ash or maple trees.
The warm and dry conditions of summer are not favorable for most anthracnose fungi, and trees tend to put out additional leaves that appear healthy. The fungi that cause leaf diseases of trees may survive on fallen leaves, so raking and removing fallen leaves can be helpful in reducing the disease, but is only practical in some urban landscape situations. Fungicides sprays for control of leaf diseases are typically not warranted on established shade trees. Fungicides cannot be used to “cure” infections once they have already occurred.
Good cultural practices that promote tree vigor can help trees recover from disease problems. Young trees should be properly pruned to promote good branch structure and good air circulation. To reduce the risk of oak wilt infection, remember to prune oak trees only during the dormant season. Mulching with an organic mulch such as wood chips and watering during drought spells help to promote a healthy root system.
Understandably, some owners of anthracnose-infected trees were very panicked about losing their white oak trees. Their worry centered on possible presence of oak wilt, a very serious disease of oak trees caused by a fungus unrelated to the anthracnose fungi. Oak wilt can result in death of trees. The oak wilt fungus invades the water-conducting tubes of oak trees, eventually causing leaves to wilt and die. All oaks are susceptible to the disease, but oaks in the red oak group, such a red, pin and shingle oaks are more susceptible than oaks in the white oak group, such as white, bur and swamp oaks. Trees in the red oak group typically die within a few months of infection. Infected bur oak trees may survive a number of years with the disease, and white oak trees may survive for 20 years or more with the disease.
Oak wilt causes loss of leaves and death of branches. Anthracnose, on the other hand, can cause defoliation, but new leaves typically emerge as time progresses. Anthracnose is most commonly observed on lower branches in the spring, especially if extended wet conditions occur, whereas oak wilt symptoms may appear throughout the tree. Symptoms of leaf loss and branch death related to oak wilt are most often observed as conditions become hot during the late spring and summer months. In contrast, the seriousness of anthracnose tends to lessen as the summer progresses.
Diagnosing oak wilt can be difficult. Other factors, such as construction injury, drought stress, advanced tree age and borer activity can contribute to dieback of branches and can be confused for diseases caused by fungi. A close look at a sample in a laboratory can be helpful. Anthracnose can be diagnosed by using a microscope to look for specific fungal structures on the underside of leaf tissue. Oak wilt is diagnosed by culturing branch segments from affected areas in the trees.
Samples of diseased oaks can be submitted for diagnosis to the Iowa State University Plant Disease Clinic. Samples can be submitted through your local county extension office or mailed to: Plant Disease Clinic, Iowa State University, 351 Bessey Hall, Ames, IA 50011. Samples should be collected for several different symptomatic branches. Place the samples in a plastic bag and ship in a sturdy container. They should be kept cool and shipped as quickly as possible. Include detailed background information about the tree and the surrounding site conditions. Pictures or digital images are very helpful in diagnosing oak problems, but are not a substitute for a good sample of affected tissue.
There is one photo for this week's column: Anthracnose7706.jpg, 4.28 MG