Anthracnose is a common foliage disease of shade trees in Iowa. Symptoms occur on sycamore, ash, maple, oak, walnut, linden, hickory, and other deciduous trees. Anthracnose is caused by a number of different but closely related fungi. Each fungus is specific to the host tree it affects.
In most cases, although symptoms may appear serious, damage caused by anthracnose is minimal and does not seriously harm established shade trees. Symptoms vary from small, circular to irregular spots that are tan, dark brown, or black, to larger blotches that are usually associated with midribs and veins. When immature leaves are infected, these leaves may become distorted from abnormal leaf expansion. Young leaves may die and fall soon after a heavy infection. If a severe infection occurs early in the growing season and the trees defoliate, a new set of leaves may emerge. Sycamores may also show bud, shoot, and twig blight in addition to blighted leaves.
Anthracnose in Sycamore Trees
Early loss of sycamore leaves can be alarming. A close look at the fallen leaves will reveal brown areas that typically follow along the veins of the leaves. These areas of browning are often V-shaped.
Anthracnose tends to be most severe when extended cool and wet weather occurs in the spring. The fungus that causes sycamore anthracnose needs wetness in order to infect the leaf tissue. Leaves are most vulnerable to this fungus during the first weeks of growth.
Unfortunately, the anthracnose fungus can also cause death of buds and twigs on sycamore trees. The death of new shoot growth for repeated years can result in a gnarled or crooked branch growth as side shoots take over as the new leader branches. The fungus can survive the dormant season on diseased leaves that have fallen to the ground or on diseased twigs that remain on the tree.
Damage caused by the anthracnose fungus is usually minimal so fungicide use is rarely warranted. Chemical control can be problematic because proper timing can be difficult to predict from year to year and adequate coverage of big trees is difficult.
As hot and dry summer conditions arrive, the fungus is suppressed. New shoots emerge and new leaves appear.
Anthracnose in Oak Trees
Anthracnose on white oak can be common in the spring. Leaf symptoms range from large areas of browning, especially on the leaf margins, to scattered small necrotic spots. The leaves have an overall scorched appearance. The lower branches tend to show the most severe symptoms.
The disease is caused by the fungus Apiognomonia quercina and is favored by rainy spring weather. Oak anthracnose can occur over a wide range of temperatures, but lower temperatures promote the most severe symptom development. Midsummer conditions tend to cause outbreaks to subside.
Acervuli, fruiting structures of the pathogen, eventually become visible on the undersides of leaves, especially in the necrotic areas next to leaf veins.
Twig infections may also occur, causing dieback before the buds open in the spring. The fruiting structures on the dead twigs can provide a source of spores to infect emerging leaves.
Although unsightly, anthracnose is a minor problem on established trees. Basic cultural practices such as mulching, proper watering, and removal of fallen leaves will help maintain tree vigor. Remember, oaks should not be pruned during April, May, or June. Pruning wounds can attract the beetles that spread the oak wilt fungus.
Minimizing Severity and Impact on Tree Health
- Clean up and destroy as many fallen leaves as possible. This will help reduce the overwintering population of anthracnose fungi.
- Prune the tree to remove diseased twigs and branches (primarily for sycamore anthracnose) and to open up the canopy for better air circulation and light penetration.
- Maintain tree vigor with proper watering, fertilization, and other cultural practices such as mulching.
- Select species that are resistant or less susceptible to anthracnose.
- Apply a labeled fungicide when warranted. Research and experience show that fungicide control is rarely warranted because anthracnose usually does not seriously damage tree health and adequate control is seldom achieved.