You may have noticed that sycamore trees are looking more stressed this spring than usual. Two factors are involved—a disease and the weather.
Many sycamores are infected with anthracnose, as is expected in Illinois every spring. The intensity and duration of infection is dependant on the weather. As mentioned in issue no. 4 of this newsletter, anthracnose diseases of shade trees require cool, wet conditions for infection, especially during the 2 weeks following bud break, because succulent new plant growth is most susceptible. Many trees have slower than normal growth this spring because of unseasonably cool temperatures, so they have been vulnerable to infection for a longer window of time.
Sycamore anthracnose actually appears in three rounds (phases). The first is the canker phase, which begins when the host is mostly dormant. Cankers develop most when the host is least able to fight back. In the spring, we see the shoot blight phase. Cankers may kill small twigs at this time, or spores may infect and kill new shoots. Direct infection of new leaves is the leaf blight phase. Scientists have shown that when the average temperature during the 2 weeks after bud break is less than 55?F, anthracnose will be severe. When the average temperature in that time frame is 55? to 60?F, the disease will be less severe. At temperatures greater than 60?F (average), there should be no injury. Looking at temperatures around the state, one can see that we have been in the serious to less serious range. Not only were conditions ideal for initial infection, but continued cool, wet weather has allowed secondary leaf infection. The image shows a sycamore branch from the South Farm on the University of Illinois campus, not far from the Plant Clinic. Cankers, shoot blight, and leaf blight of anthracnose are present on this branch and on all of the branches in that tree.
Sycamores are tough trees and will recover with warmer temperatures. Still, infections cause dieback and cankers that cause unsightly trees with lot of twig drop during the season. I think this disease is one reason sycamores are considered by many to be messy trees.
We do not usually recommend fungicides for sycamore anthracnose. If you like the tree and want to plant a similar, less susceptible species, consider one of the resistant plane trees. Oriental plane tree is resistant to anthracnose. London plane trees vary in their resistance, so be sure you are buying a resistant hybrid. Vegetative propagation of a resistant clone is the safest way to obtain a London plane with resistance to anthracnose.