This mature sycamore has not fully leafed out as of May 27, 2020. Photo credit: Julianne Schieffer
Mysteries of our native Sycamore Resilience
Authored by: Julianne Schieffer
The Pennsylvania State University
Agricultural Sciences
You may have wondered if we would ever see the full-bodied greenery of our beloved sycamore. Let’s look closely at the characteristics of this adaptable tree to decipher its success at withstanding cold temperatures and attack by a fungus.
Remember the hard freezes of this past winter? Since late frosts tend to kill sycamore buds, the damaged trees characteristically have long dead twigs with bushy masses of leaves around their bases by midsummer. Frosts and low temperatures in the North, and drier conditions further west, probably limit the sycamore’s natural Eastern North American range. Many of our sycamore trees exhibited this damage in the past and have since re-foliated. Although low winter temperatures may also injure the cork cambium of the bark, the tree's overall health remains unaffected.
Signs of early anthracnose fungal infection include the dead areas of one leaf and the fruiting bodies already on the dead twig. Photo credit: Julianne Schieffer
However, sycamores still face damage from the naturally occurring fungus Apiognomonia, which causes sycamore anthracnose. Anthracnose refers to dead areas on leaves, twigs, or fruit. This fungus attacks as young leaves unfurl, forming large, irregularly shaped dead areas along the leaf margins and between the veins. The ends of twigs may be killed back 8 to 10 inches. Cankers or dead areas may develop on the tree trunk and main branches. Affected leaves fall early, only to be replaced by new leaves mid-season. Killing buds, young shoots, and leaves stimulates new bud formation and more bud death, and twigs result in a “witches' broom” proliferation of branch ends and an overall contorted branching pattern across the tree canopy.
Weather like frequent rains and cool temperatures promotes the disease. Little or no anthracnose will occur if average temperatures during bud break are above 60 degrees.
Each branch node has a stipule (three total) encircling a bud on this twig. Photo credit: Julianne Schieffer
Few trees die from this yearly attack. Anthracnose may weaken a tree, making it susceptible to attack by other diseases. Frost damage can both mimic and complicate anthracnose symptoms.
How does the sycamore tolerate these stresses that thwart healthy growth? One way is the presence of many dormant buds beneath the bark. As a bud or twig dies back, other buds become released from their dormancy to take over, thereby creating that witches’ broom or heavily twiggy appearance. This becomes a common identification feature for the species. Or, in the case of heavy flooding along a waterway, scouring vegetation from the banks and leaves off trees, a sycamore can rely on its ability to re-foliate along its entire living surface to continue to survive.
Microscopic view of hair on sycamore leaf. Photo credit: Jim Conrad
Another unique feature is how the base of a leaf stem encircles next year’s bud until the leaf drops, thereby protecting it from infection and bad weather until fall. The tiny buds then grow in spring as another interesting protective leaf feature takes over.
After bud break, you may have also noticed that a sycamore twig develops peculiar little modified leaves called stipules encircling its buds or at twig junctions. These stipules protect the newly forming buds from damage. Another tree with ancient lineage, the tulip poplar, employs the same tactic but loses its stipules after the leaf unfolds. Imagine the stipule as a little umbrella shielding water or wind, containing the fungal spores of the fungal invader from the bud.
However, the hairs at the leaf's margins, seen from their side, reveal that they're more "plumose," or "feather-like".Such hairs protect the developing leaf's delicate tissue from water evaporation, intense sunlight, temperature extremes, and physical damage.
Surely an insect would think twice about chewing into that hair or finding a good foothold. But how about a fungal spore gaining access to the actual leaf surface? This hair remains on the sycamore when it is most susceptible to the anthracnose fungus.
Citations
Hartman, J. 2001. Sycamore and planetree diseases. Pages 355-359, in: Jones, R.K. and Benson D.M. eds. Diseases of Woody Ornamental and Trees in Nurseries. APS Press, St. Paul, MN.
Silvics of North America. Volume 2. Hardwoods. USDA Forest Service Handbook 654.
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