Mysteries of Our Native Sycamore

After hard freezes in mid-April and May of 2020, you may have wondered if we were ever going to see the full-bodied greenery of our beloved sycamores. Let's look close at the characteristics of this adaptable tree to decipher its success at withstanding cold temperatures and attack by a fungus. 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 dryer conditions further west probably limit the sycamore's natural Eastern North American range. Many of our sycamore trees exhibited this damage in 2020 and have since re-foliated. Although low winter temperatures may also injure the cork cambium of the bark, the overall health of the tree remains unaffected.

The same sycamore tree in May and July of 2020. On the left, the tree on May 27, 2020 has little foliage as a result of late spring frosts. On the right, the tree on July 20, 2020 has pushed out a new flush of foliage. Photo credit: Julianne Schieffer.

However, sycamores still face damage from the naturally occurring fungus Apiognomonia, which causes sycamore anthracnose. The term anthracnose refers to dead areas on leaves, twigs, or fruit. This fungus attacks as young leaves unfurl and forms 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 in the season 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 results 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 promote the disease. Little or no anthracnose will occur if average temperatures during bud break are above 60 degrees. Few trees die from this yearly attack. Anthracnose may weaken a tree, however, making it susceptible to attack by other diseases. Frost damage can both mimic and complicate anthracnose symptoms.

Signs of early anthracnose fungal infection in the dead areas of one leaf and the fruiting bodies already on the dead twig. Photo credit: Julianne Schieffer.

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 giving 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.

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 continue their growth 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.

Each branch node has a stipule (three total) encircling a bud on this twig. Photo credit: Julianne Schieffer

Finally, let's get up close to the fuzz which completely coats new twig growth and all young leaves. The "stellate" or "star shaped" hair, when looked at head on; can lodge into our nose tissues to cause irritation and allergies upon inhalation. However, the hairs at the leaf's margins, seen from their side, reveal that they're more "plumose," or "feather-like,".  Such hairs certainly 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 during the period when it is most susceptible to the anthracnose fungus.

Microscopic view of hair on sycamore leaf. Photo credit: Jim Conrad

Despite the environmental and biotic stresses hitting our sycamores, its long record and effective countermeasures assure us that it will remain one of the fast growing and largest diameters tree species in our area. Small plant features add up to long-lived success!

References

Conrad, J. Excerpts on Sycamore from Naturalist Newsletter. Backyard Nature website. 

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.

Moorman, G.W. 2016. Sycamore Diseases. Penn State Extension website. 

Silvics of North America. Volume 2. Hardwoods. USDA Forest Service Handbook 654.