Green Horizons

Every spring and early summer I get calls about "sick and dying" sycamore trees. This year was no exception. The culprit is anthracnose, a common name given to a group of fungal pathogens which cause dark, usually sunken lesions on the leaves.

Cankers on young twigs are another sign of sycamore anthracnose.

The sycamore anthracnose fungi attack sycamore trees in early spring, causing a rapid wilt of newly emerging leaves. New twig growth may be killed back 8 to 10 inches. Larger, more mature leaves develop a brown growth along the main veins. Infected leaves often curl and eventually fall, littering the ground.

Cankers often form on the twigs and branches at the base of blighted leaf clusters. These cankers become active the following spring and produce spores that reinfect the tree and spread the disease to other sycamores in the area. Eventually, these cankers girdle and kill the twig. Repeated annual killing of twigs results in clusters of old dead twigs and live branches called "witches’ brooms."

Weather determines the severity of anthracnose. Frequent rains and cool temperatures promote the disease. If the average daily temperature during the two-week period following emergence of the first leaves is below 55 degrees, the shoot-blight phase of the disease will be serious. Disease intensity decreases as the average temperature increases from 55 to 60 degrees. Little or no anthracnose will occur if average temperatures are above 60 degrees.

The American sycamore is much more susceptible to anthracnose than its related cousins, the London and Oriental plane tree.

Characteristic browning and curling of sycamore leaves infected with anthracnose.

Funigicidal sprays are usually not recommended because (1) the treatment must be re-applied every 7 to 14 days until conditions for disease development are no longer favorable and (2) they are not practical for large trees.

A second crop of leaves is normally produced from mid- June into July after the loss of the first set. By that time, the average daily temperature is above the 60-degree threshold for disease development. Only in the case of repeated defoliations of both leaf flushes over a couple of years will the tree be weakened to the point of death.

Now that you know the facts, relax. Your sick sycamores will recover.