Anthracnose-Twig Canker and Shoot Blight, Apiognomonia (=Gnomonia) veneta (Sacc. & Speg.) Höhn.

Leininger, T.D; Solomon, J.D.; Wilson, A. Dan; Schiff, N.M. 1999. A Guide to Major Insects, Diseases, Air Pollution Injury, and Chemical Injury of Sycamore. Gen. Tech. Rep. SRS-28. Asheville, NC: USDA Forest Service, Southern Research Station. 44 p.

Importance.-Anthracnose is one of the most important diseases of sycamores and Platanus species worldwide. It occurs in sycamore trees throughout North America. Severe defoliation in consecutive years can weaken trees, making them susceptible to attacks from other agents and even causing death.

Identifying the Disease.-This fungus causes cankers on branches, blighting of twigs and shoots, and blighting of leaves, which can, in some years, cause near total defoliation. Newly emerged leaves and young shoots are killed in early spring, often following cold weather, which causes the symptoms to be mistaken for frost damage (fig. 30a). Repeated shoot dieback can cause an angular branch pattern in older trees as shoots from lateral buds replace dead terminals. Cankers (3 to 8 cm long) form on small branches (fig. 30b), and those that do not girdle a branch in a year have callous ridges at the margins.

Figure 30a. - Shoot blight, caused by sycamore anthracnose, can be mistaken for frost damage.

Identifying the Fungus.-Colorless, ascospores, which have two unequal cells, are released in spring from black perithecia on fallen leaves. Conidia are produced in small cups on dead leaves in spring and in small (0.5 to 0.9 mm) black pycnidia in the bark of twigs.

Biology.-Cankers form in the dormant season when temperatures are warm enough for fungal growth. Within 2 weeks after bud break, temperatures above 15º C favor healthy shoot development, whereas temperatures below 12º C slow shoot growth and favor the killing of shoots and twigs. The anthracnose fungus probably enters sycamore trees through their leaves; canker formation and shoot blight occur in subsequent years.

Figure 30b. - Branch canker, with a callus ridge, from a sycamore anthracnose infection.

Control.-Wet conditions favor foliar infections. Sycamore in plantations may be protected by increasing the space between rows to ensure air movement. Landscape trees may be protected from initial infections by applying a contact fungicide when buds begin to swell and, again, 10 to 14 days later. Injections of systemic fungicides offer protection as well (fig. 22b). Disease resistant varieties should be planted if possible. There is no practical control for this disease in forests.