Foliar Diseases
Sycamore Anthracnose
Apiognomonia veneta (Sacc. & Speg.) Höhn.

Hosts:  Arizona sycamore

Figure 179. Figure 179. Leaf blight caused by sycamore anthracnose

Symptoms/signs:  Sycamore anthracnose has a range of symptoms corresponding to the three phases of this disease. The symptoms include: cankers on buds and twigs; shoot blight following a period of cold spring weather; and leaf blight from direct infection of leaves. Foliar lesions characteristically extend along the veins and involve interveinal tissue. Sometimes large irregular marginal lesions develop.

Biology:  Apiognomonia veneta enters twigs via petioles during the growing season and remains quiescent until host dormancy, when it colonizes and kills bark and cambium. In the early spring, spores produced in these cankers infect new shoots and leaves. Shoot blight, the second phase of the disease, involves the rapid death of expanding shoots and leaves. This phase tends to develop suddenly during a period of cold spring weather. The third and final phase of sycamore anthracnose is leaf blight, which results from direct infection of leaves. It starts out most severe on low branches, and intensifies and spreads upward during wet seasons, causing premature leaf drop. Leaves are most susceptible during the first few weeks of growth. Hyphae of the fungus often grow down petioles into the twigs, setting the stage for the next year’s damage.

Figure 180. Figure 180. Chronic infection by sycamore anthracnose results in repeated dieback of shoots.

Effects:  Repeated twig dieback alters the form of sycamores in two ways. First, when the terminal twig on a branch is killed and a lateral takes over as the new leader. Since infection takes place repeatedly during the life of a susceptible tree, the branch axis changes direction again and again, and crooked branches result. The second altered form is the development of a cluster of twigs around a common point on a branch because of the repeated killing of terminals.

Similar Insects and Diseases:  Sycamore anthracnose may occasionally be confused with injury by late spring frost. However, frost damage may affect several species in the same area and Apiognomonia veneta affects only sycamore.

References:  93

Figure 181. Figure 181. Severe defoliation by sycamore anthracnose.
Figure 182. Figure 182. Infected trees refoliate but branch dieback is evident.