Anthracnose is a group of diseases found on many deciduous and evergreen trees and shrubs; some trees such as sycamore, ash, and evergreen elms can be noticeably blighted. Often called leaf, shoot, or twig blight, anthracnose results from infection by any of several different fungi, including Apiognomonia errabunda, A. veneta, Discula fraxinea, Glomerella sp., Gnomonia sp., and Stegophora ulmea, depending on the tree attacked. Infections on deciduous plants are more severe in areas where prolonged spring rains occur after new growth is produced. Anthracnose fungi need water to be disseminated and infect; they do not spread under dry conditions.
IDENTIFICATION AND DAMAGE
Anthracnose symptoms vary with the plant host, weather, and time of year infection occurs. The fungi affect developing shoots and expanding leaves. Small tan, brown, black, or tarlike spots appear on infected leaves of hosts such as elm or oak. Dead leaf areas may be more irregular on other hosts such as ash. Sycamore anthracnose lesions typically develop along the major leaf veins. If leaves are very young when infected, they may become curled and distorted with only a portion of each leaf dying.
Generally, mature leaves are resistant to infection, but when conditions are favorable, they may become spotted with lesions. Heavily infected leaves fall prematurely throughout the growing season, and sometimes trees are completely defoliated. Early leaf drop is usually followed by production of more leaves. Twigs and branches also may be attacked and killed, resulting in a tree with crooked branches.
On some trees, cankers (infected areas that may or may not be surrounded by callus tissue) are another symptom of anthracnose infection. Cankers develop on twigs, branches, and the trunk, occasionally resulting in girdling and dieback. If defoliation, branch dieback, or cankering does not occur every year, anthracnose will not seriously harm plants. In California, anthracnose rarely causes permanent damage to plants except for elm trees.