Common Tree Diseases: Types of Anthracnose
Anthracnose is a general term used to describe various diseases that occur on many deciduous and evergreen tree species. The following examines the most common types of anthracnose, and the symptoms associated with each.
Causal Agents: Discula Fraxinea
Hosts: Fraxinus (Black and white ash are the most susceptible; green ash may be impacted, but it is generally more resistant to infection)
Symptoms: Symptoms develop on newly expanding shoots and leaves in spring. Brown lesions form on infected leaves. At first, they appear as moist spots on the leaf’s surface. As the infection progresses, leaves can become distorted, and drop prematurely. Branches that have shed their leaves typically produce new shoots and leaves by mid-summer.
Causal Agents: Kabatiella apocrypta
Hosts: Acer (Japanese, Norway, Sugar, Red, Silver, and Sycamore Maple)
Symptoms: Symptoms of maple anthracnose vary by species. On Norway and sycamore maples, narrow streaks develop along the veins of leaves, causing them to turn a purple to brown color. Large, brown patches form between the veins on sugar maple leaves. These patches sometimes appear as small spots between the veins before enlarging. Symptoms on Japanese maples appear as light tan spots in the leaf, or as tan areas along the margins of the leaf. Silver maples develop spotted or curled leaves in late spring and early summer. Red maples form angular shaped spots along the veins of their leaves. On all species of maple, infected leaves will develop a brittle, paper-like texture, rendering them prone to ripping or tearing. Infected maples may experience significant leaf drop in late spring. Affected trees will generally re-foliate by mid-summer.
Causal Agents: Discula umbrinella
Hosts: Fagus (American and European Beech)
Symptoms: Early symptoms appear as irregular, brown lesions on or between the leaf veins. As the infection progresses, the necrotic tissue spreads to all interveinal portions of the leaf, with large sections of the leaf becoming necrotic. Infected leaves will typically drop from the tree.
On beech trees, symptoms of anthracnose can resemble leaf scorch, an abiotic disorder associated with heat and drought. With anthracnose, the necrotic tissue forms within the inner portion of the leaf, and expands outward. Leaf scorch causes necrotic tissue to appear along the margins of the leaf first, before expanding inward.
Causal Agents: Discula platani
Hosts: Platanus (Sycamore and London plane; London plane is generally more resistant to infection)
Symptoms: Sycamore is highly susceptible to anthracnose. Sycamore anthracnose occurs in three phases: twig and branch cankers, shoot blight, and leaf blight. Each phase exhibits a different set of symptoms. In the first phase, the fungus overwinters in the tree’s twigs and buds. This results in the formation of cankers on the twigs and branches, as well as chronic bud death. During the second phase, new shoots are killed by the fungus as they emerge. This causes the tree’s canopy to become sparse, with little new growth. This symptom is most noticeable in late spring, when tree growth is generally more pronounced. During the third phase, newly expanding leaves are infected and killed. Leaves are most susceptible to infection during the first few weeks of growth. Lesions appear on the veins of infected leaves, often enlarging to v-shaped, necrotic areas between the veins. As the infection progresses, the necrotic tissue expands across the leaves, and into the petioles. Severe infection can result in significant leaf drop in late spring or early summer. Most trees will re-foliate with healthy leaves by mid-summer.
Causal Agents: Discula quercina
Hosts: Quercus (White, black, pin, burr, and scarlet oak)
Symptoms: White oak is the most susceptible of all oak species. The most common symptom on oaks is a blighting of newly expanding shoots, and emerging leaves. Necrotic lesions develop on the leaves, often forming along the veins or at the margins of the leaves. Lesions may also develop on petioles and stems as the infection spreads. Severe infections can cause leaves to become distorted. This often results in premature leaf drop by late spring or early summer. Affected oaks typically re-foliate by late summer.
Causal Agents: Gnomonia leptostyla
Hosts: Juglans (Eastern Black Walnut, Butternut, California, and Rinds Walnut)
Symptoms: Eastern black walnut is the most susceptible of all walnut species. Dark brown or black lesions appear on infected leaves, gradually becoming more numerous before coalescing into large necrotic spots. Infected leaves tend to fall prematurely, although in some instances they may remain attached to the tree. Light brown lesions may develop on shoots. This eventually leads to the formation of dead, sunken areas on the affected shoots. Walnut anthracnose influences the quality and growth habit of nuts. Sunken, necrotic spots appear on the husks of infected nuts, often causing them to drop prematurely. Infected nuts will form dark, shriveled meat.