Symptoms

The most common symptoms of anthracnose are tan, brown or black blotched areas on leaves. These blotches may develop along the leaf veins or along the margins of the leaves. In some cases, the leaves may become distorted.

In severe infections, young leaves may die and fall off soon after infection, but most trees re-leaf by mid-summer. Tiny fungal masses can be seen through a magnifying lens on the underside of the leaf. Leaf injury is most noticeable on the lower branches.

Anthracnose is a more serious infection on plants whose twigs and buds are susceptible, such as sycamore. In the spring, spores are dispersed to new shoots and buds, often killing buds before new leaves emerge. Sycamores may lose all their young leaves due to a severe attack from anthracnose. However, they will re-leaf by summer. Since anthracnose will damage buds on sycamore, proliferation of shoots, called witches’ brooms may form along the branches.

Disease Cycle

Anthracnose is a generic name for a disease caused by several species of fungi in genera including Discula, Apiognomonia, Gloeosporium and Colletotrichum. During winter, these fungi reside in diseased leaf, bud and stem tissue, and on the ground in fallen leaves. In early spring, spores of the fungus are carried by rain and wind to newly emerging leaves. Additional spores may be produced from recently infected leaf tissue, causing further spread of the disease during the growing season.

Temperature and rainfall are the two key factors determining the severity of anthracnose. Mean daily temperatures (the average of the maximum and minimum temperatures) between 50°F and 57°F during bud break and early leaf development are crucial for spore production and infection to occur. Higher or lower average temperatures during this period will reduce disease severity. Frequent rain aids the dispersal of spores and also allows for a greater number of infections.

Management

Cultural Management

The severity of anthracnose varies each year with weather. Even in those years when the disease is severe, anthracnose will not result in tree death. Most trees are able to withstand infection and push out a new crop of leaves by mid-June. Therefore, the most practical control is good sanitation practices to keep trees healthy. Discard fallen leaves to reduce the potential for reinfection, prune infected twigs and branches (with cankers) back to healthy wood, and water affected trees to help them recover from severe defoliation.

Chemical Management 

Spraying for anthracnose is often not warranted. Spraying with fungicides can help reduce the severity of anthracnose, but by the time injury is apparent, fungicide sprays are usually ineffective. If fungicide sprays are used, timing is critical, and thorough coverage is necessary. For large trees, consult with a certified arborist.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).