Anthracnose of Shade Trees
One of the most common and unsightly diseases of shade trees is anthracnose. Anthracnose is a foliar disease caused by several species of fungi whose spores, when released, infect newly emerging leaves. The disease, which becomes active in spring, can become severe when cool, wet spring weather persists. Unfortunately, trees most likely to be affected are quite common, such as ash, dogwood, elm, hickory, maple, oak, sycamore, and walnut.
The most common symptoms of anthracnose are tan to brown or black blotched areas on leaves which develop along the leaf veins. Frequently, the infected area will expand outward to the leaf margin causing irregular, brown patches and distortion of the leaf.
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. Numerous cankers (localized, injured areas) may occur on stems and branches.
Anthracnose is a more serious infection on plants whose twigs and buds are susceptible, such as sycamore and flowering dogwood. In the spring, spores are dispersed to new shoots and buds, often killing buds before new leaves emerge. Sycamores appear more dead than alive in early spring because of the severe attack from anthracnose. However, they will re-leaf by summer.
Anthracnose is a generic name for a disease caused by several species of fungi in genera including Discula, Apignomonia, and Colletotrichum. During winter, these fungi reside in diseased leaf and stem tissue, and on the ground in fallen leaves. In early spring, infectious spores are produced which are carried by rain and wind to newly emerging leaves. Additional spores are 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 50o and 57o 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.
The severity of anthracnose varies each year with weather; however, 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, fertilize lightly, and water stricken trees to help them recover from severe defoliation.
Spraying with fungicides can help reduce the severity of anthracnose, but by the time injury is apparent, fungicide sprays are usually ineffective. Thus, spraying is generally not warranted. If you choose to spray, timing is critical, and thorough coverage is necessary. For large trees, consult with a certified arborist. Typically, three applications are necessary, beginning in early spring when buds first start to open, and then two additional sprays should be made at 10-14 day intervals.
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