What Is It?
Anthracnose is a group of diseases that cause dark, sunken lesions on leaves, stems, flowers, and fruits. In our area the most common hosts are dogwood, sycamore, ash, and maple trees. Anthracnose symptoms vary with the plant host, weather, and the time of year infection occurs. Cool weather (50° to 55° F, average daily temperature) during leaf bud break and emergence greatly favors disease development whether it is wet or dry. Although wet weather tends to favor this disease, temperature is the most important factor. If daily temperatures average above 60° F at bud break, anthracnose is usually not severe. 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 can be more irregular on other hosts such as ash, while sycamore anthracnose lesions typically develop along major leaf veins. If leaves are very young when infected, they can become curled and distorted with only a portion of each leaf dying.
Why Don’t We Like It?
Generally, mature leaves are resistant to infection, but when conditions are favorable, spotty lesions can occur. Heavily infected leaves fall prematurely throughout the growing season, and sometimes trees become completely defoliated. New leaf growth usually occurs after an early drop.
Anthracnose also can attack and kill twigs and branches. On some trees, cankers—sunken, infected areas with swollen edges—develop on twigs, branches, and the trunk, occasionally resulting in girdling (the destruction of the nutrient and water conducting tissues all the way around a branch or twig) and dieback. Regrowth from lateral buds can give branches a gnarled or crooked appearance. If defoliation, branch dieback, or cankering doesn’t occur every year, anthracnose won’t seriously harm plants.
What Can We Do?
- Prune out and destroy dead branches and twigs.
- Rake and destroy fallen leaves around susceptible trees that have branches close to the ground.
- When a specimen tree must be protected, fungicides can be applied. Spraying must begin at bud break and be repeated on a weekly or biweekly schedule until the weather warms, daily temperatures averaging above 60°F.
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