Anthracnose on Plants

Symptoms

Dark lesions on leaves are the sign to look for first. Anthracnose is a loose term for a group of related fungal diseases, and these lesions are what they typically cause. The lesions are described as dark, and the damage on twigs is described as sunken.

Young leaves show a different sign. They curl and become distorted, and only part of each leaf dies, so part of the leaf stays alive. Twigs and stems can show sunken lesions and cankers.

Defoliation runs from the bottom of the plant upward, and a rim of undamaged foliage is left at the top. Premature leaf drop can occur throughout the growing season.

In short, watch for dark lesions on leaves, curled and distorted young leaves, and early leaf drop. Also watch for sunken lesions and cankers on twigs and stems. These symptoms are described on hosts such as ash, maple and willow.

Causes

The name anthracnose covers a group of related fungal diseases. These fungi are not tied to one plant, since the group affects hosts such as ash, maple and willow. Weather is the first cause to look at.

The disease is worse in cool, wet weather. Prolonged spring rain after new growth appears is another named cause. Rain after new growth appears is the timing detail to remember.

Irrigation that splashes foliage also makes the disease worse. Irrigation systems that wet leaves are the kind to avoid, and splashing is the reason the disease is worse. Weather and watering both put water on the plant, which is the common thread in these causes.

Taken together, the causes listed are a group of fungal diseases, cool and wet weather, splashing irrigation and prolonged spring rain after new growth. Dark lesions on leaves are what these fungal diseases typically cause. If your plant is one of the hosts named and the weather has been cool and wet, anthracnose is a reasonable suspect.

Diagnosis

The facts on this page name no look-alike problems, so the best test is the pattern of symptoms and conditions. Start with the leaves and look for dark lesions. Anthracnose is a group of related fungal diseases, so a fungal problem is what these lesions point to.

Next, look at young leaves for curling and distortion where only part of each leaf is dying. Then check the twigs and stems for sunken lesions and cankers. Look at the whole plant for the direction of leaf loss. Defoliation from the bottom upward, with a rim of undamaged foliage at the top, is one of the signs described for anthracnose.

Then ask whether the weather has been cool and wet, and whether irrigation has been splashing the foliage. Prolonged spring rain after new growth appears is another condition that fits. Several signs together, such as leaf lesions, twig cankers and wet weather, fit anthracnose better than a single sign alone.

Check the host too. Ash, chinese elm, dogwood, maple, oak, sycamore and willow are the named hosts.

Treatments

The cultural steps come first. For new plantings, choose cultivars that are resistant to anthracnose-causing fungi. The advice on resistant cultivars is for new plantings. Space plants far enough apart to maximize air circulation and increase sunlight. Spacing is aimed at air circulation and sunlight.

During fall or winter, prune and destroy or bury infected leaves, twigs and branches. During the growing season and in fall, rake up fallen leaves and twigs and dispose of them. Raking up fallen material is a separate step from pruning infected parts. Avoid irrigation systems that wet leaves.

On London plane trees, pollarding can be used to help control anthracnose. Pollarding is mentioned for London plane trees specifically, not as a general method.

Chemical products are the second type of treatment on the list, and the first type is cultural. Fungicide sprays protect only healthy tissue, and they do not eradicate existing infections. These fungicides are listed for protection, not for curing.

The active ingredients listed are chlorothalonil, copper diammonia diacetate, propiconazole and thiophanate-methyl. Chlorothalonil is a protective fungicide spray. Copper diammonia diacetate is a copper spray fungicide. Propiconazole is also a fungicide spray. Thiophanate-methyl is a systemic fungicide, and it is for professional use only.

The product label is the legal instruction, so follow it for the crop and site being treated, and use it for any rate or timing.

Prevention

Prevention is mostly about the conditions around the plant. Keep irrigation off the leaves, because irrigation that splashes foliage makes the disease worse. Give plants enough room so that air can circulate and sunlight can reach them.

When you plant something new, choose cultivars resistant to anthracnose-causing fungi. Clear fallen leaves and twigs through the growing season and in fall. Prune and destroy or bury infected leaves, twigs and branches in fall or winter. Pollarding is used on London plane trees to help control the disease.

Cool, wet weather and prolonged spring rain cannot be changed, but they tell you when to watch closely. Watch young leaves after new growth appears, since prolonged spring rain at that stage is a named cause. Plants such as ash, maple, oak and willow are named hosts, so they deserve the closest watch.

Plants most often affected

Anthracnose is described on ash, chinese elm, dogwood, maple, oak, sycamore and willow. The name covers a group of related fungal diseases, and they typically cause dark lesions on leaves. The facts on this page name these hosts and no others. Look first for dark lesions on the leaves of these plants.

• American Sycamore
• Asian Pigeonwings
• Black Elderberry
• Black Walnut
• Boxelder
• Bur Oak
• Camellia
• Cantaloupe
• Cayenne Pepper
• Edible Banana
• English Ivy
• English Oak
• English Walnut
• Garden Lettuce
• Garden Tomato
• Highbush Blueberry
• Japanese Maple
• Mango
• Monstera
• Nightblooming Cactus
• Norway Maple
• Ohio Buckeye
• Purple Coneflower
• Purple Granadilla
• Sapodilla
• Silver Maple
• Snake Plant
• Southern Catalpa
• Strawberry Tree
• Sugar Maple
• Tuberose
• White Oak