Brown or black lesions on leaves, stems, flowers, fruits, and other plant parts may be symptoms of anthracnose. But not all anthracnose is created equal. The term anthracnose refers to a group of fungal diseases that can affect a wide range of plant species, trees as well as shrubs, both ornamentals and edibles, and also garden crops. While the symptoms are similar, the fungi that cause the disease are different from host to host.
Here is an overview of some of the most common types of anthracnose.
Anthracnose on Deciduous Trees
Shade trees commonly affected by anthracnose are ash, dogwood, elm, hickory, maple, oak, sycamore, and walnut. The fungi that cause it, mostly from the family Gnomoniaceae, vary depending on the tree species.
Anthracnose can affect the buds of a tree early in the season before it has grown any leaves. When the buds or the tips of twigs die as a result, it might look like frost injury, which can make it tricky to diagnose anthracnose at this stage.
The symptoms of anthracnose are easier to identify once the tree has leafed out. You’ll notice small, circular or irregularly shaped dark or brown dead spots on the leaves, dead leaf margins and tips, and large dead blotches along the leaf veins or in-between the veins.
When the tree is heavily infected early in the season, the leaves may be distorted, shrivel and fall off prematurely. Sometimes the foliage regrows after defoliation. Other symptoms are girdled dead twigs with areas of sunken bark.
To determine whether it’s anthracnose, take a look at the underside of infected leaves with a magnifying glass. You’ll see fungal fruiting structures that protrude like pimples, especially along the leaf veins. There are similar fruiting structures at the tips of dead twigs.
Anthracnose overwinters in infected branches, twigs, and leaves. In the spring, wind carries the pathogens to young leaves and twigs, where it forms new spores. These spores then move by wind or water, splashing to neighboring foliage, infecting it and thus continuing the disease cycle.
Cool spring weather with temperatures between 50 and 55 degrees F is especially conducive to spreading the disease.
Anthracnose on Roses
Sphaceloma rosarum, the fungus that causes anthracnose on roses, is different from the fungi causing tree anthracnose.
A characteristic symptom of the disease are small, reddish-purple spots or lesions on the leaf veins. As time passes, the spots develop thin brown margins. Then they turn gray and the tissue disintegrates, leaving tiny bullet-like holes in the leaves. The leaves also turn yellow, wither, and eventually fall off.
To distinguish anthracnose from black spot, another rose disease that causes defoliation, take a close look at those lesions. Those caused by anthracnose have distinct edges whereas the lesions from black spot have irregular fuzzy edges.
In addition to the leaves, rose canes and stems can also be affected. The fungus produces cankers that girdle the stem, literally choking it to death. Dieback usually starts at the tips of the stems and moves towards the center of the plant.
Climbing roses, wild, and rambler roses, as well as some hybrids and shrubs are reported to be more susceptible to anthracnose.
How to Control Anthracnose on Trees and Roses
The good news is that even when a tree or a rose is severely infected with anthracnose, it will not kill it. But keep in mind that it weakens it and makes it more susceptible to other diseases, frost injury, environmental stress such as drought and extreme temperatures, and insect damage. For these reasons, it is important to control the disease early.
Keep a close eye on your roses. As anthracnose progresses and the lesions turn into those tiny bullet holes, they are easily mistaken for insect damage and possibly treated improperly.
Good sanitation is, as usual, your first line of defense. In the fall, rake and safely destroy all fallen leaves from infected trees and roses. This way the anthracnose spores won’t have a place to overwinter. Remove any infected twigs and cankers and disinfect any tools with a 10 percent bleach solution (one part bleach to nine parts water) between making the cuts to prevent the fungus from spreading onto the same tree, or onto other trees.
Tool sanitation is especially important when you grow roses for cut flowers so make sure you disinfect your tools when moving from one rose to another to prevent the disease from spreading. Safely dispose of any infected plant parts by burning them or throwing them in the trash.
Fungicides with chlorothalonil and copper may be used as a preventative. For trees they are only recommended when the infection is severe and recurs every year, resulting in a lot of twig dieback. The fungicide must be applied to the tree at bud break in early spring and repeated weekly or biweekly until the daily average temperatures are consistently above 60 degrees F. Roses may also be treated with fungicides containing copper, sulfur, or chlorothalonil. Follow the manufacturer’s instructions for frequency and dosage.
As all fungi, anthracnose thrives in humid conditions. While you cannot change the weather, you can ensure good air circulation by leaving ample space between your rose plants, as well as by regular and proper pruning. Both are ways that help the foliage to dry quicker from dew and rainfall. Also, when watering, water only the roots and avoid getting the foliage wet in order to decrease the chance of the fungus spreading.
Anthracnose on Edibles
Anthracnose can also befall garden crops, small fruits, and tropical fruit trees, all of which can considerably decimate the harvest.
Anthracnose on tomatoes, eggplant and peppers is caused by species of the Colletotrichum fungus, most commonly Colletotrichum coccodes.
Tomato anthracnose occurs mainly on overripe fruit. The tomatoes show small, circular, sunken spots, often in concentric rings. As the spots grow larger, they cluster together to form large blotches, which often start to ooze.
Cucumbers, watermelons, certain melons such as honeydew, and sometimes pumpkins can also get anthracnose. In members of the cucumber family it is caused yet by another fungus, Colletotrichum orbiculare.
The fungus can affect the leaves, stems, petioles and fruit of cucurbitae. The symptoms vary and often resemble other foliar diseases such as leaf blight, leaf spot, downy mildew and powdery mildew, which can make it difficult to diagnose. The name of the disease – anthracnose means "coal disease" – can give you clues what to look for: dark spots on leaves, leaf stalks, stems and fruit, oftentimes sunken, that later coalesce.
The symptoms of anthracnose in grapes, caused by the fungus Elsinoe ampelina, start as small, circular reddish spots and can appear on all parts of the plant but are most common on young shoots and grapes. Later these spots develop into sunken lesions that grow together.
Anthracnose on black, purple and red raspberries as well as blackberries, is caused by the fungus Elsinoe veneta. Like in grapes it starts with small, often sunken reddish spots that gradually coalesce and turn grey in the center, which also earned the disease the nickname “gray bark.” The disease results in stunted deformed berries, and the canes often die.
Tropical fruit trees such as mango isn’t spared by anthracnose neither. The fungus Colletotrichum gloeosporioides can affect mango, banana, avocado, papaya, and passion fruit.
The pattern of the disease on mango is similar to anthracnose on other plants. It all begins with the typical small spots that coalesce to larger lesions which then become dead areas on fruits, leaves and flowers. If the fruits don’t drop off before ripening, they have large dark spots that go beyond the surface and lead to rotting of the entire fruit.
How to Control Anthracnose on Edibles
Similar to ornamental trees and roses, the best way to deal with anthracnose on edibles is control and prevention following good gardening practices.
Start out with healthy seeds, seedlings, and plants from a certified reliable source.
As part of a yearly crop rotation in your garden, don’t plant any members of the same crop family, such as tomatoes, peppers, eggplant and potatoes, or cucumbers, melons, watermelons, squash and pumpkin, in the same location for at least two years in a row.
Anthracnose spreads well in humid conditions and moisture so avoid overhead watering and provide good air circulation within the plants and in-between through proper plant spacing, regular weeding, and pruning.
In the fall remove and safely discard any diseased plant material and debris from the garden, around berry and grape plants and fruit trees, making it less likely for the fungus to overwinter.
Because anthracnose has so many different forms depending on the underlying fungus and the host plant, it is best to take a sample of an infected edible plant part to your local Extension Office for a proper diagnosis.