Anthracnose, even the name sounds bad! This fungal disease affects the leaves and new shoot tips of trees and shrubs. It can severely weaken the plant if left unmanaged and is just plain ugly! In some cases where a plant if left to suffer with the disease for many years, the reduced vigor caused by Anthracnose can leave the tree open to other more deadly infections.
The greatest impact of anthracnose is in the urban environment. It can result in reduced property value because of the decline or death of shade trees that become infected. Beautiful landscape trees such as flowering dogwood, sycamore, oaks, ash trees, and walnuts are just some of the species affected by this disease.
A brief history of the Anthracnose fungus
If you break down the meaning of the word, “anthra” means coal or carbon while “noso” means disease. The action of anthracnose on a leaf kills part of or the whole leaf, leaving it black as coal. That’s my theory on the reason coal is in the name.
Anthracnose, one of many tree diseases, was first discovered in the 19th century when unsightly oval-shaped black legions were found on bean plants growing in Germany. The disease was identified as a fungus and beans growing all over the word were suffering from this awful infection. In fact, scientist discovered there were many strains of this fungus, each with a different effect on particular varieties of bean.
Flash forward to the 21st century and you’ll find anthracnose infects much more than bean plants. It is found on trees, shrubs, grape vines, and many vegetable and flower crops. It is a widespread problem because the fungus that causes it moves around so easily. Learning to identify anthracnose as well as how the disease spreads and thrives will help you prevent it from infecting your landscape.
What is Anthracnose?
As mentioned above, Anthracnose is a fungal disease that causes dark, sunken spots on leaves, stems, flower, and/or fruits. It also attacks developing shoots (i.e. new growth) and expanding leaves. Anthracnose is actually a general term for a variety of diseases that affect plants in similar ways. Each strain of anthracnose-causing fungi will infect a particular kind of plant while leaving many other alone, or causing them minimal damage. It is what scientists call “host-specific”- they like what they like. I shouldn’t personify a fungus, it’s just a fungus, or is it?
Anthracnose Fungus Life Cycle
Fungi spread by releasing millions of spores into the environment. Spores are like seeds; they are the beginning stage of a fungus. These spores hope to land on a moist green leaf or young twig where they can start the next phase of their life cycle. Once on a tender green surface they germinate and produce hyphae that penetrate the leaf’s cell walls. More spores are produced in tiny, sunken, saucer-shaped fruiting bodies known as acervuli.
Fungi produce millions of spores that are sent all over the place when water splashes on them, by the wind, or on the bodies of insects. It spreads particularly quick during a rainy season.
At the end of the growing season, when temperatures drop, anthracnose can over-winter in infected leaves that have fallen from the tree. It therefore remains a presence in your landscape if diseased leaves and twigs are not disposed of. The best method of disposal is in a trashcan or a burn pile.
How to Identify Anthracnose
On leaves, it generally appears first as small irregular, yellow or brown oval-shaped spots. These are usually located close to the leaf veins. These spots darken as they age and may also expand, covering the entire leaf. It causes wilting, withering, and plant tissue death. On young leaves, the infection may cause them to look distorted, cupped or curled. A severe infection can result in leaves falling from the tree in spring.
The spots or blobs caused by a fungal disease such as anthracnose are different from those caused by a bacterial infection. Fungal infections cause irregularly shaped spots while bacterial spots are consistently round. This is an important difference because a treatment for bacteria will not be effective against a fungal infection. An arborist specializing in tree health care will be able to identify the problem-causing agent in your tree and help you make a plan to manage it.
On green twigs, the infection can be small orange brown blisters or may form a brown band that encircles the young twig. This girdles the twig and results in death. Those particular symptoms are most common on oak and ironwood trees.
The tree disease is most often severe on the lower and inner branches of the tree but may progress up through the canopy. The disease is most common during cool wet springs.
Oak Anthracnose & Sycamore Anthracnose
Infection is these trees begins with the leaves. The fungus grows through the veins, down the leaf stem, and into the branch. The fungus survives through the winter in cankers. There is waits until conditions are favorable to infect the tender new growth of emerging spring leaves. The impact of this disease is much worse during years with heavy rainfall and low temperatures. Trees are not particularly healthy can also suffer worse than those who are strong and healthy. For images of anthracnose infected sycamore trees click here.
Dogwood anthracnose was discovered fairly recently (1987). Millions of flowering dogwoods were killed and disfigured by the disease. This particular strain of anthracnose-causing fungi often develops a purple border between the dead infected area of the leaf and the healthy tissues. In dogwoods, dieback of twigs and branches occurs in the lower crown and many small branches shoot from the main stem of the tree. This is called epicormic branching and is a tree’s reaction to stress.
In dogwood anthracnose, the fungus causes cankers that can kill the tree. Large trees often die 3 to 4 years after the first symptoms appear. Young trees usually die the same year. For images of dogwood anthracnose click here.
It is important to remove diseased limbs and any leaves that have fallen. The disease can overwinter on fallen leaves. Removing them will reduce the risk of a new infection the following spring. Prune the tree or shrub to remove infected twigs, increase light penetration, and improve air circulation throughout the canopy.
If a tree is suffering from anthracnose try and reduce any other stressors including drought, and improper nutrition. A soil test can identify any soil amendments that are needed for optimal health of your tree.
Fungicides are only necessary if the tree has been completely defoliated several years in a row. Applying a fungicide will protect leaves before the fungi infect the tree. Therefore the timing of application should be before symptoms appear. For large trees, help from an arborist specializing in tree health care who can apply the fungicide with high-pressure spray equipment is needed in order to cover the whole tree.
How do I prevent an infection of Anthracnose?
Airflow and drip irrigation! Those are two important terms you should know when planning and managing your landscape. Good airflow means that plants are not too close together and overlapping. Leaf surfaces will dry more quickly when there is good airflow, reducing the risk of anthracnose-causing fungi to find a place to land and grow.
Using irrigation that delivers water to the base of the plant, where the roots are, is much better than a sprinkler system that sprays water onto the leaves of the plant. Again, wet leaves are far more susceptible to an anthracnose infection than dry ones.
Also, planting the right tree in the right place can go a long way to creating a healthy landscape. For example, plant flowering dogwood in a shady spot where they are protected from the afternoon sun. Trees such as sycamores will grow to become very large. Plant these guys in a place where they have room to grow. Learning about the particular needs of the plants in your landscape can be fun and will help you improve the quality and beauty of your yard. It will also help you prevent diseases such as anthracnose and to identify it early so that steps can be taken to help prevent its spread.