A variety of things may cause your trees to suffer from poor health, ranging from insects to severe weather to improper pruning practices. But diseases are some of the most significant threats, as some may cause your tree to decline or die in a matter of weeks or months.
Because some of these diseases are quite serious, it is vital that homeowners remain vigilant and inspect their trees routinely for any signs of trouble. This will allow you to have a certified arborist initiate treatment quickly or, in those cases in which a treatment is not possible, have the tree removed to prevent the further spread of disease.
7 Common Tree Diseases Michigan Homeowners Need to Know
There are countless diseases that affect trees in Michigan and the rest of the upper Midwest, but the following seven are among the most noteworthy. Try to familiarize yourself with the signs and symptoms of each, paying particular attention to the ones that are likely to affect the tree species present in your yard.
As you’ll notice, many of these diseases are caused by invasive species, which are not native to our state.
1. Oak Wilt
A fungal disease caused by Bretziella fagacearum, oak wilt can strike fear into the heart of just about any tree lover, given that it can kill some oak trees in as little as four weeks. The disease is present throughout most of the Midwest, as well as parts of Texas, but no one is sure exactly where the pathogen came from or when it arrived.
Bretziella fagacearum spreads via beetles that feed on oaks and through root grafts. This is part of the reason that you should never prune oak trees between April and July, as the wounds may attract beetles and promote the spread of the disease. Additionally, trenching to sever existing root connections is generally recommended before having trees with oak wilt removed.
All oaks are susceptible to the disease, but oak trees in the red oak group are at the highest risk. This includes the northern red oak (Quercus rubra), black oak (Q. velutina), and northern pin oak (Q. ellipsoidalis), among others.
The fungus kills oaks by clogging the vascular tissue of the trees and triggering the tree’s defenses, which clogs the vascular tissues even further. This prevents the tree from drawing water up into the canopy and leads to its eventual decline. There is no effective cure for affected trees, but systemic fungicides are often helpful at preventing healthy trees from becoming infected. They may also help to prolong the life of infected white oaks (though they will not cure them).
The first symptoms of oak wilt are usually dieback in the canopy, with entire branches becoming covered in dead, brown leaves. In some cases, the tree’s bark of a tree may also split, due to the internal pressure fungal mats create.
2. Anthracnose
Anthracnose is a catch-all term that is applied to trees exhibiting a similar set of symptoms, rather than a specific causal agent. In fact, the disease can be caused by an array of different fungi, which often attack a specific species or group of species. Oaks (Quercus spp.), sycamores (Platanus spp.), ashes (Fraxinus spp.), maples (Acer spp.), and walnuts (Juglans spp.) are among the most common victims, but many other tree species are susceptible.
Primarily a foliar disease, anthracnose causes a variety of different symptoms, which vary slightly from species to species. However, most affected trees will exhibit some type of leaf damage, ranging from spots to broad areas of discoloration to curling. Twig dieback and twig cankers may also occur in some trees.
Anthracnose is spread via fungal spores and is often most prevalent following cool, wet springs. It rarely kills trees outright, but it can weaken them, making them vulnerable to secondary pathogens and reducing the amount of shade and beauty they provide your yard. Trees that are repeatedly attacked over consecutive years are most at risk of serious consequences.
Fungal treatments can be administered, but they are rarely necessary. Instead, badly affected portions of the tree may need to be pruned, and any leaves on the ground should be raked up carefully and burned or otherwise disposed of to prevent further spread. Fertilization may be recommended in some cases to support the tree’s vigor and bolster its defenses.
3. Apple Scab
Apple scab is a disease that primarily afflicts fruit trees in the rose family (Rosaceae). In Michigan, crabapple trees (Malus spp.) are the most frequently affected.
Apple scab is a foliar disease caused by the fungus Venturia inaequalis. As the fungus takes hold, it begins causing brown lesions (which are often described as appearing “velvety”) to appear on the leaves. By mid-summer, most of the affected leaves will begin turning yellow and falling off the tree, with some trees experiencing complete defoliation.
Apple scab rarely kills crabapple trees outright, though it can cause them to drop a significant number of leaves. However, if this happens over the course of several years (as it often does), the trees will become stressed. Eventually, this can cause them to decline and make them vulnerable to secondary threats.
As with anthracnose, apple scab outbreaks are most common following cool, wet spring weather. Repeated applications of a spray-on fungicide can be useful for controlling apple scab, and properly collecting and disposing of shed leaves will also help. Nevertheless, the fungus is ubiquitous and difficult to eradicate entirely.
4. Black Knot
Black knot is another fungal disease that affects Michigan trees (sensing a pattern, yet?). Caused by a fungus named Apiosporina morbosa, black knot primarily attacks trees of the genus Prunus — especially plums and cherries.
An aptly named disease, black knot causes large, swollen, dark-colored knots to form on a tree’s twigs, branches, or trunk. However, the knots initially present as olive green spots, and they often start at leaf bases. Once fully developed, the knots will begin producing and releasing spores, which will affect other portions of the same tree or entirely different trees during wet weather.
Black knot can be treated, though the process typically extends across several years. The first step in treating the disease is having a professional arborist carefully remove the knots (and a border area of uninfected tissue). This is best conducted in the winter, while the trees are dormant. Then, in the spring, fungicides can be applied to the tree.
5. Dutch Elm Disease
An invasive disease that first reached the U.S. in the 1930s, Dutch elm disease attacks most North American and European elm species, but Asian elms — such as the Siberian (Ulmus pumila) and lacebark elm (Ulmus parvifolia) — are largely resistant to its effects.
Unfortunately, most of the large American elms in Michigan died from the disease when it first became common within the state’s borders. However, a few remain standing, and scientists hope that they’re resistant to the disease. If so, they could be used to develop cultivars for planting. Young American elms do pop up in forests and residential properties, but they almost always die before reaching maturity.
Dutch elm disease is caused by either of two fungi: Ophiostoma novi-ulmi and Ophiostoma ulmi. They’re transmitted by bark beetles, who travel between trees while feeding and depositing eggs. Root grafts are also a method by which the pathogen spreads. The disease also moves throughout trees as it is carried by water moving through the xylem, which helps spread the disease within a given tree.
The symptoms of Dutch elm disease are relatively vague and usually take the form of canopy dieback that progresses from the outside of the tree toward the inside. Premature leaf drop is another common symptom. Treatment using a combination of fungicide injection and aggressive pruning is possible, though not always effective.
6. Beech Bark Disease
Beech bark disease is yet another disease caused by fungal pathogens, in this case, Neonectria. A native fungus, Neonectria is a normal part of U.S. forest ecosystems and doesn’t normally cause problems for beech trees (Fagus grandifolia).
However, a tiny scale insect named Cryptococcus fagisuga was introduced to North America in the 1800s, and it has changed the situation. As the insect chews through the bark to feed on the sap of beech trees, the fungus gains entry. The fungus causes small cankers to form on the bark, and reddish-orange fruiting bodies emerge from the cankers in the late summer and fall.
Unfortunately, beech bark disease isn’t treatable once it has become established, and it will almost always kill infected trees within a few years. However, it is still imperative that homeowners regularly inspect their trees so that infected trees can be removed to help slow the spread of the disease.
Some insecticides may be helpful for reducing scale insect infestations, and growers are trying to develop disease-resistant cultivars.
7. Powdery Mildew
Powdery mildew is a disease that affects trees across the entire United States. A variety of species are susceptible, but some of the most commonly affected trees in Michigan include apples (Malus spp.), pears (Pyrus spp.), maples (Acer spp.), and birches (Betula spp.).
An aptly named disease, powdery mildew is generally easy to identify, as it causes white, powdery areas to develop on a tree’s leaves and stems. Generally, the lower leaves on a tree are the most likely to exhibit signs of the disease, but it can occur anywhere in the canopy.
Caused by various ascomycetes (sac fungi), the powdery mildew tends to appear during periods of high humidity. It is especially common in trees that receive poor airflow, which is one of the reasons it is always important to plant trees with plenty of space between them and to prune them properly.
Fortunately, powdery mildew is rarely fatal to a tree, though it can cause leaf distortion, twig dieback, and defoliation. However, if this occurs several years in a row and affects the entire tree, it may lead to stress and eventual decline. Powdery mildew is treatable with several common fungicides, though treatment is not always required.
Two Threats on the Horizon
We’ve discussed some of the most common tree diseases in Michigan above. However, there are two other tree diseases that have either recently been documented within the state or are likely to arrive in the coming years.
Beech Leaf Disease
A disease that affects American beech trees as well as some European beeches, beech leaf disease causes a tree’s leaves to exhibit striping and develop thick patches between the leaf veins. This disease has been documented in Michigan and threatens the entire beech population within the state.
The disease is usually fatal within a few years, but there are a few treatments available, including foliar sprays and injections. As with most tree diseases, prompt treatment provides a better chance of recovery.
Scientists aren’t certain what causes the disease, though infected specimens are associated with Litylenchus crenatae — a microscopic nematode. It isn’t yet clear whether any other organisms are involved in the disease pathology.
Thousand Cankers Disease
A disease of black walnut trees (Juglans nigra), thousand cankers disease has been present in the southwestern U.S. for some time, but it has begun turning up in the eastern U.S. over the last decade or two. It has not yet been detected in Michigan, but many scientists feel as though it is only a matter of time before it is discovered within the state. Accordingly, homeowners are encouraged to report any potential sightings of the disease to the Department of Natural Resources.
This disease is caused by a combination of the walnut twig beetle and the Geosmithia morbida fungus. The beetle’s tunneling activity allows the fungus to infect the trees, causing tree cankers to form. The most commonly noted symptom is branch dieback, so it is important to have an assessment performed anytime a tree starts showing brown or dead leaves in the summer without an obvious cause.
There is currently no treatment for thousand cankers disease.
Help Keep Your Trees Healthy and Protected from Disease!
No matter which disease is affecting your trees, prompt action is imperative. This is the only way to save trees that can be salvaged, protect unaffected trees when possible, and remove any that may infect other trees on your property. But you can’t just call any tree service for help; you need a professional, ISA-certified arborist – like the ones at Longtree Tree Service. Contact us at the first sign of trouble, so we can inspect your trees and provide a comprehensive plan for treating the disease, preventing further spread, and keeping your trees healthy and looking their best for years to come.
