The Care of Trees – Major Diseases of Trees

As we head into autumn we need to start preparing for next season’s round of diseases. Some of these diseases of trees will affect us now and into winter, while others pop up during spring and summer. Knowing which disease is most likely to be a problem for our trees is important to preventing major damage.

While most diseases are aesthetic by nature, and cause little damage, some diseases are true killers of trees. There 6 major diseases of trees that will kill or weaken trees to death. These are common throughout the Midwest and Central Great Plains regions.

There are some diseases, such as chestnut blight (CB), which have had significant stories in the last 100 years, but are not a factor today. This is because there are few American chestnuts left for the disease to affect. Unlike CB, Dutch elm disease (DED) is still prevalent in American cities and forests where the trees still grow.

Anthracnose

This group of tree diseases affect a variety of trees in early spring, causing stunting and twig blighting, and leaf shedding, blighting, or mottling. Trees that are most often affected by various forms of anthracnose include sycamore, birch, maple, ash, oak, and walnut.

Although anthracnose causes mostly aesthetic damage on mature trees, young trees can be severely impacted, resulting in a weakened state. Thus weakened, the tree can be killed by a secondary pest or pathogen, such as a boring beetle or canker disease.

What causes anthracnose?

Anthracnose is a host-specific fungal infection. This means that each anthracnose fungi affects only certain species of trees. For example, an anthracnose that affects birch will not infect oak, and vice versa. The fungus overwinters in fallen leaves and twigs.

What can I do to limit or stop anthracnose?

A good cultural practice to limit anthracnose (as well as bacterial leaf infections) is to remove all fallen leaves and twigs. If you know that your tree has had anthracnose in the spring, leaves and twigs in the fall should be removed to a burn pit or pile (not composted).

Another method for preventing bad infections of anthracnose are yearly injections of a fungicide. This application should be done by a certified arborist. Injections of a fungicide at or before the infection period will prevent major damage to the tree.

Fungicides can also be applied to the tree by spraying. However, this application method may affect insects in the area and is more difficult to get good coverage of a large tree.

Cedar-Host Rusts

One of the most annoying tree diseases is actually several diseases usually grouped under one label. Most people treat cedar apple rust, cedar hawthorn rust, and cedar quince rust as the same, when they are all different. We see all 3 of these tree diseases in the landscape. These fungal infections cause orange, rusty spots and formations on pear, hawthorn, crabapple, and apple.

What is the life cycle of cedar-hawthorn rust?

This fungal infection starts on junipers, including Eastern red cedar, as small brown galls that produce orange horn-like fruiting bodies. The spores from the fruiting bodies are carried via wind to the host plant. The infection is worse in warm, wet springs.

The fungus attaches to leaves and twigs of the host plants and form rust-colored patches above and cupped lesions below. The cupped lesion produces spores that re-infect the junipers, completing the life cycle of the disease.

How much damage does cedar-hawthorn rust cause?

Cedar-hawthorn rust rarely cause anything but aesthetic damage to junipers. The brown and then orange galls are unsightly, but not damaging.

However, the rust damage to the alternative host plant is significant. In certain species and cultivars, the rust can defoliate the entire tree. It can also weaken the plant and cause fruit drop on apples, pears, and hawthorn.

What can I do to limit cedar-hawthorn rust, or prevent it?

The best method for controlling cedar-hawthorn rust is by planting resistant cultivars. This does not mean that they will never get any rust, but that the infection will be light if at all. Check plant labels when buying apples, crabapples, hawthorns, pears, and serviceberries for rust resistance.

For those of you who have these trees already growing, the best help is to apply a preventative fungicide. This is best done at flowering time in spring. To avoid hurting bees with the fungicides, apply at petal-drop or slightly after. Copper fungicides work best for preventing rusts.

Oak Wilt

The care of oak trees is important, as most gardeners and homeowners have at least one oak in their yard. I myself have several, and 3 different species. It is important to note that there are 2 different groups of oaks, red and white.

The red oak group has glossy, smooth leaves with “pins” at the end of the margin tips. The white oak group has soft, slightly hairy leaves and no pins on the margin tips. I will discuss them further, later this winter in A Guide to Growing Oaks. Look for that in January 2020.

Oaks have a number of diseases that affect them. But none are quite as concerning as oak wilt.

What causes oak wilt and how does it spread?

Oak wilt is a systemic fungal infection that causes wilting and eventual death of members of the red oak groups. This disease primarily affects red, pin, scarlet, and black oaks.

Initial spread may be from infected trees via sap beetles that travel to healthy trees that have been pruned in the months of April through August. AVOID pruning oaks during this time if possible, to prevent this disease.

Secondary spread is through root grafts underground. When roots pass each other, they often form natural grafts, linking them together for the passing of nutrients, water, diseases, and chemicals. This mostly occurs in woodlots and orchards, where there are often trees of the same species planted closely together.

What are the symptoms of oak wilt?

Symptoms first appear in May and June and continue through the summer. It starts with a bronzing or wilting of leaves on individual branches within the tree or on a portion of the crown. There is also a brown-streaking of the sapwood, but it is difficult to identify in smaller diameter branches and twigs.

In a few weeks after initial wilting, leaves will begin to drop. (NOTE: If the leaves stay on the tree, it is not likely oak wilt but another issue). The wilting will spread throughout the rest of the canopy as summer goes on. Then the tree dies.

What can I do about oak wilt?

Unfortunately, there is no curative measure for oak wilt. Infected trees should be removed and burned, but do not take firewood from the infection area, lest it be spread to other trees.

While there is preventative fungicides for oak trees, it can be costly. I would recommend not applying a fungicide unless the tree is important for shade or is in a woodlot, and only if there has been infections of oak wilt nearby.

Thousand Cankers

This quick-spreading disease of walnuts has yet to be found in our area, but knowing about it will help keep it out. This tree disease is a canker or fungal infection native to the Western United States, that is being delivered via walnut twig beetles. Both black walnut (Juglans nigra) and butternut (Juglans cinerea) are highly susceptible to this disease.

What are the symptoms of thousand cankers?

From Ohio State University:

The first symptoms of TCD are yellowing of the foliage and upper crown thinning (Figure 1). These symptoms are followed by dieback of twigs and branches. A tree must experience many sustained, separate attacks (infection events) over time by G. morbida-contaminated beetles for tree dieback and overall decline to become apparent. However, once external symptoms are detected tree death may occur in as little as three years.

The progression of crown symptoms is preceded by the formation of many small areas of dead tissue (cankers) underneath the outer bark of branches and stems (Figure 2). This symptom gives the disease its name. Initially, small (a few mm to 3 cm), roughly circular or oblong cankers, caused by G. morbida, form at the feeding or tasting sites of WTB in twigs, branches, and tree trunks. During this phase of infection, cankers are restricted to the inner and outer bark tissues. In most cases, there is no external indication of tree damage except for tiny beetle entrance holes, but sometimes areas of outer bark surrounding holes may be darkly stained and smaller diameter branches may form cracks, giving them a rough appearance.

As fungal colonization progresses, cankers spread and eventually reach the actively growing layer of tree tissue (cambium) between the inner bark and the wood (turning tissue brown or black). During the later phases of infection, multiple cankers coalesce. Once coalescence occurs the supply of nutrients and water to twigs and branches is cut-off, resulting in dieback. Severe dieback, and multiple main stem cankers, eventually results in tree death.

What can we do about thousand cankers disease?

Be vigilant. If you think that you have walnuts that have any of the above symptoms, contact your local extension office and the department of agriculture for your state. There may be preventative measures to takes before the disease spreads into neighboring communities.

Pine Wilt

This disease was one of the first diseases I ever learned to identify. This disease affects mostly non-native pines, including Scots (Scotch), Austrian, Japanese Black Pine, and Bosnian pine. In the 1980s and 1990s, windbreak recommendations were for the use of scots and Austrian pines. And then pine wilt, which is a native disease, started killing out windbreaks.

Pine wilt is a fungal infection vectored by the pinewood nematode, which is carried by the pine sawyer beetle. The beetle feeds on the tree, the nematode is released into the tree, which in turn releases the fungus as it feeds.

What are the symptoms of pine wilt?

When a pine gets pine wilt disease, it starts by the needles turning a dull to gray-green color, then brown. If you suspect your pine of having the disease, snap of a branch. Sap flow is restricted in diseased trees and will not seep from broken branches. On healthy trees, if you break a branch, sap will flow almost instantly from the wound.

After infection, depending on temperatures, drought, and climate, the tree will die in 3 weeks to 6 months. Certainly, stressed trees will die quicker than ones with adequate moisture.

What can I do about pine wilt?

The best thing to do for the future is to plant native pines which are resistant to the disease. These include eastern and southwestern white pine, Virginia pine, ponderosa pine, and other native pines. Also, using other species of evergreens for windbreaks would be beneficial.

If you have a larger scots or Austrian pine that is an important part of the landscape, you may want to treat this tree to prevent pine wilt. This is also important is you have a Japanese black pine or umbrella pine, which are also susceptible.

A yearly injection of an insecticide by a certified arborist with a systemic insecticide is necessary to prevent pine wilt. The systemic insecticide kills the beetle and the nematode before they can deliver the fungus to the tree. This application can be costly, so make sure it is a tree worth saving.

Dutch Elm Disease

This fungal disease affects native elms including American, slippery, and winged. Dutch elm disease (DED), was first discovered in the United States in the 1920s. It came over from Europe with elm being used for cabinetry. It spread rapidly through the east coast of the U.S., as American elms were often planted in monocultures along neighborhood streets for shade.

Today, DED is still a costly disease as there are still many hundreds of large elms throughout the country, primarily as shade trees. Many of these trees are isolated away from locations where DED was prevalent. This is why they are still alive. Even though there are many new hybrids and resistant cultivars, DED prevention is still important for these large, old trees.

What are the symptoms of Dutch elm disease?

Elms that are infected with DED exhibit yellowing, browning, and wilting of leaves on a branch or several branches within the canopy. This is a “flagging” effect, where branches in the canopy, but not the whole canopy is affected.

Infected trees may day die within a few weeks or take a whole year or more to die. Diseased branches develop a brown streaking just under the bark on the sapwood.

The fungus is spread by European and American elm bark beetles and can also move between trees by root grafts.

How can I control Dutch elm disease?

Trees that are infected that are showing less then 10 percent of crown loss can be saved. This can be done with applications of a systemic fungicide by a certified arborist. Proper remove of any nearby diseased elms should also be done to prevent reinfection. A yearly application of a systemic fungicide should be done on large, important trees, such as state champions or historic trees.

When planting new elms, choose disease resistant varieties.

Conclusion

While there are many diseases that affect trees, these mentioned above are some of the most severe in the United States. We should stay vigilant to protect valuable, historic, and champion trees for future generations. Remember: “Life is a shade better under a tree“.