Many arborists are puzzled and plagued by the presence of Verticillium wilt in their woody ornamental trees and shrubs. This devastating condition is caused by two forms of soil borne fungi:
- Verticillium albo-atrum
- Verticillium dahliae
In this article, we will look at this malady and discuss some smart solutions to help you cope with and prevent its occurrence. Read on to learn more.
Which Trees & Shrubs Are Most Commonly Affected?
Verticillium fungus is very common in a wide range of soil conditions. Because of this, literally hundreds of woody and herbaceous species of plants are affected by it.
The fungi tend to prefer specific species under certain conditions, so a particular type of tree may be a much more susceptible host in one environment than in another.
This may cause a plague of fungal infection among a certain type of tree in a given area; however, it is also possible for trees to host the fungi without showing signs or symptoms of disease.
What Sorts Of Trees Are Most Susceptible?
Below is a short list of highly susceptible trees. It is very important to note that some of these trees are only susceptible in certain settings.
For this reason, some species listed both in this list and on the list of trees hardy against fungal infection. In these cases, adjustments in abiotic environment make all the difference.
- Kentucky Magnolia
- Ohio Honeysuckle
- Russian Olive
- Black Spirea
- Smoke Tree
- Coffee Tree
- Cork Tree
It is important to keep in mind the degree of susceptibility or resistance a given specimen may exhibit depends a great deal on abiotic environment, the type of Verticillium present and the cultivar, itself.
How Serious Is Verticillium Wilt Disease?
Verticillium wilt is considered very serious, indeed, because it can spread through a plant systemically in a very rapid fashion and kill it off without much warning.
However, it is worth noting this is a problem which generally affects planted specimens in a yard or garden. It is highly unusual for Verticillium wilt to affect forests and natural stands of trees. For this reason, this disease does not present the urgency of very severe arboreal illnesses (e.g. Dutch Elm Disease).
What Are The Symptoms Verticillium Wilt?
You should know, the symptoms of this disease are similar to those of a number of other problems, maladies and conditions. For example, lack of water, use of herbicides, negative environmental conditions and physical damage can all cause sudden wilt.
One way to rule out some of these causes is to consider the time of year when symptoms occur. Verticillium typically strikes in the hot months (July and August), so wilt symptoms in other times of the year are likely to be caused by something else.
Symptoms appearing in mid-late summer may very well be attributable to Verticillium wilt. These symptoms may present as acute or chronic problems.
1. Acute Symptoms: If acute, you may suddenly see curling leaves, an abrupt change of color (from green to red or yellow), falling leaves, branch dieback and general wilting. Entire sections of the bush or tree may die off.
Some types of trees exhibit sections of dead bark on the trunk and branches because the inner bark has been killed by the fungus. Other types may suddenly drop green leaves without any wilting, scorching or yellowing. It is not unusual for Verticillium to cause a tree or bush to simply die suddenly and collapse.
2. Chronic Symptoms: If chronic, you may see spots of yellow and/or scorching on the leaves. Your tree or shrub may grow slowly and produce an abnormally large number of seeds or nuts, but few leaves. Branches and shoots may die back abruptly.
For a very definite diagnosis, you can examine the vascular tissues of the wood of the tree by cutting a section and examining it for discoloration. In recent infections, you may be able to find discoloration by peeling the bark off sapwood.
If Verticillium is well established, you will see streaking and discoloration of the wood. It may be isolated to a trunk cross-section, or it could be scattered throughout. In a recently infected tree, you may find discoloration only in new sapwood.
You should recognize this discoloration when you see it; even though, it will vary in color from one type of tree to another and depending upon the degree of establishment of the fungal infection.
Discoloration in infected areas may range from shades of red to brown, green, gray or black. Nonetheless, you should be able to tell that something is not quite right when fungal infection invades your trees and bushes.
How Does Verticillium Get Into A Tree Or Bush?
The fungus enters the organism through schisms in the root system. Your plant may experience injury to the roots due to digging around it, chewing by pests or simply through the friction of growing through the soil.
When an opening is made, fungus quickly invades susceptible plants. Once inside it produces a toxin and invades the tissues of the plant that produce water. The fungal spores travel throughout the plant, carried by the water in the plant.
Spores lodge themselves in the vascular tissue of the plant and spread infection. Not only is the infection spread within the host plant, it can also be spread to neighboring plants via the toxins produced by the fungus. This is why it is fairly impossible to isolate the fungus within a plant and prevent it from doing further damage.
Can’t Trees & Bushes Protect Themselves?
The fact the fungus is able to enter a plant at all indicates the plant is somehow compromised. If this compromise is caused by a trauma, the plant may still have a very strong immune system to help prevent further damage.
When a pathogen enters a healthy plant, the plant produces tyloses (gum) to help close off the pathways the fungus might travel. This prevents contaminated cells and the fungus from moving around within the plant.
If defenses fail, the tree or shrub may be able to go into remission and recover entirely. If you see your tree exhibits symptoms of wilt every year but then bounces back, this is what is happening.
In this case, you should replace the specimen with one more naturally resistant to fungal infection. A tree must fight off disease year after year will never be able to perform well. Furthermore, a tree repeatedly infested with fungus will only spread it to other plants in your yard or garden.
Although Verticillium comes from the soil, you must realize a host tree contributes to its presence in the soil when it drops its infected leaves. These leaves will be filled with mycelia or microsclerotia.
These tiny, thread-like growths can persist in soil for a decade, and if you keep a chronically infested plant, their numbers will be regularly replenished and never be rid of them.
Nip Verticillium In The Bud
The longer any kind of infection lingers, the stronger it can become. If your soil is riddled with Verticillium, microsclerotia may gather on the roots of resistant plants and grow stronger and better able to take those plants down.
This is why it is very important to eradicate affected plants from your yard.
It is also important to avoid bringing infested plants into your yard. Be careful of your sources of all sorts of plants and plant products, such as:
- Bare Root Trees
Bringing infected plant materials into uncolonized soil will quickly wreak havoc in your garden. For this reason, you should also be careful to keep your tools and equipment clean and to sterilize them upon return if you happen to loan them out.
Is There No Way To Rescue An Infected Tree Or Bush?
If you suspect one of your trees or bushes is experiencing wilt and you just can’t bear to part with it, there are steps you can take.
Begin by confirming your diagnosis by having some lab testing performed on a tissue sample. It would be a shame to pour years of time, effort and resources into fighting Verticillium wilt only to find that was not the problem at all!
If you find your tree infected with fungus, you’ll want to determine exactly how severe the problem is. One important aspect of this determination is the length of time the tree has been in place.
Newly planted trees may exhibit true signs of Verticillium wilt, but removing or treating them may not be necessary.
These symptoms may have more to do with the trauma of replanting than anything else. In this instance a program of regular watering and use of a low-nitrogen/high potassium fertilizer supplement may do the trick.
Keep in mind for both recovery and prevention of fungal infection, it is important to be sure your trees and bushes are always well-watered. Drought conditions and severe water stress actually promote the spread of the fungus.
Be sure to keep your affected tree properly pruned. Remove all of the affected areas and dispose of them properly by putting them in a plastic bag in your trash can far from other plants. Don’t chip these branches for mulch or add affected leaves to your compost. Remember to clean your shears and tools thoroughly after each pruning.
Replacing A Tree That Succumbed To Verticillium Wilt
There are a number of trees and shrubs naturally resistant to this problem. Here are some trees you may wish to consider:
- Mountain Ash
- Honey Locust
- White Oak
- Bur Oak
As noted, some types of trees are resistant under one set of conditions and not under others. For this reason, these appear on both lists. To choose properly, you must take steps to determine the experience of other arborists in your area. Consulting your local gardening center or horticulturists’ club can be helpful.
Vigilance, Diligence, Patience & Regular Care Deter Verticillium Wilt
The occurrence and severity of Verticillium wilt varies from plant to plant and place to place. Because it is so very difficult to deal with and can spread very vigorously, it is best to eliminate affected plants whenever possible.
If you do decide to treat an infected specimen, focus on helping it to build up its own resistance. You can do this with regular watering and protection from punishing afternoon sun.
Be sure to set up a consistent schedule of watering that includes a high phosphorous/low nitrogen fertilizer, and remember to be diligent and sanitary with your pruning.
If you have removed an infected specimen, focus on getting the fungus out of the soil. One good way to do this is to “solarize” the soil. In this process, you would heat the soil by tilling it, wetting it thoroughly and covering it with clear plastic.
Be sure the edges of the plastic are firmly held down or buried to prevent cooling and evaporation. Keep this setup in place for a month to six weeks during hot weather to heat up the soil and kill off the fungus.
Even though Verticillium wilt can be devastating to your yard and garden, you can overcome it with diligence, patience, hard work and smart choices.