Verticillium Wilt, A Disease That Attacks Trees From Inside Out

Verticillium Wilt, A Disease That Attacks Trees From Inside Out

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

It’s a disease that becoming more of a problem in our region.  Its name is verticillium wilt.  Verticillium wilt is a fungus disease that attacks over 300 woody and herbaceous host plants.  While it can be a problem in the vegetable and flower garden, its attack of woody plants is what creates the most havoc for home gardeners. 

Imagine a lovely, healthy maple tree in your front yard... growing well with the proper watering, fertilization, and good care that you’ve provided over the years.  One summer half the tree suddenly wilts and dies.  This scenario could happen if your tree is attacked by verticillium wilt.

There are lots of tree species it can attack, but there are some that it commonly attacks in our region.  These are maple, ash, catalpa, redbud, smoke tree, sumac, and Russian olive... with maple being the tree most often attacked.

Verticillium wilt fungus is a sneaky disease, entering a plant through the roots in the soil.  Infections are not obvious like some other diseases, such as powdery mildew or sycamore blight.  Symptoms can be acute... with leaf curling and drying, abnormal red and yellow coloring of the leaves, partial defoliation, wilting and dieback of branches.  This wilting and dieback will typically develop on one main branch, a sector of the crown, or an entire side of the tree.  Chronic symptoms are stunted growth, yellowish leaves, crispy brown edges on the leaves, slow and stunted growth, heavy seed crops, and branch dieback. 

Trees with verticillium wilt may limp along for years, exhibiting symptoms some years and other years not showing up at all.  However, the disease can suddenly attack a completely healthy tree causing it to wilt and die in a short period of time.

The first outward symptoms of verticillium wilt are leaf scorch, abnormal coloring, and dieback of branches.  However, there are many things that cause the same symptoms.  Girdling and encircling roots, root and crown rot, drought stress, compacted soil, trunk injury, and improper planting can all cause similar symptoms.

When verticillium wilt is suspected, a pocketknife should be used to make a slanted cut on an affected branch.  When verticillium wilt is present the cut may reveal streaking or discoloration of vascular tissue (transport) tissues in the wood.  In some species, like maple, this streaking is olive-green, but it varies from tan or brown to green or even black in some species. 

What causes this streaking?  The verticillium fungus once inside a tree, invades the xylem which is the water conducting tissue in roots, trunks, and branches.  The fungus produces toxins that can kill cells, even cells not close to the infected tissues.  The disease spreads in the plant by spores.  New fungal spores move upward with water in the vascular tissue.  The spores then lodge in healthy vascular tissue and new infections begin.  Dead and discolored tissues develop at the sites of these infections, first appearing slightly discolored and then developing the characteristic color for the species.  It’s this discoloration that causes the diagnostic streaking in vascular tissue.

The infected tree reacts to this fungal invasion by trying to block its spread.  It does this by producing gums and tyloses (chemical substances) in the vascular tissue to block fungal movement.  However, these gums and tyloses also impede the flow of water.  This results in the external symptoms of verticillium wilt that can be confused with other causes that also deter a tree’s uptake of water.

When looking for streaking in a recently wilted branch, the first place to look for streaking is directly under the bark.  If no streaking if found there, a deeper into the sapwood may reveal streaking.  However, streaking may not be evident even if the plant is infected with verticillium wilt.  It typically is not present in recently infected sapwood and is usually not found in twigs of one inch in diameter or smaller.  Streaking will most likely be found near the base of the tree, since the initial infection route is usually through the roots.  As you move up the tree, you’re less likely to find streaking.  To make diagnosis even more difficult, there is no detectable streaking of vascular tissues on certain species, like ash.

Before we talk about management of verticillium wilt, let’s talk a just a little more about it’s life cycle and how it enters a tree.  The wilt’s microsclerotia, which are tiny black resting structures, can be found in many soils.  They spread by wind or water.  They can also be introduced into “clean” soil from contaminated seed and plants or from contaminated soil on root balls, hand tools, or machinery.  They can exist in the soil for many years, up to 15 years, without contact with a host plant.  Lying in wait, the microsclerotia will germinate and infect a new plant when they come in contact with the roots of a susceptible host.  They invade the roots through a wound or by direct penetration.  Once inside the host, the fungus gets into the vascular tissue and spreads throughout the plant by spores. 

So how do we control the disease?  Plain and simply, you can’t control verticillium wilt once a tree is infected.  No fungicides have been found to be effective against the fungus in trees.  Injections of fungicides into a tree have been tried, but the results have been inconclusive.  You may not be able to control verticillium wilt, but you can try to manage it.  Just because a tree is infected, doesn’t mean it will die.  It may recover and be able to live with the disease... with careful management.

Trees should receive plenty of water to promote growth and avoid stress.  Infection is less severe when trees are not drought stressed.  A good deep soaking once a week during hot summer weather will help.  Fertilization with ammonium sulfate, when symptoms are first noticed, is recommended by many “verticillium wilt” experts.  Research in Michigan indicates that you should avoid applications of nitrate fertilizers because they ineffective in management of the disease.  While fertilization is part of the management program, you should avoid excessive fertilization, which apparently can increase problems with the disease.

You will want to promptly remove branches killed by the disease but you shouldn’t remove them until you give the water and fertilizer a chance revive recently wilted branches.  If they don’t come back in response to the extra care, remove the branches, cutting well below any of the streaking or discoloration and back to a main branch or limb.  Be sure to make proper pruning cuts and don’t leave stub cuts.  In between cuts, disinfect your pruning sheers with 70 per cent rubbing alcohol or a 10 per cent bleach solution.  Wood from infected branches should be removed and destroyed.  Chipping the wood and using it for mulch can spread the disease to other areas.

If a tree dies and needs to be replaced, keep in mind that the microsclerotia can remain in the soil for over 15 years.  When replanting, select a species of tree that is resistant to the disease.  Trees reported to be immune or resistant to verticillium wilt include beech, birch, ginkgo, honeylocust, sycamore, hophornbeam, sweetgum, pear, mulberry, apple, hawthorn, willow, white oak, bur oak, and sycamore.  Dogwood and linden appear to be susceptible to some strains of verticillium and resistant to others.  Fir, spruce, pine, juniper, and arborvitae are also resistant.