Clematis have fairly few pests or diseases but they do suffer from the dreaded clematis wilt and it can be devastating. A plant that is growing great and ready to flower one day, is almost dead the next. All the leaves go black and the flower buds wilt. So much for this season.
Clematis wilt was first reported in 1885 and we have been trying to understand it ever since. A lot of progress has been made, but even today it is not fully understood. Some believe it is caused by a fungus, and others believe it is a watering issue. Confounding the whole thing is the fact that many gardeners think they have clematis wilt when in fact they don’t.
In this post will summarize what we know and don’t know about clematis wilt, and in the process help gardeners better understand the culture of this fantastic plant.
Classic Symptoms of Clematis Wilt
This is how the wilt is described by most authorities. One or more stems of the clematis suddenly wilt. The leaves and stems go black and the flower buds droop. The plant looks as if it is devoid of water. The whole process happens very quickly, in a matter of a day or two.
The disease does not affect the roots, so normally the plant will regrow new shoots, although this may not happen until the following year, depending on the type of clematis.
Large flowered clematis are more susceptible to the disease and most small flowered cultivars are immune.
Once the disease hits, there is no chemical treatment to stop it, but it rarely kills the plant.
Clematis tend to lose their lower leaves later in the season and they usually go brown or black before falling off. Provided that the upper sections are green, this is not clematis wilt. It is normal for them to drop older leaves and some gardeners cut the plant to the ground at this time.
Identifying a Problem Using Symptoms
Gardeners identify most problems, including this one, by looking at visible symptoms and this works in many cases. However it does not work in this case because a variety of issues could be causing the symptoms. It is quite clear that not all reported cases of clematis wilt are caused by a disease.
Clematis stems are woody, but quite thin. They are easily damaged while you tie them to a trellis, or by a strong wind. When the stem is cracked, water can’t get to upper leaves and flowers and they wilt showing symptoms similar to what I have described above.
Slugs, snails and small rodents can also damage these fragile stems, usually close to the ground. If the damage is extensive enough the plant will wilt.
All of these abiotic problems cause wilt which leads gardeners to conclude that they have the disease; clematis wilt. But the term “clematis wilt” is best reserved for the disease.
Does the Clematis Wilt Disease Really Exist?
Jim Fisk, a clematis expert who was instrumental in making them popular in gardens and who has written several books on clematis, is of the opinion that clematis wilt is not a disease, and he is not alone. Jim felt the wilt “is a failure of the very thin stems to cope with a sudden demand for moisture from stems, leaves and flowers, resulting in a breakdown of the tissues at a certain spot.” This defective stem is then no longer able to move water up the plant and it wilts.
This explanation is supported by gardeners who report that plants grown in soil where there is a good subsoil water source, never get clematis wilt.
The problem usually happens just as the plant is ready to open a lot of flowers and all those expanding flowers put a big drain on water resources. This fact, coupled with a very thin stem, seems to support the idea that clematis wilt is a watering issue.
The disease angle has also been examined and there is now clear evidence that a known fungi can infect clematis and cause the observed symptoms. That fungi can be extracted from wilted plants and used to infect other plants.
I’ll discuss this disease in more detail below, but it seems as if this is the most likely explanation for the disease. Most authorities no longer believe it is just a watering issue, although a lack of water might explain some instances in gardens. I think it is best to consider watering issues along with slugs and wind as a possible explanation of the symptoms but not the true cause of the disease, clematis wilt.
The Clematis Wilt Disease
Several fungi have been found on clematis showing the disease symptoms but the most likely candidate is Phoma clematidina (syn Ascochyta clematidina). The RHS calls it Calophoma clematidina.
This is considered to be a wound pathogen that gains entry in plants when the tissue is damaged. For example, a partial break in a stem allows the fungus to infect the plant. This is common near the soil where moisture levels are higher.
Infection can also occur on leaves and on leaf petioles even without a wound. When the pathogen lands on a leaf its hyphae enters the leaf, causing a visible leaf spot. This leaf spot has a characteristic ring formation as shown in the above picture.
Once the leaf is infected, the fungal hyphae grow inside the leaf and work their way down the leaf petiole, causing the petiole to turn black. They then enter the leaf node on the main stem, causing stem girdling, a blocking of the plant’s vascular system. When this happens the stem above the infection starts turning black, but the stem below the infection stays green. The pathogen has essentially turned off the water that is normally moving up the stem and everything above the infected node, wilts.
The time between infection and wilting is in the order 10 to 40 days. Infection occurs more often on leaves that are wounded.
Some clematis seem to be resistant to the disease and initially it was thought that the fungus could not penetrate resistant varieties, but does not seem to be the case. The fungus does infect resistant leaves but these leaves prevent proper growth of the hyphae and/or are less sensitive to the toxins produced by the fungus.
This process is not nearly as fast as gardeners think, because they normally only see the final wilting stage, which happens quickly.
Life Cycle of Clematis Wilt
Phoma clematidina can survive on dead plant tissue and it can survive in soil. Water splashing on these materials will move the spores onto plants. Garden tools can also move spores between plants. It is also suspected that some insects might move them and this could explain how the disease moves from garden to garden.
Plant roots are not normally infected when planted in the ground, however, they can be severely damaged when the pathogen infects roots of containerized plants. The reason why roots in the ground are not affected is that this pathogen is not systemic; it does not travel throughout the plant. Once it has girdled the stem, it stops growing.
Testing for Clematis Wilt
Researchers have developed a fairly simple test to check the susceptibility of a clematis to the disease. They apply the fungus to some wet filter paper which is applied to the stem and wrapped in plastic, not unlike the stem air-layering process. The wrap is left on for 60 days to see if the stem wilts.
This test demonstrated that stems can be infected without a wound. The extra water and plastic wrap keep the spores unusually wet and it is unclear how often such stem infections happen in a normal garden environment.
Wilting in this test took anywhere from 25 to 40 days. Compare this to the gardeners impression, that clematis wilt happens overnight.
Small Flowers vs Large Flowers
Not all clematis get clematis wilt and some are more prone to it than others.
Gardeners talk about resistant types which usually include the small spring clematis like aplina and macropetala, as well as the viticellas. Some of the large flowered clematis such as Nelly Moser, Henryi and Elsa Späth are considered prone to clematis wilt.
Researchers classify them a bit differently. They use the terms large flowered, which are prone to the disease and small flowered which are not.
In 1997 a research group tested a variety of cultivars, using the above mentioned clematis wilt test, for sensitivity to the disease and at the same time they did a British survey among amateur gardeners and commercial growers, to see which plants they thought were more resistant to wilt.
When actual test results were compared to the opinions of commercial growers, there was good correlation, except for C. viticella. In this case, the test showed higher infection rates than the survey. These plants are vigorous growers and they might be able to outgrow the infection in the garden, but it is incorrect to say that they are resistant.
The flower size really has nothing to do with infection, except that many of the larger flowered varieties are also more highly inbred and perhaps breeding programs are moving us towards disease susceptible plants. It is also possible that breeding has focused on larger flowers at the expense of stronger stems to support the flowers, both in terms of weight and water resources.
Here are some general rules for resistance.
- Most species are resistant, although some wild populations do get clematis wilt.
- Small flowered varieties are more resistant than large flowered ones.
- Late large flowering cultivars are more resistant than early large flowering ones.
- Hybrids with C. lanuginosa in their ancestry are most susceptible.
The recovery rate after infection is higher for the late large flowered group which may suggest that at least some of the wilt seen in early large flowering plants is due to other causes. This latter group also tends to have thinner brittle stems.
Why Do All Stems Wilt at the Same Time?
We see two scenarios in the garden. In some cases, one or two stems wilt, maybe at different times. The rest of the stems stay healthy and flower.
In the second scenario, all stems seem to wilt at the same time, or certainly within a few days of each other.
If a couple of stems wilt it could very well be mechanical damage to leaves or stems. Or it could be an infection in a limited part of the plant. But how does one explain the case where all of the stems wilt at the same time?
It is unlikely that all stems were damaged at the same time by the gardener. A sudden very strong wind or some extensive rodent damage might do the trick, but even in this case one would expect only some wilting stems. Disease infection would also happen on different stems at different times and infection takes many days before it shows up. Fungal infect does not seem to move from one stem to another. Disease infection does not seem consistent with the observation that all stems wilt at the same time.
I am still looking for an explanation for this.
Shading Clematis Plant Roots
A very common gardening myth is the idea that clematis roots need to stay cooler than other types of roots, but this is completely false as I’ve explained in Keep Clematis Roots Cool.
Many people plant things at the base of clematis to keep their roots cool and this may encourage clematis wilt to take hold. The following is my suggestion and experience.
In several cases I have planted things like Geranium X magnificum at the base of clematis, not to keep the roots cool, but to hide the ugly lower stems. This geranium grows as a vey thick clump which increases humidity and reduces air movement at soil level. These were added to clematis that had been growing well for several years and showed no signs of clematis wilt. Within a year of hiding their base, they developed clematis wilt. Several of these were larger flowered plants such as Nelly Moser, but one was a small flowered plant, Princess Dianna.
These clematis had wilt for a couple of years until I removed plants from their base. Once that was done, the plants stopped having wilt. In the case of Princess Dianna it was immediate, but with Nelly Moser, the amount of wilt in the following year was less but still present. A year later there was almost no wilt.
It is known that the clematis wilt fungus requires a certain level of humidity to grow and in research, leaves or stems are placed in high humidity environments to encourage their growth.
I’d like to suggest the following. The common cultural practice of planting at the base of clematis could be increasing the incidence of clematis wilt.
What Should You Do if You Get Clematis Wilt?
Cut off all infected stems right to the ground and discard the plant material in the garbage. Clean up all dropped leaves from the ground.
It is not clear to me that any of this cleanup will prevent further infection, but since this is a fairly rare disease compared to something like powdery mildew and it is not noted to be air borne, it might help.
Since the pathogen does not invade the roots, your plant should grow normally next year.
Using a fungicide will not help once you see the wilt.
I did not look into this extensively, but one report showed that “The fungicides chlorothalonil and fenpropimorph were the most effective of those tested for inhibiting spore germination and mycelial growth, respectively”. Note that fungicides don’t work once the plant is infected.
Preventing Clematis Wilt
There are several things you can do to help prevent or at least control the disease.
Always plant clematis a couple of inches below the soil surface. This will reduce the likelihood that the fungus infects the roots.
I see this suggestion a lot but following the advice might result in planting too deep. Many purchased clematis are already planted deeper in the pot and you plant it deeper still. If you then move this plant and follow the advice you will plant it even deeper. When is it too deep?
A better suggestion is to plant so that the crown of the plant is 4″ below soil surface.
Keep the Ground Clear
Clematis roots do not need to be kept any cooler than other types of roots. Don’t plant things at the base of a clematis. A 1″ high ground cover, or mulch is not be a problem, but you want lots of air movement and sunlight at the base of the plant so that this area stays dry.
Plant Smaller Flowered Clematis
Stick to smaller flowered clematis. That might work, but most people love the large flowers so you should probably plant what you like. Even with large flowers you may never see clematis wilt in your garden.
Plant Newer Cultivars
Some of the new breeding is producing plants that are claimed to be wilt free. I don’t think anyone has identified a gene that prevents wilt, so it is not clear how these new plants got the “wilt-free” label. It might just be marketing hype.
Handle Plants Carefully
Be careful when you handle the stems of clematis. Tie them up gently so you don’t break stems.
If you are located in a very windy area, you might need to provide protection from the wind.
Thinning the Plant
This is recommended a lot for other types of fungal infections, and is also sometimes recommended for clematis. Personally, I would not do this once the clematis has grown more than a foot above the ground. Clematis vines grow very thick together and trying to remove some stems will almost certainly damage the remaining ones.
If you want to give this a try, you need to cut the stems out early in the season before they start clinging to each other.