Recently Ms. Jan Pavlinak, who is customer service representative at Bluestone Perennials, Inc., contacted me about which clematis were considered to be disease resistant which led me to write this article.
Clematis, to some extent, can be susceptible to diseases such as powdery mildew, slime flux, etc., but I suspect when most gardeners are looking to find a disease resistant clematis they want one that will not succumb to the dreaded stem rot (aka wilt). So, the following documents the experiences I have had with clematis when it comes to stem rot.
Over the years I have regularly seen it on clematis in the Patens Group and the Jackmanii Group in my own garden, in other gardeners’ yards, as well as in retail and wholesale nurseries. I have also seen it on clematis in the Florida Group (you would know this first hand if you have ever tried growing C. ‘Sieboldii’) and occasionally on a few stems of clematis in the Texensis Group. With clematis in the Viticella Group I am happy to report I have not observed any wilt to this point with the exception of C. ‘Polish Spirit’. Even though Clematis ‘Polish Spirit’ is considered a non-wilter since it is a member of the Viticella Group, which is considered by many (including me) to be a group of clematis that does not easily succumb to wilt, I have witnessed it strike this allegedly semi-immune cultivar. Out of the 18 plants of this variety our society was selling at our annual fundraiser in 1998, four totally crashed from wilt. To date my two Clematis crispa plants and my Clematis pitcheri (both in the Viorna Group) have never shown any signs of this malady.
Practicing my due diligence, I turned to the renowned clematarian Wim Snoeijer’s book: Clematis Cultivar Group Classification with Identifying Key and Diagrams for more information about this question. On page 148 he wrote: “But, as we know today, many cultivars from different Cultivar Groups may be infected by the fungi and wilt while the other Groups, known as non-wilters, can host the fungi, like the Atragene Group and the Montana Group”. I then decided to email Wim asking his thoughts on this important subject and here is his very enlightening response:
Good Morning Edith,
About disease resistant clematis. Well, there are no disease resistant clematis. Any plant can be attacked by a fungi and die.
But you can keep in mind that there is a basic splitting in clematis with reference to the roots. Those that have fleshy roots (except Armandii & Forsteri Group) need deep planting and can suffer from “wilt”. The first cause of “wilt” is lack of water. So, when you have a “susceptible” Patens or “strong” Viticella Group cultivar next to your front door planted in between the wall of the house and the foot path and the plant wilts, the reason is probably lack of water.
When a plant suffers from lack of water (and feeding) the plant, any plant, is more susceptible to a fungi attack. In such a situation it is quite obvious that 90% of what is blamed to be “wilt” is actually caused by lack of water and not by the stem rot fungi.
Some of the Patens Group cultivars such as ‘Vyvyan Pennell’ are very vulnerable to the stem rot fungi. At the nursery there is hardly any problem because of enough water and feeding and spraying with chemicals against fungi. But the stem rot fungi can survive on any clematis including on the Atragene and Montana Group cultivars. But these cultivars will not wilt from the stem rot fungi just like many other cultivars from fleshy roots groups. For instance ‘Emilia Plater’ will not wilt from the stem rot fungi. When attacked the leaf might turn black but the attack stops at the leaf stalk and so the stem will continue to grow and flower. Others, like ‘Polish Spirit’, can show wilt problems and part of the stems will or can die like a Patens Group cultivar.
So, for these fleshy roots cultivars (except Armandii and Forsteri), it is important to keep the plant healthy by providing enough water, feeding and deep planting them.
Overhead watering seems to be the main way the spores of the stem rot fungi spreads. So your infected ‘Vyvyan Pennell’ gets overhead water from your irrigation system, the spores spreads with the drops that splashed on the leaves of ‘Vyvyan Pennell’ on the next door ‘Pamela Jackman’ or ‘Mayleen’. Here the fungi cannot attack the plants (do not know the reason) but the fungi survives and the spores are spread with overhead watering to their next door ‘Polish Spirit’. The fungi can attack ‘Polish Spirit’ and some stems will die. Entering of the spores takes place via the leaves.
When you use a source of watering for overhead watering that is infected by the stem rot spores it means that you are spreading the fungi yourself.
The Groups with the fibrous roots, including Armandii and Forsteri, do not want deep planting as these plants might suffer from Phythophthora a fungi that attacks the stems at soil level. When attacked the plant usually dies (opposite of the stem rot fungi; a plant will never die from that fungi). The main cause seems to be too wet soils in winter.
That is in short what I can write first. I suppose this information will raise more questions than it provides answers, so please do not hesitate to write again. I am surely most pleased when I can be of any help.
All my best wishes from Holland,
Wim
As Wim points out, when watering your clematis it is extremely important to give them a regular and a sufficient amount each time to keep them healthy and strong. That alone is not enough as you should also be regularly feeding them (see Feeding Your Clematis Correctly www. clematisqueen.com/content/feeding-your-clematis-correctly) to help give your clematis a fighting chance. Sadly, it appears that sprinklers can be a problem in spreading the wilt spores. So, what is the alternative? Other than hand watering, the obvious choice would be to install a drip irrigation system. I’m sure I’m not the only one out there who is unwilling to change my current sprinkler method to a drip system. For me they are a lot of work, expensive and to be quite honest, they are unattractive (okay ugly), so you won’t be seeing any at the Queen’s Castle. Not only that, heads clog easily and parts deteriorate in the sun.
So, when watering your clematis, try to keep the water from splashing as much as possible. I guess even rain can be considered a spore scattering culprit. Maybe the next time precipitation is predicted I will go out and put up an umbrella over each one of my babies to protect them (wink). As we do not have any chemicals (fungicides) specifically available to treat for wilt in the U.S I want to stress the importance of spraying the affected area with Physan 20 to help curtail the spreading of any remaining spores. For more about spraying see: Rx for Wilt (aka clematis Stem Rot) (www. clematisqueen.com/content/rx-wilt-aka-clematis-stem-rot)
When growing clematis we have to face the hard cold fact that wilt is in their DNA. However, the good news is that wilt is usually not a fatal occurrence (if you have planted it deep). Philosophically speaking, it is a very disappointing incident that we may have to endure, but the upside of being able to behold the beauty of the genus’ gorgeous flowers far outweighs the downside.
If you would like to learn more about Clematis Groups and which clematis are in them here are two outstanding sources: Wim Snoeijer’s Book: Clematis Cultivar Group Classification with Identifying Key and Diagrams and J. van Zoest Clematis nursery (http://www.clematisinfo.nl/1280_1024/).