About 120 years ago, because of 'stem rot' the growing of clematis ceased throughout Europe. The fungus responsible had found an ideal medium in the many new clematis; the more clematis were grown the greater the epidemic and the stronger the fungus got.
In the last 50 years clematis has made a comeback. A major cause has been the use of fungicides that controlled the fungus in the nurseries; they could now properly claim to be selling healthy clematis. But the situation in the garden had not changed at all. For three weeks after the plant leaves the nursery the effect of the fungicide has worn off. Thus the apparently healthy plant is still prone to develop 'stem rot'. One investigation showed that 80% of members of BCS had met 'stem rot'. Only a small fraction of the clematis sold are sold to knowledgeable BCS members. The bulk is sold to the public. 80% of these will also meet 'stem rot'. This is why we all too often meet people who say "I don't grow clematis. Mine wilted".
Just at the moment there is another concern. The fungicides are getting less effective in controlling 'stem rot' in the nurseries. The fungus is getting immune to the fungicides. Last year I saw the fungus of about five years ago growing strongly on ager. A fungicide was introduced, the fungus died immediately. On another plate of ager was a colony of today's fungus. The fungicide was introduced. It had no effect! The fungus of today is becoming immune to the fungicide. However, there is hope that a new lot of fungicides are on their way. They will allow the nurseries to produce healthy clematis (but still prone to wilting). But in the garden the situation will not change at all. The effect of the fungicide used at the nursery will wear out in three weeks. Even if allowed to use the fungicide, the public will not do so, especially the organic gardeners. The major cause of loss of interest in roses was the advent of 'black spot'. Could we see a drastic reduction of interest in clematis because of `stem rot'? We could. The public is unhappy at loosing money on expensive clematis that wilts and dies on them. Could the event of 120 years ago be repeated today? It could, but I don't think it need do for one reason we know more about 'stem rot' than they did 120 years ago. If we use this knowledge wisely we could increase rather than decrease interest in clematis. I will now review this new knowledge and then outline an action plan.
When I entered the clematis world some 20 years ago, the nature of clematis wilt was a mystery which was best not mentioned. However, it soon became clear that the public was well aware of this weakness in clematis and once having experienced it, were avoiding this beautiful plant. Surely it was best to tackle the weakness so that our plant could prosper. Having spent all my life as the head of a research institute at the University of Cambridge, I knew that the first step was to review all the known information about the disease. This revealed a vast amount of interesting data. The most striking was that Gloyer, in the USA in 1915, in a thorough piece of scientific work (he established Koch's postulates for the fungus involved) had found the cause.
I also came across a list prepared in Holland of susceptible clematis. Looking at the list, it looked as if the big wilters came from the Early Large Flowered Group. Was this true of the UK? I asked 20 knowledgeable clematarians. They were asked to comment on the Dutch list and make a list of their own. Both answers agreed with my evaluation of the Dutch list. So three investigations pinpointed that the Early Large Flowered Group was the vulnerable group. The Derby investigation some years later asked two groups of clematarians, amateurs and growers the same question. Both groups agreed with the previous three groups. Hence five groups of people pinpointed the same group, the Early Large Flowered, as vulnerable.
My review strongly hinted that the Early Large Flowered Group were vulnerable because of a flaw carried by two clematis used for hybridising this group, C. lanuginosa and C. fortunei. I won't detail the work here as more work is continuing.
The review also threw up a number of research areas which would require laboratory facilities that I had not got. I approached the Horticultural Development Council in 1991. There was cautious interest. It was agreed to leave things until my review was complete. By 1994 we were able to meet and in 1996 work began at the University of Derby. Excellent supervision with an able Dutch post-graduate student brought useful findings. Being Dutch was valuable for the researcher was familiar with the very important Dutch literature. Other recent areas of research were a new venture in Holland precipitated by the worsening situation in the Dutch nurseries as the fungus became immune to the fungicides, a large study in New Zealand, smaller studies in New Zealand and another in Herefordshire, UK.
Now I shall attempt to summarise the knowledge from all this work and now available to us;
Summary of Findings
This was elucidated in 1916 by Gloyer in the USA in a thorough research over three years. He gave an accurate description of the fungus responsible, phoma clematidina, described its actions, and established Koch's postulates for the fungus. Work by Ebben and Last in the UK, initiated by Christopher Lloyd, confirmed Gloyer's findings. Subsequently it was confirmed by three separate investigations in Holland, one in New Zealand and by two centres in the UK. There is no room for doubting the cause of 'stem rot'.
The spores of the fungus are spread by water splash and by insects. For best growth the fungus requires a temperature of about 20oC, but it can grow at lower temperatures. It will survive winter in cold climates. It requires humidity for growth.
The fungus will make colonies on the leaves of any clematis, including the wild species of clematis. It attacks the lower side of the leaf most often possibly because it is wetter. In susceptible clematis (Group VI, the Early Large Flowered Group and less frequently in Group VII, the Late Large Flowered clematis) the fungus moves down the petiole of the leaf to the node on the stem and the plant cannot resist its entry. In the resistant ten groups the fungus cannot gain entry at the node. Investigation on plants of the montana group showed that even before the fungus in the petiole had got to the node, the plant abcised (shed) the leaf to frustrate the fungus.
Once it has gained entry to the stem the fungus destroyed and area of about 1in (2.5cms) on either side of the node. The damaged area has a characteristic jet black colour. When it dries it makes jet black powder. The walls of the stem will sometimes disintegrate when the damage can be seen by an observer. The damage is right across the stem. Therefore no sap can pass this area. The stem above the node is deprived of sap, flops, wilts, turns brown, dark brown and dies. The effect is the same as cutting across the stem at the node. If immediately the stem flops it is cut across above the node, the stem will, of course, recover if placed in water. If the plant is particularly valuable cuttings could be taken from the rescued stem.
The most attacked node is the lowest node on the stem. This may be below ground level. Other nodes low on the stem can be attacked. Low nodes have leaves more likely to be humid than higher nodes. Young stems, before they become woody, are most likely to be attacked. When the stems of the clematis have become woody they may not be attacked but branches higher up the plant may still be damaged. Internodal areas if damaged by trauma or insects may be attacked; the area of damage is as at the node. Nodal areas are most frequently attacked in the garden.
Roots in clematis can be damaged by the fungus. There may be spread of the fungus down the stem to the roots. This is most likely from the lower node. How often this happens requires further investigation. In my experience it does not necessarily happen. However it is prudent if the lowest node is involved to cut the stem away below the lower node. If the top of the stem left in the ground is green, then the cut is satisfactory as there is green stem below the cut.
The Vulnerable Groups
The most vulnerable is Group VI of my classification, the Early Large Flowered Group ('Nelly Moser' group). All clematis in this group are persistent wilters. Investigation points to the fact that there is no guarantee that any clematis in the group is free of wilting. Damage to the plants can be severe. 'Vyvyan Pennell' and `General Sikorski' are examples of clematis that are very susceptible.
The Late Large Flowered clematis, Group VII, will occasionally succumb to 'stem rot'. Some such as 'Hagley Hybrid' are almost completely resistant, 'Jackmanii' will occasionally be attacked in this group. Viticella in the parentage encourages resistance to the fungus as the attack is occasional and the damage less severe. It probably is a reasonable approach not to employ special preventive measures with this group. Arguably, and hopefully, the gardener will accept this group as being in the same range of risk of disease as the average plant in his garden.
As a global statement it can be said that all the other ten groups are immune to 'stem rot'. Where this appears not to be so, then great care must be taken not to necessarily assign the wilting to 'stem rot'. I will make the point with two examples. My C. montana 'Mayleen' wilted. Inspection showed a dirty smelly area on the stem 3ft (1m) from the ground. This was 'flux'; when it was cut away the plant threw up strong branches from below. The montana wilted but not due to 'stem rot'. In 1999 my 'Recta' gave a wonderful display. But one day all the stems had wilted and died. Inspection showed it was due to phytophthora. The plant had wilted but not due to 'stem rot'.
One group, the Viticellas, need special mention. In my book The Viticellas. Trouble free Clematis I included a small number of Large Flowered Clematis. To be complete in my book I had to do this as they are so classed by the eminent authorities, Ernest Markham and also Mathews and Wilson in their books. I pointed out that they can wilt. It would be satisfactory to reclassify these clematis into Group VII the Late Large Flowered Group. 'Venosa Violacea' in the Viticella Group can wilt. Its heredity is obscure and 'Lanuginosa' may have crept in.
C. florida is best regarded as a member of Group VI and therefor susceptible. I have heard claims against C. 'Triternata Rubromarginata', C. texensis 'Princess Diana', C. montana 'Broughton Star', the herbaceous 'Aljonushka', and C. viticella 'Polish Spirit'. Here we must exercise great care. They may wilt but are they doing so because of 'stem rot' of phoma clematidina, or one of the other causes of wilt. I shall discuss shortly, especially phytophthora.
To the public it is important to mention that ten groups are free of 'stem rot'. Using the numbering in my classification these are Group I - Evergreen, Group II - Alpina, Group III - Macropetala, Group IV - Montana, Group V - Rockery, Group VIII - Herbaceous, Group IX - Viticella, Group X - Texensis, Group XI - Orientalis, Group XII - Autumn.
Other Causes of Wilting
Now that we know more about the action of phoma clematidina, it becomes apparent that there are many causes of flopping and wilting of clematis stems. A number of agents can attack clematis roots and cause wilting - vine weevil, phytophthora, nematodes, ants, wood lice, etc. Physical damage to a stem will cause it to flop and wilt, and so will the action of frost, heat, chemicals, water shortage and excessive watering. We must not confuse wilting due to these with wilting due to 'stem rot'.
To establish that damage is due to 'stem rot' is simple. Having cut off the stem, chip away at the suspected node with a sharp blade. The jet black area will immediately be seen - usually in the lowest node. Failure to undertake this simple action has retarded our knowledge of 'stem rot' for nearly a century.
Much confusion is caused by using the term 'wilting'. There are many causes of this and each should be spelt out. The action of phoma clematidina is to cause rotting of the stem. Gloyer called this 'stem rot'. This succinctly describes what happens to the plant. We should use this term so that there is no confusion with other causes of wilting.
Action at the Nurseries
1. Growers are already taking the most important single step in combating 'stem rot'. In Group VI, the Early Large Flowered Group, they are selling plants with more than one stem, often up to six stems. It means that if 'stem rot' involves one stem or even two, the remainder will produce a crop of flowers that may well satisfy the customer. This is highly desirable and makes redundant the undesirable notion that deep planting can produce extra stems from under ground (see later).
2. Fungicides will control the fungus in the nursery. It is worthwhile to sell a healthy plant as long as it is realised that the effect of the fungicide wears off in three weeks after sale and any clematis in Group VI is prone to develop 'stem rot'. Two or three fungicides should be used in rotation to prevent development of immunity to fungicides in the fungus.
3. Displays at nurseries tend to concentrate on the eye catching Early Large Flowered clematis. Those may remain in favour but there are eleven other groups; these can give the customer strong clematis flower displays all year long. These eleven groups need to be given prominence. I was surprised to read that an occasional reviewer of my book The Viticellas. Trouble free Clematis had failed to see the significance of the book; the disappointed customer is having his attention focused on a highly desirable alternative group rather than abandon clematis.
4. All researches point to the desirability of strict hygiene in the nursery. All dead material should be removed. Cutting tools should be sterilised.
5. Watering should be from below. Overhead watering increases the risk of infection as the fungus prospers on humidity and spores can be spread by splashing. Wesphal in Germany waters from below in his tunnels and he has no 'stem rot'. However, once the plants go outside and thus into rain he finds that infection can occur.
6. The closer plants are together, especially if they grow into one another, the greater the risk of transmitting infection.
Action in the Garden
1. When buying plants from Group VI, the Early Large Flowered Group, buy plants with more than one stem. If 'stem rot' attacks one stem the rest will survive to give you flowers.
2. Do not plant deeply. Plant a clematis as you would any other shrub. Disadvantages of deep planting are a) it is not necessary if you buy plants with a number of stems, b) the lowest node on a plant is most likely to develop 'stem rot'. If you plant deep the node will be in the soil where it is wet. This encourages 'stem rot', c) clematis are hungry feeders and need the nourishment in the top layer of soil, d) we will grow more clematis if we don't have to give them special treatment.
3. If a plant develops 'stem rot' cut the stem away below the node that is affected. Go down the stem until you see healthy leaves coming from a node. Cut the stem above this. You may often find there are no healthy leaves; if so then the lowest node is affected. Cut below this. If the top of the stem left in the ground is green you know that you have left healthy stem in the ground. This damaged stem should be burnt or destroyed. Chip away at the affected node on the stem to find the 'stem rot' and prove to yourself that damage was from 'stem rot'.
4. Continue to water your plant. The plant is not dead. Remaining stems will flower and new stems may appear from the ground.
5. Avoid wetting leaves when watering. The ideal watering method is the 'leaky pipe' system where the pipe is laid about 4ins (10cms) below soil level. Another effective method is to fix a pipe down to root level and water through that.
6. If fungicides become available to you then you must use them every month in the growing period. Alternate two to three different fungicides.
7. Put sand or grit round collar of plant to encourage quick drainage of water and thus reduce humidity.
8. If you have continuing bad luck with the Early Large Flowered clematis, switch to trouble free groups. The less vulnerable Late Large Flowered Group and the Viticellas will produce even more colour at about the same time of the year. You can have a trouble free clematis in flower every month of the year.
9. A plant in Group VI, the Early Large Flowered, planted away from other clematis will not develop 'stem rot' for a long time. Planted near other clematis it will develop 'stem rot' after a while. Planted amongst other clematis it will develop 'stem rot' quickly.
No doubt there is a place for the tiny phoma clematidina in the scheme of things. But it's getting out of hand. Understanding it we can better control it and retain our beautiful clematis in their proper prime place in the garden.