How to Treat Clematis Wilt

In the book Clematis as a Garden Flower by Thomas Moore and George Jackman, published in 1877, there is no mention of the word wilt! Was there no such thing as Clematis Wilt in those days? It seems that such was the case judging from their enormous popularity a hundred years ago. Since then, however, wilt is a word to strike terror into the hearts of all clematis devotees. Did Clematis Wilt exist one hundred years ago? or was it hushed up in the catalogues and in Jackman’s and Moore’s book? If not, when did it start? It was prevalent in the early 1900s as William Robinson wrote a slim volume on clematis which I once possessed but have now lost. In this, writing I think about the year 1914, he accuses Nurserymen of bringing wilt into clematis by the practice of grafting. This has since been proved to be quite wrong, as when clematis are grafted, the stock is only used as a nurse-stock; by the time clematis are planted out into the garden the scion has produced its own roots and the stock is discarded. The modern method of producing clematis in most Nurseries these days is by cuttings and these have wilted just as easily as grafted plants. A gleam of hope has emerged, however, with the introduction of Benlate, a new fungicide which seems to exercise a certain control over Clematis Wilt, as one of my customers remarked in his letter, ‘Benlate than never.’ Gardeners would be well advised to spray the base of their plants with Benlate in the autumn and again in the spring and once a month during the summer. It seems to be at the base of the plants where this attack takes place. When wilt occurs it is as though the plant has been cut through with a knife or hoe. Everything above this spot hangs limply and it is especially infuriating when the plant is in full bud or bloom and was an apparently healthy-looking plant. The only thing that can be done when this happens is to cut the plant right down to the ground, no good watering and spraying the plant, the damage has been done and all that remains to do is to remove all the wilted stems and foliage. Provided the clematis has been planted deeply, then there is every chance of shoots soon coming up from the buried first pair of nodes. Water and feed well to encourage these nodes to develop.

Many theories have been advanced as to the cause of Clematis Wilt. Ernest Markham in his book Clematis, published by Country Life Ltd., even has a name for the fungus that is supposed to be the cause of the disease. He says, ‘In 1915 a Bulletin was published which dealt with Ascochyta clematidina, as this fatal disease is named. This Bulletin was written by Mr. W. O. Glover and published by the New York Agricultural Experimental Station in Geneva, New York, U.S.A.’

The action of the fungus was described as follows: ‘The plants are killed by the growth of the fungus down the petiole into the stems, thus girdling the plant at the node. The stem may be girdled also by the lesions anywhere in the internodes. Dead stubs left on the vines are a means of holding the disease over a period of time. New shoots may be formed below the girdled region, but the downward progress of the fungus ultimately kills the plant if the diseased tissue is not removed.’

He goes on to say that various spraying experiments were carried out, such as dusting with sulphur, spraying with Bordeaux mixture and with a mixture of 1 lb. soap and 6 lb. sulphur. The last-named seemed to be the most effective, but that sulphur alone was quite useless. He also goes on to say that clematis may become damaged in gales with vines cracking and allowing fungus to enter into the broken tissues.

In the book Garden Clematis by Stanley Whitehead, published by John Gifford Ltd., the author says that the disease has been known for over fifty years (his book was published in 1959), which seems to support the idea that a hundred years ago it was unknown. He says that the fungus Ascochyta clematindina described by the New York Experimental Station is not the same as Clematis Wilt, but is a form of stem rot or leaf spot, which leaves Clematis Wilt unidentified with no clue as to the cause or cure.

Christopher Lloyd in his book, Clematis, published in 1965 by Country Life Ltd., says, ‘My own unsubstantiated suspicions fall on our old enemy the Grey Mould fungus Botrytis cinerea. It is so like it to enter living plant tissue by way of a damaged or dead leaf stalk, since the large-flowering types never shed their leaves cleanly. Against this is the evidence that the fungus sometimes does its damage not at the node, but in between nodes.’

My own personal pet theory, which may be quite wrong, is that it is not a disease at all but a failure of the very thin stem to cope with the sudden demand for moisture from the stem leaves and flowers which results in a breakdown of the tissues at a certain spot. These die, causing everything above to be suddenly cut off from its life-giving sap, and consequently everything collapses. These collapses usually occur at times of high humidity when the plant is growing rapidly and requires several pints of water a day. So my answer is to make sure they have a good supply of water at the base of the roots by giving them a good soaking two or three times a week. Then as water washes out the nutrients from the soil they must be replaced by a liberal application of a general liquid fertiliser once a week. One reason that leads me to support this theory is that clematis grown by rivers always seem to be beautiful plants, and several people who live in such spots have told me that their plants never suffer from Clematis Wilt. With their soil, one has only to dig down a foot or two to come on to water, which means that the plants have a constant supply of water. The stems of their plants are also much thicker than plants of those less fortunate people who have to contend with hot, dry, hungry soils, and so perhaps are more able to draw up the extra moisture demanded at peak periods, without causing damage to the tissues. One way to give the plant a constant source of water is to bury a container about 2 feet underneath the clematis before planting. Fill with stones, top up with water and cover with peat. Then plant in the normal way. A pipe driven down to the container will enable you to top up with water during drought periods and your clematis should have a constant supply of moisture. Even a pipe driven down to below the roots will enable one to get water down to what must be a very dry spot.

Another way to help to avoid Clematis Wilt is to layer two or three shoots around the plant, which provides the plant with extra roots. The only snag about this idea is that one has to grow the plant a suitable size before it can be layered, and then it takes a year for the layer to root, and it is usually during this period that wilt occurs! Once a plant has developed a good thick stem, or, as with a Jackmanii variety, has several stems coming out of the ground, then wilt seldom happens.

In 1965 experiments were tried out in Holland and the following report was given in the Boskoop Annual Report:

Small-scale trials were carried out at an Experiment station. Large-scale trials on commercial holdings.

Infection trials were with isolates from two fungi: Ascochyta clematidina and Coniothyrium clematis-rectae. The following treatments were given: 1.: Mixing diseased plants from the previous year with potting soil of this year. 2.: Shoot wounded and agar with fungus mycelium applied. 3.: Agar with mycelium applied to shoot without wounding. 4.: Spore suspension of fungus injected into shoot. 5.: Spore suspension of fungus rubbed into leaf. 6.: Culture filtrate injected into stem. (No fungus injected, filtrate only.) Results: Treatments 1 and 6 gave no wilting.

Field trials with fungicides have been inconclusive but it is suggested that control might be possible with 0.1 per cent Tuzet applied to the base of the plant.

We often get letters from customers on the subject of wilt, and Mr. Jim Hodgkinson of Frodsham near Warrington has two theories. His first suggestion is that it is caused by a symbiotic union getting out of hand but he writes: ‘I’ve been looking into the reason why I seem to be so lucky in not getting wilt on my clematis, whilst others seem to get ‘it’ so frequently, either I do something, or don’t do something, which others miss. One thing I have noticed is that, as you know, when the stem swelk in the autumn, the epidermis splits longitudinally to reveal the tender cortex and inner portions. At this time I always spray with a fungicide (Benlate of Karathene) as, until the tissue hardens, the stem is open to attack. It could just be possible that spores enter the plant at this time and only reproduce in the spring, so that by the spring and early summer we already have a fungi within the plant, which have entered, not at the height of the growing season, but in the autumn.’

Mr. Hodgkinson’s theory, then, is supported by the findings of the Dutch trials when they suggest that the base of the plant should be sprayed with a fungicide.

Mr. R. W. Sidwell of Ashton-under-Hill, near Evesham, writes: ‘Wilting of leaves is due to failure to replace transpirational water losses. This may be due to infection with plant pathogens and there seems no doubt that under some conditions this can be the cause with Clematis Wilt. Cases are, however, known where it seems improbable that infection is the primary cause. ‘Most plants are capable of developing a certain amount of root pressure which may be sufficient to force water to the tops of the shoots but under conditions of high transpiration this is usually insufficient to replace lost water and the water columns then become in a state of tension rather than compression. This is certainly the case with clematis when at maximum water demand. ‘Water travels up plant stems mainly through the wood vessels which are literally water pipes. In the case of clematis these wood vessels are few in number and very large in size. They remain functional for about one year only and are replaced annually by fresh cambium growth. This exceptionally large size of wood vessel makes the water transport system of clematis particularly vulnerable. Stems with large numbers of small vessels are much better able to cope with heavy stresses in water demand. ‘All the cases of wilting that I have observed on planted-out plants could be explained by water shortage in the root zone, either through competition from other plants or other reasons. Plants grown in deep rich moist soil free from immediate competition have never wilted in my experience. ‘Wilting that I have observed is often just at peak growth, reached in early summer. This might possibly be connected with the ageing of the wood vessels before the newly developing ones are able to take over. This is pure theory but is worth investigating.’

Mr. Sidwell seems to support my theory, then, especially when he notes that plants grown in moist soil have never wilted as I have noticed that plants grown near rivers never wilt. So the answer here is lashings of water if your soil is light and dry.

My answer to Clematis Wilt then is, plant deeply with 3 or 4 inches of the stem below the soil, spray the base of the plant with Benlate in the autumn, again in the spring and during the summer at regular intervals. Keep the plants well watered and fed, especially during humid and quick-growing spells, make sure the soil is moist below the roots, and with a bit of luck this should prevent any attack of so-called wilt.

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