The term clematis 'wilt' has always been unsatisfactory as the stems of clematis, like those of other plants, can 'wilt' for a number of reasons, the commonest of which is lack of water. Additional causes of 'wilt' are physical damage to the stem, vine weevil damaging the roots, excess of fertilisers, lack of essential minerals, and root damage by ants and wood-lice.

Now that the pathology of the disorder is clear, and the lesion described, it is helpful to use for the lesion a particular term which gives a general description of the lesion. 'Stem rot' would seem to be appropriate.

Cause of 'Stem Rot'
'Stem Rot' results from the action of fungi on a plant vulnerable to the action of those fungi. The first clue to the likely vulnerable parent of clematis hybrids came from the result of the first hybridisation by Anderson-Henry in Edinburgh in 1855. He crossed C. fortunei and C. lanuginosa to produce the hybrids 'Henryi' and 'Lawsoniana'. Both hybrids are high wilters. Thus both C. fortunei and C. lanuginosa were suspects as the original host to vulnerability. Soon G Jackman crossed C. lanuginosa with C. viticella to produce the popular hybrid 'Jackmanii'. C. viticella and C. lanuginosa, again, were suspects. 'Jackmanii' also wilts but to a lesser extent than 'Henryi' and 'Lawsoniana'. But C. viticella is known not to wilt and it may be able to contribute a degree of protection against 'wilt' to its offspring. This left C. fortunei and C. lanuginosa as suspects. The plant responsible for wilt acted on a huge scale as in 20 years the clematis trade was in disarray because of the massive incidence of 'wilt' as it was called. Clematis lanuginosa was employed on a huge scale in hybridising and thus it had to be the prime suspect. But it was necessary to test the hypothesis that it introduced vulnerability to 'wilt' by systemic investigation.

Studies in Holland listed the degree of susceptibility to 'wilt' in a number of popular clematis.1,2,3 With the help of prominent growers in the UK it was possible to make an up-to-date list of susceptibility in popular clematis in the UK. The lists in Holland and UK were in general agreement and from the two lists it was possible to make a joint list. It was crucial to know what was the difference in the genetic background with 14 low wilting clematis. The result was dramatic and clear cut.4 All the high wilters on which there was information had C. lanuginosa as a direct or indirect parent. The vulnerability to 'wilt' therefore came from C. lanuginosa.

Another finding of the investigation was that most wilting occurs in the early Large Flowered clematis; in the UK list 100% of the high wilters came from the early group. The late Large Flowered group, the Jackmanii group, is much less affected.

The fungi causing 'wilt' have been identified as aschochyta clematidina (transferred to genus Phoma in 1979) and coniothyrium clematidis-rectae. There is a recent suggestion that the two fungi may be identical.5


The lesion of 'stem-rot'
The damaging fungi are frequently found on the leaves of clematis and may even make lesions in the leaves. However, lethal damage is only done when the fungi penetrate the stem through the nodes; this is especially prone to happen if the stem has been damaged. Young green clematis stems in conditions of humidity and at a temperature between 20-25oC are unable to summon defence mechanisms to counter the invasion by the fungi. A characteristic lesion is then formed at the node. For a distance of about 1_-2in (4-6cm) near the node the tissue is destroyed. The damaged area at the node becomes a characteristic black colour. As the area around the node dries the tissue falls out as a dry black powder. The area above the node turns greeny-brown, then brown and brittle and dies. The stem below the affected node remains green. The lesion extends right across the stem; this is why the action is so damaging as no sap can pass the node. Thus the stem above the node wilts as if the stem had been physically severed.

The selected node in my experience is invariably close to the ground in the bottom four nodes. It can however start at an upper node or even a node on a branch stem. In the past there were many speculations on the nature of clematis 'wilt'; had the stems been cut open the obvious and incontrovertible cause of the stem death would have been seen.

Management of 'Stem Rot'
When a stem 'wilts' it should be severed across the green part of the stem below the dead area of the stem. This is invariably low down on the stem. Thus cut the stem just above soil level if the whole stem is involved. The end of the stem left in the ground will show its green face to you and you will know that the lesion is above this level. The lesion can be identified using one of two ways after placing the stem lengthways on a table:

1. Chip away with a sharp knife at each node starting at the bottom node of the stem. If wilt is present you will quickly see the black area in one node. Below that node the stem is green, at the affected node it is black and the stem is brown above.
2. With care and using a sharp knife or razor blade knife it is possible to start at the bottom of the stem and slice up the middle of the stem dividing it into two parts. Sharp scissors will also do this work. As you start up the stem you will be in green tissue, at the affected area this gives way to the characteristic black lesion and above is the brown dying stem.

If you do not find the black area then your clematis has not died from 'stem rot' but from one of the other causes of wilting listed above.

Your next action is crucial. Stem rot rarely kills clematis. The gardener is the killer. The inexperienced assume the clematis is dead and give it no further care. Thus it dies. The correct action is to gently explore the top half-inch of soil around the stem. Frequently you will find that the roots are already sending up shoots. So often does this happen that it is tempting to speculate that the roots have already re-acted to the fungal pruning of the stem by throwing up shoots. To find the shoots will confirm your belief that the plant is alive and needs your care. If no shoots are apparent then keep up your usual care, especially watering, of the plant and shoots will soon appear. Shoot appearance can be encouraged by deep planting of the clematis which leaves some stem nodes below ground from which extra shoots can appear.

Additionally, of course, fungicides should be employed for preventive purposes and the long-term solution to stem rot is to remove C. lanuginosa and its progeny from the hybridising programme. This, and other approaches, are too lengthy to be followed in this report.

References:
1. STEEKELENBURG VAN, N.A.M. 1971. Verrwelkingsziekte in clematis. Jaarboek Proefstation Bookwekerji, Boskoop, 1971. Pp88-91.
2. STEEKELENBURG VAN, N.A.M. 1972. Verrwelkingsziekte in clematis. Jaarboek Proefstation Bookwekerji, Boskoop, 1972. Pp103-105.
3. SLAVEKOORDE, S.M. 1973. Verwelkingsziekte in clematis. Jaarboek Proefstation Bookwekerji, Boskoop, 1973. pp82-85.
4. HOWELLS, J. 1994. The genetic background of wilt in clematis. The Clematis pp.62-67.
5. CHARTIER-HOLLIS, J.M. (1996). Personal communication.