John Howells' Work on Stem Rot (Clematis Wilt)

RESEARCH

While clematis has become a popular garden plant in the last 20 years its popularity is threatened by a virulent fungal infestation known as "clematis wilt". Soon after planting in the garden, the plant collapses and to the gardener appears to be dead.

The author reviewed the international literature on clematis wilt from 1860 onwards. Between 1860--1880 hybridising clematis became very popular. But the advent of clematis wilt by 1880 killed all interest. The responsible fungus was discovered by Gloyer, in New York State, USA in 1915. However no explanation was forthcoming as to the reason for the vulnerability of clematis to the fungus.

From his review of the literature the author formulated a hypothesis that the vulnerability to wilt was introduced to the hybridising programme by the use of Clematis lanuginosa and related clematis brought from China by Robert Fortune in 1851. See: Howells, J. 1993. Clematis wilt: A Review of the Literature. The Plantsman. 15. 148-160.

By using a published Dutch study and instigating two British studies, it was possible to identify 12 clematis accepted to be "high" wilters and 12 clematis accepted to be "low" wilters. The high wilters were from Group 6 of Howells' classification, the Early Large Flowered Group. Group 7, the Late Large Flowered Group is much less affected by wilt. No other Groups are affected. See: Howells, J. 1994. Vulnerability to Clematis Wilt. The Clematis. 51-61.

In the next study, the geneology of the two samples was examined. Clematis lanuginosa was present in the background of every "high" wilter. Clematis lanuginosa was rarely present in the background of the "low" wilters. Thus Howells' established that the vulnerability to stem rot was introduced by using Clematis lanuginosa and related clematis in the hybridising programme. See: Howells, J. 1994. The Genetic Backgroung of Wilting Clematis. The Clematis. Pp.62-67.

 


THE LESIONS OF 'STEM ROT'

The damaging fungi are frequently found on the leaves of clematis and may even make lesions on 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 (See plate - later). For a distance of about 1.5-2ins (4-6cms) 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. See: Howells, J. 1996. The Lesion of Stem Rot. The Clematis. P.54.

This photograph illuminates the fungal 'stem rot' (wilt) in clematis. Two clematis nodes (marked with an arrow) are seen here. The right node has healthy green tissue above and below it (the flow from the roots upwards is indicated by red arrows). The left node has healthy green tissue below it but at the node itself this tissue has been destroyed by fungus leaving a black mass for about 2in. (5cm). Above the left node the tissue is cut off from nourishment by the fungal infection and the tissue turns brown as it dies.

See: Howells, J. 1996. The Rose and the Clematis as Good Companions. U.K. Antique Collector's Club. P.170


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.

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.

Additionally, of course, fungicides should be employed for preventive puposes but the long term solution to stem rot is to remove C. lanuginosa and its progeny from the hybridising programme. See: Howells, J. 1996. The Lesion of Stem Rot. The Clematis. P.55.

 


For more clematis information by John Howells click the highlighted titles below.

Howells' Gardeners' Classification of Clematis

Pruning the Twelve Groups of Clematis

Round the Year Care of Clematis

Five Ways of Combining Roses and Clematis

Twelve Easy Combinations of Roses and Clematis

Exploding Myths About Clematis

Pronouncing Clematis

Clematis for North Facing Walls

A Brief History of Clematis



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