UNDERSTANDING CLEMATIS WILT

by Dr John Howells

First published in Garden News, Nov. 3rd 1999


New knowledge has brought hope to managing clematis wilt. We now know that only one group of clematis out of 12 is vulnerable. Another occasionally wilts but too rarely to need treatment. Ten groups are entirely wilt free. Furthermore, the fungus almost never kills a plant. Ignorance fuels alarm, understanding brings promise.

The Enemy
Faced with an opponent, it helps to know that opponent. Better still put yourself in the place of the enemy - in this case a tiny fungus - phoma clematidina.

This little chap can only feed on clematis. For centuries it had a thin time. Then Jackman in 1860 hybridised Jackmanii. This beautiful plant had such an impact that clematis took off. By 1880 Jackman had over 400 new clematis in his catalogue. Other nurseries followed all over Europe. Now the tiny phoma never had it so good. The best way to start an epidemic is to pack people together. The same applies to clematis wilt. It spread and spread so that it brought the industry to a halt by 1880. But the gardeners and growers then had little notion of germs and fungi. Now we do.

The Fungus
Gloyer, a scientist in New York State, USA, discovered the fungus in 1915. His finding was ignored until recently when his finding was confirmed in the UK, Holland and New Zealand. But there was another conundrum. Working with a group of expert gardeners and growers we found some years ago that the fungus only attacks one group of clematis - the Early Large Flowered group with clematis such as 'Nelly Moser', 'Elsa Spath', 'Countess of Lovelace', 'W.E. Gladstone', etc. This finding was confirmed by research at the University of Derby. So why is this group vulnerable? I found that it was due to the fact that C. lanuginosa, the woolly clematis, from China had been extensively used as a parent in the last century. This is very vulnerable to wilting and it has parented most of the clematis in the Early Large Flowered group, the vulnerable group.

How It Destroys
All fungi need moisture for growth. So does this chap. He also likes a warm day, ideally about 70oF (23oC). He starts his growth on a leaf. This extends and grows down the stalk of the leaf to the node on the green stem where the stalk starts. Ten of the clematis groups in my classification repel the invader. The eleventh partly repels the invader - the Late Large Flowered group, the Jackmanii group. The twelfth, the Early Large Flowered group, with C. lanuginosa in its background can't defend itself and in goes the fungus.

Once in, the fungus destroys an area of about an inch either side of the node. This rots and turns black, jet-black. That is why it would be better for us to talk of ëstem rot' rather than ëwilt' as this term tells us what is happening inside our clematis.

The fungus does so much damage because it destroys right across the stem. The sap coming up the stem can't get past the node so the stem above withers (wilts) and turns brown - just as if you had cut across the stem.

The fungus finds young green stems much easier to attack. As the plant gets older and the stems get woody and brown it's more difficult for the fungus to get into the stem.

What can you do?

The most obvious thing you can do is to buy a good plant with more than one stem. If you loose one, the others will still prosper.

It was thought that to plant deep would encourage roots to strike on nodes below the soil surface and make extra plants. So if you lost the main plant you might have a subsidiary plant. But what you have done is to pull a node or nodes into the top of the soil where the ideal moist conditions for fungus growth exists. So don't plant deep. Plant at the normal depth for a shrub, leave the nodes above soil level and buy a good plant.

Reduce moisture by encouraging drainage at the crown of the plant with a 3" (7.5cm) layer of sand or grit. If you can, use a leaky pipe watering 4" (10cm) below the surface.

If wilt strikes do the 'Jim Fisk' treatment. Cut across the stem below the affected node. This is often the lowest on the stem or the next three above. This will be the lowest node that has no leaves coming out of it. After the cut, the stem top that you leave in the soil will be green, so you know you are in healthy tissue. Burn the brown stem as it has fungus on it. It is at this stage that the plant dies. It is you who could kill it by assuming the roots are dead! They are alive but need water. Keep looking after the plant and in days new shoots will appear from below the soil. The fungus pruned the plant for you, but rather too early, and you may miss flowers that summer.

There is no fungicide at the moment recommended for use by gardeners, but a new lot is coming, certainly for the growers. Whether gardeners will use them in this organic world is another matter.

Clematis plants can wither for other reasons than an attack by this fungus. For example the action of your hoe, lack of water, excessive cold or heat, vine weavil at the roots, action by ants, etc.

If you like getting to the bottom of things, then having separated your wilting stem from the plant, put it down on a table and using a razor sharp blade cut lengthways into each node starting at the bottom node. In one node, often the bottom one, you will now see the black, jet-black, area where the fungus has been at work. Give me encouragement by writing to me. Too many people still won't believe the obvious.

Reproduced by kind permission of Garden News.




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