In clematis we can see a phenomenon which is not uncommon, when the stem suddenly becomes weak and flops, droops, sags, or becomes limp, and after this the stem dies. We have chosen to use the term 'wilting' to describe this. This drooping and flopping can occur due to any serious damage to the stem or to the roots that support the stems.
The simplest damage is a mechanical injury to the stem. An injury across the stem can cause death of the stem. Common causes affecting the root are water shortage or excess of water. But there is evidence1 to suggest that a number of other agents, non fungal, can cause wilting. These are viruses, bacteria, insects and nemotodes, either damaging stems or roots.
Fungal organisms can also cause 'wilting'. Well known is the fact that a fungus phoma clematidina, can damage a stem and cause 'wilting'. Less well-known is the fact that other fungi can cause 'wilting'. Walter Hörsch2 has brought to our attention that the fungus coniothyrium clematidis rectae can do this. He is referring to the work of Block3 and Rattink4 in Holland. Both were working on the efficacy of a number of fungicides. They felt that coniothyrium clematidis rectae was more damaging than phoma clematidina. Walt refers to the beautiful catalogue of Westphal (2000) where there are illustrations of damage both by rectae and by phoma clematidina. In all probability all the illustrated damage, both to leaves and stems, could be caused by either of the two fungii. Walter's vivid description of damage to 'Vyvyan Pennell' (shown by two investigations to be most vulnerable of all clematis) is a classic description of the effect of phoma clematidina. A later investigation from Holland5 stated that phoma clematidina was the commonest cause of 'wilting'.
Two other fungi by damaging roots can also cause flopping or wilting. These are alternaria and phytothera. Both are frequently found in the roots of clematis.6 The first is probably non pathological. The second causes 'root rot' and the death of the whole plant.
Over the world phoma clematidina has been established as the commonest threat to clematis. Gloyer, in 1915, proved that phoma clematidina caused 'wilting', which he precisely called 'stem-rot'. Since then phoma clematidina has been found the commonest causative agent in the UK, Holland and New Zealand.7 It causes browning of leaves in all 12 groups of clematis. But, it can only enter by the node in the Early Large Flowered Group as this group has an inherent lack of defence mechanisms against the fungus. The damage is verifiable by any gardener - cut through the damaged node (usually the lowest node) of the one affected stem. There will be seen the dark black damage caused by the fungus, obstructing the stem at the node and causing death to the stem above the damage.
Gloyer wisely advised against using the imprecise term 'wilting'. As can be seen above, 'wilting' can be caused by a multitude of agents. He saw rotting of the stem and called it 'stem-rot'. We should adopt this term, or use the even more exact term 'phoma stem-rot'. Thus we can differentiate from phytophthora 'root-rot', nemotode root damage, viruses, bacteria, damage, etc, etc.
REFERENCES
1. Van de Graaf, Pieter. 'Non-fungal Organisms as a cause of Wilt in Clematis'. The Clematis. 1997. P.47.
2. Hörsch, Walter. 'Clematis Wilt Again'. Clematis International. 2001. P.66.
3. Blok, I. 1964. 'De Verwelkingsziekte van Clematis'. Netherlands Journal of Plant Pathology. 70. 67.
4. Rattink, H. 1968. 'Verweckingsziekte in Clematis'. Jaarboek Proefstation Boomkwekerij. Boskoop. 1969. P.66
5. Kuik, Van A., Brachter, E. 'Wilting in Large Flowered Hybrids'. De Boomkwek Erij. 1997. 9. 16.
6. Wolff, W.G. and Hall, A.M. 'The Biology and control of clematis wilt'. The Clematis. 1996. P.49.
7. Howells, J. 'Clematis Wilt. A review of the literature'. The Plantsman. 1993. 15. 148.
