Clematis wilt

What is clematis wilt?

Clematis wilt is a disease of clematis caused by the fungus Phoma clematidina (formerly Ascochyta clematidina). Many of the large-flowered hybrid cultivars are very susceptible, but the smaller flowered species appear to be much more resistant.

Wilting has long been recognised as a serious problem in clematis. And although it is known that the fungus Phoma clematidina causes clematis wilt, there are cases of wilting when the fungus is not present. Research has clarified two important points:

1. Wilting in large-flowered hybrid cultivars may be caused by the Phoma clematidina
2. Wilting in resistant hybrids and species clematis is very unlikely to be P. clematidina, and is probably caused by environmental problems

So, although all wilting in clematis is often blamed on fungal infection, it is most likely to be caused by environmental problems, (unless it is wilting of the more susceptible large-flowered hybrids).

Clematis is a plant which, in the natural environment, prefers a deep and fertile soil in a moist and shaded habitat. However, in gardens, clematis are often planted in shallow dry soils in exposed sites, often close to buildings. In such circumstances they suffer from root stress which contributes to poor growth and what is loosely described as wilt. Overall, infection by P. clematidina is a problem for the nursery trade and specialist growers, but relatively uncommon in gardens.

Symptoms

The symptoms of fungal infection and environmental stress can be similar. The following symptoms are associated with fungal infection:

• When fungal infection occurs through the leaves, these wilt and the leaf stalks turn black. Leaf infection is followed by rapid wilting of the stems
• Fungal infection can also occur through stems. Freshly affected stems show black discolouration of tissue when split open
• Young healthy shoots may be produced from the base of affected stems, sometimes from below ground

Control

Non-chemical control

• Try to create a suitable root environment by deep cultivation and mulching, to minimise root stress
• If fungal infection is suspected, cut out all wilted stems back to healthy (non-stained) tissue and promptly destroy the affected material to prevent it contaminating the soil. New healthy shoots may be formed at ground level
• Particularly susceptible cultivars include: Clematis ‘Henryi’, ‘Vyvyan Pennell’, ‘Mrs N. Thompson’, ‘Duchess of Edinburgh’, ‘William Kennett’, ‘Marie Boisselot/Madame le Coultre’, ‘Ernest Markham’. Clematis 'Comtesse de Bouchaud'; C. 'Jackmanii' and C. 'Nelly Moser' are also susceptible
• Resistant cultivars and species include: Clematis ‘Avant Garde’, ‘Bonanza’, ‘Confetti’, C. alpina, C. montana, C. orientalis, C. viticella (which is tolerant rather than resistant), C. integrifolia, C. macropetala, C. tangutica

Chemical control

There are no chemicals available to treat clematis wilt.

Biology

Phoma clematidina can remain in the soil on dead plant material for many months. Infection occurs when spores are splashed or otherwise carried to leaves or young stems. Infection spreads rapidly to the shoots and stems, which when split open, are stained black. Infection causes rapid wilting and death. The root system is often not killed and young shoots frequently regenerate from or below ground level. The disease is not immediately fatal, but susceptible cultivars will usually die eventually.