Your clematis had been growing vigorously, but suddenly a stem wilted, often just before its flowers open. Then the stem and the leaves turned black or brown. Sometimes only a single stem was touched, but in other cases, the whole plant died. What happened?
Your clematis was likely hit by clematis wilt. This used to be a very mysterious disease. Back when I was a beginning gardener 50 years ago, no one knew what caused it. Today though we know it is due to a soil-borne fungus called Phoma clematidina (Ascochyta clematidina) which blocks the flow of sap, hence the quick decline. It primarily attacks large-flowered hybrid clematis.
The spores of clematis wilt overwinter in the soil and in infested leaves and stems. When it rains or when you water, water droplets bounce off the soil, picking up a few disease spores as they go, and deposit the sticky objects on the plant’s stem. That in itself won’t cause any symptoms, as the spores are incapable of penetrating the stem on their own. However if there is any kind of injury to a stem or leaf, the fungus may penetrate and if so, starts to develop, soon blocking sap flow and resulting in wilt. Since the disease is most active in early summer when the air is humid, avoid pruning or even touching clematis at that time of year.
Note that clematis wilt is not a systemic disease: it does not circulate in host plant’s sap. That means while the part above the infected spot may die, the disease won’t migrate to lower stems or the plant’s roots… unless there are other injuries. So quite often the plant seems to recuperate fully, only to be hit again a few years later when the spores do reach a wound.
- Choose a suitable location for growing clematis: a sunny spot with rich, well-drained soil that enjoys good air circulation. Ideally, the soil would be slightly alkaline (a pH of around 7), as this will also help prevent the disease.
- While moderate air circulation is important, avoid windy sites. They can cause fragile clematis stems to move too much, provoking wounds.
- Plant disease-resistant clematis. Most species clematis and small-flowered hybrids seem little affected by the disease. Large-flowered hybrids are riskier. Some of them, like ‘Vyvyan Pennell’, ‘Duchess of Edinburgh’, ‘Ernest Markham’, and ‘Madame le Coultre’ are considered especially susceptible to clematis wilt.
- Avoid hoeing or cultivating at the base of the plant.
- Cover the ground at the plant’s base with a thick mulch.
- Grow plants at the base of clematis to shade their roots.
- When you water clematis, try not to moisten the foliage. A soaker hose covered with mulch is a good way of keeping a clematis’ roots moist without ever getting splashing water on the leaves or stems.
- Remove diseased stems rapidly, being careful not to damage other stems as you prune. If an entire stem has wilted, cut it back to under the ground. Disinfect the shears between each cut.
- Give your clematis good basic care (watering during drought, occasional fertilization, etc.).
Best of luck with this… but if you’re seriously into clematis, it’s not a disease you’ll likely be able to avoid entirely.