Clematis are flowering vines that frolic across fences, trellises, pergolas and even hedges. Clematis are either deciduous or evergreen and come in a wide range of flower colors, shapes and sizes. The vines can be early season bloomers that flower in spring, summertime or fall types. Clematis are remarkably resistant to disease and pests. Clematis wilt is a known disease that can cause serious loss of health in the plant.
The disease starts out in the clematis stems and can bring about stem collapse. It moves to leaves, where it is most obvious. Necrotic, dry brown spots pop up on the foliage. In the soil you may be able to spot small black fruiting bodies around the base of the plant. The stalks will turn black as the leaves wilt. Then the entire stalk wilts and becomes limp and dry. If you split an infected stem you will find the tissue inside has turned black.
The disease is a fungal one caused by Phoma clematidina, which was formerly known as Ascochyta clematidina. Basic wilt symptoms may indicate that cultural issues are the problem but when accompanied by the black coloring the problem is likely the fungus. It is more prevalent in the large-flowering clematis. The fungus remains in soil for months and is conveyed by water splashing onto the clematis. The plant may come back the following season with no permanent damage.
Clematis should be watered slowly to prevent muddy spatters. Sulfur can be applied in early spring to control the disease but it needs to be used before symptoms occur. Heavily amended soils with compost and organic grit are less likely to harbor the fungus. Keep the ground clean and free of leaves and dropped plant parts which might harbor the disease. Pruning implements should be sanitized between uses to kill any spores. Clematis that are planted 2 inches lower than usual seem to better withstand attack from the fungus.
Plant resistant varieties such as Clematis macropetala, alpina, montanta and viticella. Most affected clematis will recover by themselves in the following season and no control is required. Pruning out the infected stems and leaves can help prevent the disease from spreading to the entire plant. If it is severely infected the entire vine needs to be cut down to within a few inches of the soil and allowed to return the following year. Ornamental fungicides, such as thiophanate methyl or copper sulfate, can help kill off the spores before the fungus can form.