Sometimes called clematis leaf and stem spot, this fungal disease has been a problem for growers of large-flowered clematis cultivars for more than a century. Despite its devastating appearance, clematis wilt is rarely fatal. Even when all the top growth is killed, healthy new shoots often emerge from basal buds.
BIOLOGY: The fungus overwinters in soil and in diseased plant debris. Moist, humid conditions and warm temperatures trigger the germination of spores that have been splashed onto susceptible tissue. Growth of the fungus ultimately blocks the uptake of water in infected stems.
SYMPTOMS: Black, irregularly shaped spots develop on the leaves, often on the undersides. Similar spots occur on the stem. Rapid wilting and browning of foliage often occur, sometimes when the plant is in midbloom.
CONTROL: Clematis wilt is a wound pathogen. It is most likely to invade cracked or damaged stem tissues, near the soil surface. Older plants with heavy stems are less susceptible than young plants, which can be protected by carefully tying the stems to reduce the chance of wind injury.
Avoid overhead irrigation, and provide plants with well-drained soil, six or more hours of sunlight, a soil pH near 7.0, and good air movement. Planting a clematis so that the crown is two-and-a-half inches below soil level will encourage the production of new basal growth should infection occur. Remove infected stems, cutting back to healthy tissue, and burn or discard the prunings.
Large-flowered, early-blooming clematis hybrids are most susceptible to infection. By contrast, the smaller-flowered clematis, such as C. alpina, C. macropetala, C. montana, C. viticella, and their cultivars are relatively resistant.