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This fungal disease problem is called "clematis wilt" by the British, and identified in some American books as "Clematis leaf and stem spot." The British term describes what symptoms I saw much more accurately. I didn't notice leaf spotting before the plant shrivelled, although the lower leaves did look a bit yellowed. The stem dies back because it is attacked near the ground and the plant can't move water through its veins. Damp weather can increase the fungal activity, and the damage often seems to occur in early summer when the plant is growing vigorously.
The good news is that clematis plants often recover. This disease doesn't attack the roots, which will send up new shoots even after the major part of the plant is annihilated. One resource on this subject states that renewed shoots can appear up to three years after the problem, so do not lose heart if this happens. One clematis so affected in my garden, Clematis x jackmanii 'Superba,' collapsed at the beginning of the spring growing season, sent up a new shoot, and is now in glorious deep blue bloom. The British writer Christopher Lloyd notes that his 'Nelly Moser' disappeared for a year and then rebounded in health and vigor.
Prune out any affected stems, cutting as close to the root as possible. The point of attack may be just below the soil surface. Not all stems may die. It's possible to have an actively growing stem emerging from the same root as a dying stem. Several British growers have also remarked that once the plant is established enough to have a heavy, tough stem the problem seems to lessen. In my garden, however, the wilt has kept these vines from developing long-lived stems.
In general, the large-flowered clematis do best with well-drained, evenly moist soil nicely amended with organic matter. The toughest places to grow them would be on heavy clay or in sandy light soils that dry out fast. A 2 to 3 inch mulch also helps with water retention. Fertilize clematis as roses, once a month until about mid-July. In planting a new vine, set the root about two inches below the surface.
Keep the roots watered during summer, even after cutting out the dead foliage. Large-flowered clematis doesn't handle drought well. WSU's Disease Control Handbook lists sulfur as a fungicide which, if it's going to be used, should be applied before symptoms are seen. Be sure to remove all the dead foliage before allowing the plant to winter over (even if taking the whole stem off isn't the proper pruning technique for that type of clematis.) The fungus lives over winter on the affected stems and can re-infect the new spring growth.
The fungus, identified by researchers as Ascochyta clematidina, infects the larger-flowered clematis most often. To have clematis free of fungal problems, you can choose from several possibilities. Clematis montana (pink), Clematis macropetala (blue, early spring) and Clematis alpina , and Clematis viticella are reported to be resistant, as well as some of their cultivars. All of these, of course, have tiny flowers that will fail to raise the heart rate of gardeners who love the saucer-sized elegant large flowers.
One excellent book on clematis is Christopher Lloyd's, Clematis. (revised with Tom Bennett, Capability's Books, Deer Park, Wisconsin, 1989.)
Hortsense: Managing plant problems with Integrated Pest Management
