Over the years, native clematis varieties were hybridized to create larger blooms, and some of the hybrids, such as Jackmanii are very popular in the nursery trade. They produce beautiful flowers of purple, pink or white with many double flowering and bi-color varieties available. The hybrids do not grow as vigorously as the native varieties that are adapted to adverse growing conditions, such as hot and dry weather. Also, they are more susceptible to certain diseases, such as clematis wilt (Phoma clematidina).
Clematis wilt (Phoma clematidina) is also called "clematis wilt and leaf spot" and is first noticed on the leaves of a clematis vine. The leaves turn brown on the tips and brown spots form on the surface of the leaves. Occasionally, it may show up as yellowing leaves near the bottom of the vine. Eventually, the disease spreads to the stem of the plant and works its way to the bottom part of the stem at the soil line. When that happens, the fungus becomes more virulent and attacks the inner tissue of the plant stem preventing water and nutrients from reaching the top of the plant, and the plant wilts.
Clematis vines affected by clematis wilt disease usually recover as it does not affect the roots of the plant. Cut the vine at or below the soil line below where the fungus has affected the plant. You can see the black fungus where it has attacked the base of the plant. Water and fertilize as normal, but do not increase water and fertilizer applications. Fertilize once a year with a fertilizer with N-P-K ratio of 15-5-5. Spread the fertilizer outward 36 inches from the plant because of the extensive root system of the clematis vine. It can take as long as three years for the vine to send up new shoots, but it will usually recover within a year.
Clematis wilt can be prevented by planting the clematis deep enough so two set of leaf nodes are located below the soil line. Pinch off any leaves that will be below the soil line. The extra stem below the soil line is important to have if you have to cut away a major part of the infected stem. It also prevents the disease from getting close to the root section, since clematis wilt does not usually grow very far below the soil line. Water the area around the roots of the plants only and avoid wetting the leaves of the plant, especially during damp warm weather. Keep a 2-inch layer of mulch around the plant while leaving 1 inch of space between the mulch and the vine. Mulch prevents rain from splashing fungal spores from the soil onto the plant. Clean up any fallen clematis leaves from around the plant, especially if the leaves are from an infected plant. The planting location should have good air circulation and be permanent, as clematis can live many years. Clematis grow best where the top is exposed to the morning sun and the roots remain shaded. Provide a strong trellis so the vine remains upright and are not damaged by high winds.
Large flowered and double blooming clematis varieties that produce growth on old wood are most susceptible to clematis wilt. Examples of these varieties are: Henryi, Jackmanii, Nelly Moser, Marie Boisselot and Mrs, Cholmondeley. These varieties are also referred to as type 2 clematis. They are pruned in early spring before they produce leaves by removing dead top growth and cutting the live stems back to the first set of healthy green buds.