For a few years now I have heard people refer to a problem they call ‘clematis wilt’. When I look in standard pathology texts, I do not find this disease listed. Westcott’s Plant Disease Handbook (5th ed.) makes no mention of it. Pascal Pirone in Diseases and Pests of Ornamental Plants discusses a stem rot caused by a fungus, Ascochyta clematidina, infecting stems near the soil line, girdling the stems and causing tissue above to die. The same fungus also causes a leaf spot. The fungal host index used by pathologists, Fungi on Plants and Plant Products in the United States, lists fungal diseases reported in the literature. That comprehensive text reports several fungi that infect clematis stems. Some are fungi common in Illinois, including Ascochyta, Botryosphaeria, Mycosphaerella, Diplodia, Dothiorella, and Phoma. Only Ascochyta and Phoma are listed as stem cankers. There is no mention of a vascular wilt pathogen. Authors of Diseases of Annuals and Perennials show clematis with Ascochyta wilt symptoms showing early signs of the disease.
It appears that clematis wilt is a term that has evolved over time to describe the wilting symptom on stems infected with one of the fungi listed above. It is not a term that is intended to implicate a vascular wilt pathogen such as Verticillium. This clematis stem disease causes individual shoots to suddenly wilt and die, thus the moniker of clematis wilt. The causal organism is usually a fungus called Ascochyta, which can be found at the base of affected stems. We do not always find the causal fungus on plant specimens sent to the lab because often only the top portion of the stem is submitted. Look for small lesions at the soil line. The fruiting bodies of the fungus appear as black, pinhead-sized specks in the lesions.
When plants are lush and full, any type of injury low on a stem seems to make whole sections of the plant wilt and not recover. With new clematis, plant deeper than you might normally. You want at least two sets of opposite buds under the soil line. Dig the hole and lay the plant down while gently curving the stem at the right place to get two buds below the soil. Clematis planted in this manner have a much hardier root system and a better chance of recovering from stem blight. Also make sure your clematis has adequate support to help minimize wind injuries to stems. Always be careful when working around clematis. Don’t allow them to become water stressed, especially when they are filled out. Clematis species prefer neutral to slightly alkaline soil with lots of organic matter. The soil should be kept cool and evenly moist. Roots should be in shade or mulched.
If this stem disease is present in your plants, follow the suggestions given. At the end of this season, cut affected stems (or the entire plant) back to a few buds above the soil. Remove all stems and fallen leaves. This should remove most of the inoculum from the site. Chemicals are not recommended, although in severe cases a general fungicide such as thiophanate methyl may slow the disease progress.