One of the greatest attributes of clematis is the mass of blooms on such a delicate looking frame. This can at times also be its downfall.
If the plant is not securely attached to its host, the stems can be damaged. The resulting wounds provide a target for a wide range of fungi to attack. The plant then severely wilts or collapses. This malady happens most often when the plant is young. “Clematis wilt” is the catch phrase that has developed to describe this occurrence.
Phoma clematidina (formerly known as Ascochyta clematidina) is the most common fungus to attack clematis but it is not always the culprit. Clematis wilt can occur at any time, however it usually happens just as the flower buds begin to open. The whole plant can be affected or as most often happens, only one or two of the stems collapse. If clematis wilt occurs, it is important to remove and destroy the infected stems. The fungus will remain in the healthy looking part of the stem, so it’s also imperative that about one inch(2.5 cm) of the stem below the infection be removed. New shoots will soon appear at or below the soil line. At worst this therapeutic pruning will delay your flower display. It's very seldom that clematis wilt destroys a whole plant. If the problem persists a preventative fungicide can help. There are new products such as beneficial fungi coming to the market place as well. In our production we apply a product called rootshield which contains hybrid fungus Trichoderma harzianum. This helps fight unwanted fungi. P. clematidina is capable of surviving and reproducing on dead leaf plant material including common weeds. Thus, cleaning up leaf litter can significantly reduce the source of inoculum.
Clematis wilt rarely infects the smaller flower varieties such alpina, cirrosa, macropetala, montana and viticella. Also, hybrids that have resulted from viticella crosses such as Madame Julia Correvon and Etoile Violette have proved to be very resistant to the various fungi that cause this problem.
Another fungus that might affect your clematis is mildew. It is seldom a problem until late in the season and not a problem at all if there is good air circulation. If mildew is noticed in its early stages it can be easily controlled by many different fungicides readily available. Your local garden center or nursery will be happy to make an appropriate recommendation here.
Earwigs and slugs are the most common pest problems clematis have. Slugs are the most troublesome. As they will always be with us, it’s a matter of control not elimination. Slugs prefer to attack young shoots, thus slug bait early in the spring gives the best results. Earwigs usually attack in mid summer and can turn the bloom of a late flowering clematis into a lace work over night. There are many good pesticides and traps available to control earwigs.