The only serious disease of clematis is highly infective fungal infestation called Clematis wilt. Suddenly either the entire plant, one of the shoots, or just part of a shoot collapses completely, although roots usually remain healthy.
This tends to affect weak, damaged or improperly cultivated plants. Buying clematis from reliable sources and planting only strong varieties, previously grown in a container can partially reduce the risk. Clematis should be planted deep in the ground in a carefully prepared site to ensure generous rooting of the stems close to the base. Before planting and also during cultivation all dead parts, both leaves and stems, should be removed. Wilt can be counteracted and fought chemically by watering the plant thoroughly with the 0.1% solution of fungicide Switch, if it's registered in your country. Use 3-5 l of the solution for one plant and repeat this treatment 2-3 times every 10-14 days.
Sucseptable to Clematis wilt are varieties from the Viticella group and Integrifolia and Haracleifolia groups.
Powder mildew is proving an ever more dangerous disease affecting clematis. A white powdery growth appears on the upper leaf surface. With the course of time the growth gets thicker and red or brown spots may appear and eventually, the infected leaves wither. The susceptibility to the disease varies greatly from one cultivar to another. All clematis from the Texensis Group are particularly vulnerable, as well as some large-flowered cultivars e.g. 'Jackmanii', 'Romantika', 'Rooguchi', 'Arabella', 'Mme Julia Correvon'. Temperature above 20oC and frequent moistening of the leaves promotes the spread of the disease. Drought stressed plants, grown in a partially-shaded position without proper air circulation are particularly prone to powdery mildew. In case of infection spray the plant straight after you've noticed the first symptoms of the disease with one of the currently recommended fungicide registered in your country. Repeat the treatment 2-3 times every 10 days, alternating the fungicide used.
Pests don't usually present a problem for clematis as they are on the whole less prone to attack than many other garden plants. However sometimes appear:
- spiders and aphis – a similar treatment should be applied as in the case of every other cultivation appling the registered and recommended cemical threatment.
- Clematis sawfly
- mice – are another burdensome pest, they will eat the roots of the plant, which will eventually collapse. You can partially prevent the roots from damage by wrapping the rootball with a plastic or wire mesh.