POWDERY MILDEW affects some clematis more than others, and a lack of water to the root system is one of the main causes. It is a white fungus which forms on the leaf, flower and stems of plants making them look as though they are covered in talcum powder, and then causes a brown or purple discolouration of the leaves. Although it doesn't usually kill plants, it is unsightly and can be treated fairly easily. Spray an affected plant the following year with an appropriate fungicide before the mildew re-appears, and it is recommended that you do this regularly using different brands so that the plant can't build up an immunity to the spray.
PLOCKET'S CLEMATIS SITE
CLEMATIS WILT is a disease that can affect some early-flowering large-flowered hybrids, whereas many of the smaller flowered species and cultivars are more resistant. The spores of Phoma Clematidina are produced on dead tissue and dispersed in water or by insects. They usually affect the leaves or stems and the infection will spread rapidly down the stem causing it to blacken and the plant above the affected area to wilt and collapse. The clematis can often be saved by cutting back the affected stem or stems below the affected area down to the base of the plant. DO NOT put the affected stems onto your compost heap because the fungus can survive for many months. There isn't a product on the market to treat Clematis Wilt. It should also be noted that if your clematis does show similar symptoms it is more likely to be caused by other things such as lack of water, over watering, damage by ants or mice to the roots or even wind damage.
PESTS AND DISEASES
Except for Clematis Wilt, clematis are no more prone to disease than any other garden plant. If you are careful how you plant and care for your clematis it will grow to be healthy and vigorous, and capable of surviving many pests and diseases. I've listed some of the most common pests and diseases which may afflict clematis.
APHIDS Greenfly and blackfly can attack clematis particularly during spring and summer, and as with any plant, heavy infestations can stunt the growth of your plant. They can also spoil the foliage with "honeydew" and then black sooty mould can form. Check your plants regularly and treat accordingly.
EARWIGS are particularly partial to clematis buds but will also attack flowers and leaves. They hide during the day and can be trapped by providing nesting places: place a small terracotta plant pot upside down on the top of a cane, as close to the flowers as possible, and fill it with straw or crumpled newspaper. In the morning the contents of the pot can be shaken out and the earwigs destroyed. You can also try putting grapefruit skins edge down on the ground to provide a cool hiding place. If the infestations are heavy there are sprays available on the market.
SLUGS & SNAILS will damage soft young shoots of clematis, particularly the herbaceous clematis as they push out new shoots in spring, and also rasp off the outer layer of more mature stems. They will hide under rocks and slates so be particularly vigilant if this is how you shade the roots of your clematis. Mulch around your clematis with sharp sand or egg-shells (baked slowly then crushed), and put copper tape around canes and supports where possible. Garlic pellets are available as an alternative to "blue" slug pellets. Home-made sprays of instant coffee work reasonably well but will need to be repeated regularly, particularly after rain
VINE WEEVIL is another nocturnal pest which will munch the leaves of clematis, but the biggest threat is from the larvae that hatch from eggs laid on the surface of the soil. The bugs burrow into the soil and eat the roots of the plant, usually killing it if the soil remains untreated. Container plants in particular are vunerable because the larvae can't move on to other areas, and I tend to water pesticide into my pots as a precaution rather than waiting until I spot damage. Nematodes are also available to act against Vine Weevil.
SLIME FLUX is caused by a bacteria that lives on the surface of bark and enters the plant when damaged. In spring the bacteria multiply and in some occasions will be forced out of wounds with the plant sap, creating a foul smelling mass of slime. This slime can also attract insects which will cause more damage to the plant. The plant above the wound usually dies but if it is pruned back hard below the affected area it might re-grow. There isn't a chemical treatment available for Slime Flux.
