Clematis wilt is easy to spot: a portion of the vine will quickly wilt (overnight), often just as the blooms start to open. Wilt is caused by a fungus that enters the stem usually just above the soil line. Clematis wilt is a condition the seems to affect large flowered, spring blooming clematis more often than other varieties.
The cause of clematis wilt is unknown. In fact, there may be several different causes that appear identical. One suspected cause are insect damage to the outer layer of the stem allows a fungus to enter the plant. Another suspected cause may be that slugs may eat around the base of the plant which cuts off water flow to that stem causing the stem to wilt overnight.
There is no cure other than to cut the entire stem to the ground and dispose of it in the trash. Do this as soon as the wilt is noticed to reduce the opportunity of it spreading to the entire plant. If you have noticed slugs in the garden, use one of the many treatments that help to control slug populations. Be careful using these slug baits if you have pets (select a slug bait that is safe for pets).
Systemic fungicides such as benomyl or carbendazim can help prevent wilt from forming and to help prevent already infected vines sections from spreading to healthy stems if applied as soon as the infected portions have been removed. This method would only work if the cause is from a fungal disease.
Provided there are enough healthy stems remaining, the rest of the plant will usually survive the attack.
There are some clematis cultivars that have been proven more resistant to wilt. These include: The President, Ville de Lyon, Betty Corning, Jackmanii, and Nelly Moser.