Jennifer, I don't have direct experience with clematis wilt - I only grow clematis at my daughter's house in Utah and they haven't had any problems. But here in Florida I have lots of experience with soil-borne blights and fungal woes in general. (I'm assuming the wilt is soil-borne) Here's what I do:
My first try is always a hydrogen peroxide douse onto the soil. Use the 3% H2O2 that you get at the drug store, and dilute it 1oz to a quart of water. Douse the whole root area. This treatment has never hurt a plant that I've tried it on, including everything from tomatoes to orchids. I've saved several plants in quite dramatic fashion, even after half the plant has died off from the disease. If it is fungal or bacterial, the peroxide douse will help and you will see an improvement in the plant quite soon. IF that happens, I'd repeat the douse at least once more, 5 days to a week later.
Also, see if you can reduce the volume of the sprinkler head that's nearest to your clematis. There's usually a screw in the center at the top of the head that will decrease the flow if you screw it down somewhat. (choose a warm day, you will get wet doing this
)
If you get no improvement from the peroxide douse, try a fungicide douse. There are some amazing new biologic fungicides available - the latest one I'm trying because I'm always battling fungal blights on my orchids is called Actinovate. It's rated for use on organic vegetables, even! So far I've had a great result with it, both on orchids and on my rust problems on plumeria, figs, cannas and daylilies. Haven't tried it as a douse, but the instructions say you can use it this way. I got it online from Amazon, but Gardener's Supply has it too.
