I know of no fix for clematis wilt other than to cut the infected branches off and discard them. Why one plant gets the disease and the other doesn't is based on the fact that some of the hybrid type IIs are not as vigorous and more susceptible to it than the species clematis and most of the type IIIs with viticella blood in them.

Now that that has been said, did the stems actually turn brown and black and die off or did they just wilt like they needed water? The reason that I ask is that we had cool wet weather and then hot dry with high winds, and quite a few of my clematis looked wilty as if they needed water, and they straightened back up once the wind died down and the sun set that evening. In this case, the plants probably just got ahead of themselves putting out too much foliage in the cooler, moister weather and then they couldn't support all the growth once the conditions quickly turned hot, dry, and windy (which sucks water out of the plants).

Could you share with us more specifics of the type of wilt you experienced and the names of those affected?

I'm outside of atlanta so you know we have had a wet spring like you so far. Of the 3 that have wilted, 2 did turn totally black and dead so I cut it back to just above the ground where a set of buds were ok. The 3rd is just wilting like you said it looks like it needs a drink of water. All 3 first draw my attention when I see the flower drooping and dying. the 3rd one now has a flower that is obviously dying and I just saw and inch that was starting to turn black so I would say in few days it will slowly all be black. Cutting one all the way back has already led to new growth that looks ok.

I had 2 suddenly do this a few weeks back. I am in central AL. and we had rain and rain and then none for a couple weeks and now lots again. One turned black and one turned wilty looking . Because of what I had read on this forum I quick cut off everything that looked at all weak. In one case that was too the ground and in another it was a long tendril on the trellis. They both surged back and are now in full bloom and better than ever. I believe , as I have read that it is a circulatory thing and has nothing to do with disease of the plant. I never spray my clems so if they were sick it would cont. Glad yours are doing well now too. c