Posted: 28/06/2013 at 12:51
At least Clematis cannot get Fireblight which is basically systemic to Rosacease.
Also C. armandii is not normally affected by Clematis Wilt but it really does sound like that nasty thing.. It could also be wind damage or even the molluscs having rasped away the covering of the base of the stem so the stem dies back.
I hope you planted it deeply so that is can re-shoot from below soil level.
I would be tempted to cut back the affected stems to ground level and feed it with somethiong to encourage new growth.
Best of luck with it.