Answer: Based on your description, I am not certain what is happenng. It is the right time of year to see plants being affected by clematis wilt. With clemagtis wilt, in addition to blackening and drying foliage at the base the stem also wilts suddenly and dramatically. There is no cure for this, but you would cut off each affected stem below the wilt and hope for the best. Eventually (possibly as long as three years later) it may regrow and seem perfectly healthy. This tends to affect newly planted clematis with slender stems, but can also affect older established plants.
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I hope this helps.
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