Answer: Whatever the original cause of the problem I think you stopped it in its tracks with the fungicide, insecticide and fertilizer applications. You can prune off anything obviously dead now in the spring. Try not to do too much pruning of live material, though because it could encourage suckers (upright stems) to grow this summer. Then in the fall, after the tree has gone dormant, you can prune to shape it. Best wishes with your contorted filbert!
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