It is difficult to prescribe a treatment given that I do not know more about the issue your wisteria is experiencing. Can you describe the overall condition of your wisteria - can you tell me more about this fungus or provide a photo? Do your blooms and leaves look healthy? Even if a fungicide was necessary, it would not require cutting the trunk.
Here is an article on common wisteria diseases, including fungal leaf disease, that may help you: