Fumigation is a good short term management plan. Just depends on how long the crop will be on site. They can build back up to damaging levels quickly. The sclerotia can survive decades so surviving in a pond is quite possible. The key will be drawing water off the middle of the pond avoiding bottom sediment that would contain the resting structures or avoiding the very surface where other pathogens, such as Phytophthora spores, might be active. Here is a good write up of the pathogen if you have not already seen it: https://pnwhandbooks.org/plantdisease/pathogen-articles/common/fungi/verticillium-wilt-pacific-northwest