Fusarium in a flowerbed - Any suggestions?

As hard as it is, i would throw that big plant out of there immediately, as the fungus is feeding off of it and getting stronger/multiplying. I would keep the other plants but inspect them on the regular. They might be more resistant to the fungus and not get infected. Have you used that soil for growing weed before? It is important for maintaining soil hygiene to remove dead, decaying roots from the ground. Those will attract diseases and pests for that family/species of plants. Or you could change the whole substrate / location of the plant ( when growing in the ground ) You can definitely try to find a cure, but i wouldnt hope for it too much. Hope that helps, good luck! 🙏