To be safe, I would treat the suspected soil with an organic fungicide to ensure that any fungal spores in the soil are killed. In fact, most biofungicides are actually beneficial to the soil by allowing the beneficial fungi, bacteria and other plant-symbiotic organisms to thrive while targeting and outcompeting the detrimental ones. You shouldn’t have any trouble finding organic fungicides online since there are quite a few of them to choose from. Also, applied at the first sign of leaf wilt, the trees root system may even be able to uptake enough of the fungicide to kill the fungal pathogens already present in the tree’s water system.