soil sterilization

Options (other than bleach, about which I have no opinion) you might consider include Cleary’s (buy it in WP wettable powder form as Banrot) or a soil fungicide sold in granular form for lawns ( be sure the active ingredient is effective on fusarium). This assumes that the problem is basal rot, which is caused by a soil-borne fungus, fusarium oxysporum ssp narcissus. I would hesitate about using bleach unless I were sure that it wouldn’t kill all microorganisms, including the good ones.