I have a theory on this. if the "medication" is causing a mechanism in the plant then it is the plant and not the mildew that has become resistant. also because the product is only active for 28 days, there must be some kind of process that gets avoided during that time allowing for a clean finish. For example plants that are treated can focus their energy on growth, production, and building their immunity. Just like any drug the organism builds a tolerance. So maybe the plant just needs a break so next time it can get ripped!