Every year I have been plagued by this, I get to mids and have to harvest because of bud rot. I am not into the idea of metals e.g. copper. The last experiment failed. It was an 0.7% solution of sodium benzoate at pH5. I think the problem is, when it dries out it goes back into the inactive salt form. Alternates include essential oils such as Tea Tree or fungicidal agents found on plants such as blueberries and wheat. ° I've noted the method involving a temporary emulsion of Tea Tree oil and water. It is a fungicide but its terpenes I think may change the character of the bud. Each EO has its Minimum Inhibitory Concentration for a given organism. EOs such as lavender and peppermint also have fungicidal action but again, the risk is in altering the terpene profile. ° I specified the following characteristics for a good antifungal to test against bud rot and I've been testing without success until now. But it's still early days. It's good to harvest in amber for a change. 1. Effective at low concentrations. 2. Safe. 3. No reported irritant effects. 4. Preferably a natural substance or one found in nature. 5. Not pH dependent. 6. Not oily or alcoholic. 7. Metal-free. I'm still experimenting, but hopefully, a low% parabens solution applied through an atomiser in mid flower seems effective.