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Maintaining sufficient Phosphorous is especially important to helping plants fight off fungus. We noticed a marked improvement in plant health when we started using Mono Potassium Phosphate on our Blueberries. Its a non-organic water soluble powder like miracle grow (only it has no nitrogen). Sulfur (microthiol disperss specifically) has seemed to help with powdery mildew on mango and can be labeled organic. My Dad likes Oxidate (which is also labeled organic I believe). We also used Aliette once when we had a bad outbreak of stem blight in our blueberries. As I recall, it stopped it dead in its tracks. Make sure to rotate fungicides whatever you do to prevent build up of resistance. It is also good to apply microrizae after fungicide applications.