Straight Zinc Sulfate is too hot for Prunus species plants, unless you would like to knock off the leaves for creating early dormancy. We generally wait until a couple weeks before normal leaf fall, which will depend on your area. If you knock them off too early you can force next year’s leaves to push early, put considering you are likely close it shouldn’t be a big deal. Knocking off the leaves is always a good idea as it will help time next year’s leaf out to be more synchonized rather than straggle on. If you are looking for a mid-season zinc product that won’t burn try Neutral Zinc 52%, which is a powder mix of Zinc Oxide and Zinc Sulfate that has a neutral pH. Also, Manganese Sulfate won’t likely help much unless you successfully chelate it with the amino acids, citric acid, or organic acids incidentally chelating it. We have had success with amino acid chelates after years of manganese sulfate applications that didn’t do anything. Also, consider lo-bi urea as a foliar spray, it will add N to your trees without soil or water contamination issues, and if done late enough it can help with frost protection next spring.