I have used bonide Sulfur, and nutritional sprays for powdery mildew of the mango. Generally, I use the nutritional foliar sprays trough the season and well before the panicle emerges. I feel that they help to reduce the problem. Then as soon as the panicles emerge but before flowers, you can apply sulfur. As soon as peas size fruit appears, then you can continue using fungicides. If no powdery mildew is present, then apply sulfur again. If its present consider switching to Potassium Bicarbonate. Sulfur is a preventative but Potassium Bicarbonate is an eradicant. Sometimes if I see powdery mildew before the fruit is formed I use Potassium Bicarbonate. But I only do this if I have to.