don't know if this is natural enough for you, but ive used it many times in the past. works like a charm...
two teaspoons of baking soda, mix in a quart of water, and add about four drops of dish detergent. Spray it on the new leaves on both the tops and bottoms when they're halfway out. Spray one more time when they're all the way out. spray as soon as you see mildew developing.
