I just received my Gardening Life magazine and noticed this recipe:
Acetic acid fights rust, black spots and powdery mildew. To make this spray, mix two tablespoons (30 mL) of cider vinegar in two quarts/litres of water. This spray should only be used in the morning or early evening, when it's cool and there's no direct light on the plant.
Has anyone tried this?
They also recommend the usual baking soda, garlic, horsetail, seaweed, onion greens and horticulutal oil.