This is what my herb book says about powdery mildew...

"A common fungal disease occurring in hot and dry conditions. Prevent by watering during dry spells.

If your plant does suffer, destroy all affected leaves before spraying with elder"

"Elder leaves have fungicidal properties and may be useful against mildew and black spot diseases"

I haven't come across elder leaf spray before, but you can make your own.

I would be tempted to either ignore it or cut the plants back and water well to encourage new growth.

All parts of elder other than the ripe fruit are poisonous. The toxic agent is hydro-cyanic acid and spraying it around the garden is not something I would recommend.

Pictures would help us to ID what the problem might be. They are very different diseases, thrive in different types of environments, and should be treated differently.

You can also try spraying with 1/3 milk and 2/3 water with a drop of dish soap.