It's powdery mildew, I should think, Cuddles. The weather this year has encouraged it to go over the top on loads of plants, because of the warm, wet days. Are the plants close together? Lack of air circulation doesn't help. Give them a couple of doses of fungicide, and see if that does any good. I've no doubt somebody will have another remedy--I've an idea somebody said something about vinegar and water being a good idea, but I'm not sure. Sorry I can't be of more help. Annie

They did, but somebody else said not - doesn't Bamboo recommend water with a little milk in it? But you can buy fungicide sprays if you don't mind using chemicals.

1 part milk to 9 parts water mixed in a sprayer is said to be very effective - it's been trialed officially with good results, I've not tried it myself.