The only product I have found that works extremely well on this fungal disease is called Green Cure. Potassium bicarbonate is one its main ingredients. Apply this fungicide as directed to treat powdery mildew.

Roseclear is a good treatment, but it must be used regularly, I think its fortnightly as I recall, according to the instructions on the bottle, or mildew will recur. If its powdery mildew, you can use 1 part cow's milk to 9 parts water and spray with that weekly, but for black spot, you'd need 3 parts milk to 4 parts water, again weekly, so that's a lot of smelly milk dripping off your bushes. The milk treatment is ineffective on Downy Mildew though.

Thankyou. I'll look out for Green Cure, & will certainly try both suggestions. I am very conscious of not harming bees & I read on the Roseclear container it should be used early in the day. Not so easy if it's raining!but of course that applies to all treatments. Hopefully with care & vigilance I'll get a good rose arch this year.