You need to alternate fortnightly between two different fungicides. Make sure that they have different active ingredients though. No point in buying two different product names with the same active ingredient! It should be on the front of the container and one will almost certainly be Myclobutanil. So long as the other is different.

Feed the rose at least twice a year with a high potash rose fertiliser. Potassium gives the plant a degree of resistance too but the fungicide is the main agent to prevent it.

Thankyou very much Fractal