I would say they have a heavy bout of " rose mildew"

There are a number of things that can cause this and their are sprays for curing it.

For an explanation and treatment have a read of this article I have written on my website;

http://www.thegardenersalmanac.co.uk/Data/Mildew-Rose/Mildew%20-%20Rose.htm

Powdery mildew - spray with 1 part milk (cows milk) to 9 parts water.

Thanks Bamboo!

Can I add that piece advice to my website please?

Yes, I have no rights to it, its general knowledge - I learned it off the web myself, lol. Doesn't matter whether the milk is skimmed, semi skimmed or full fat...

Are you sure that this idea is ok,bamboo?

Yes - scientifically proven. Give it a try, better than reaching for the fungicide straightaway. If it doesn't work, use a fungicide spray which treats for mildew, but the milk treatment should work. Spray thoroughly, till run off, including under the leaves and the stems.

Thanks Bamboo I have added it to my article on Powdery Mildew.

In doing so, it reminded me of the scientific cure which is;

"Fatty acid sprays have some affect on contolling the disease."

Particularly when you consider milk contains lactic acid and fats (fatty acid)

Thanks again!..Tg

Full cream milk by the way, not the white water most folks drink these days.

Hi Bamboo, Many thanks for that tip as I had asked the very question about mildew on my roses a few days ago and was about to buy some Roseclear Ultra. Will the milk and water remidy help with Black Spot also?
Thankyou

For recurring blackspot in roses read this

http://www.armillatox.com/

No, it won't treat anything other than powdery mildew. Suggest you get yourself some Roseclear if there's a problem with blackspot.
Doesn't have to be full fat milk, Owdboggy - I checked just last week.