Circular black spots surrounded by a yellowish halo-- eventually leaves turn yellow and fall frome the plant as the spots get larger-- if this is what you are experiencing then your plant may have a case of "Black Spot"-- its a fungus-- spread by over-head watering or rainfall. Its most common in humid and rainy conditions. The fungus spores germinate in water-- once a plant is infected the fungus will stay in old leavesleft on the ground or int the canes through the winter and reappear on the next season's growth. -- There is no chemical cure-- in early spring after an infection, prune the canes back lower than normal to eliminate the fungus spores that may have survived over the winter-- apply a lime-sulfer spray before the leaves open. Remove all infected leaves from plant and ground and destroy. Water in the morning so the plants will have time to dry before evening and do not water plants from above. Wet leaves promote fungus growth.
Try the following site for organic treatment measures:
http://www.gardensalive.com/category.asp?c=15

I hope this helps you some...
BTW- WELCOME TO THE FORUM

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Each day is a new beginning...What you begin is your choice.
The pleasure you receive from life is equal only to the attitude you put into it.

Go ahead and pick of any diseased leaves as this will help eliminate further infection. And since the condition is so close to the bottom of the plant(it appears to be in the photo) you might try the sulfur treatment around the lower part of the plant maybe as a deterant to futher contamination. You may be able to contact someone via the website I offered and ask them if it would help... Best of Luck with the roses!

* * * *

Each day is a new beginning...What you begin is your choice.
The pleasure you receive from life is equal only to the attitude you put into it.