Treating Black Spot can be much more effective when you understand the life cycle of the disease. The disease starts in late Spring when Black Spot spores, which have over-wintered in the soil,  are blown or splashed by rain onto the rose bush. When the weather warms up (especially if it is also damp), the disease spreads throughout the plant.

The affected leaves die, fall onto the soil where they further re-infect the rose. Leaves which fall in Autumn stay on the soil, allowing the spores to over-winter in readiness to infect the plants again next Spring.

GO TO THE GARDEN PEST AND DISEASE MAIN PAGE FOR
PESTS AND DISEASES CLASSIFIED BY PLANT

The key to controlling Black Spot is to attack it at two points in its life cycle. First when the infected leaves fall to the ground and second in late Spring when spores already within the plant begin to do their dastardly work.

First attack -  pick up and destroy all leaves (infected or not) which are on the soil surface - try to do this as often as possible. This will prevent re-infection during the season and reduce the number of spores which over-winter in the soil. 
Frequent turning over of the top 3 cm / 1 in of soil will help expose the spores to the ravages of frost and will kill some.

Second, spray with a systemic fungicide such as Rose Clear in late Spring and again two weeks later. Spray at this time of year even though there may well be no sign of Black Spot. This will attack the disease well before it has gained a stronghold and is at its weakest. Further spraying should only be necessary when signs of the disease are apparent later in the year.

Another weapon against Black Spot is to grow varieties which are naturally resistant to this disease. Ask you nursery for advice.