How should Rose plants be watered ? How often and how much ? Should water be applied on leaves too ?

The black spots are probably a common fungus that attacks roses, known helpfully as "black spot". You definitely don't want to get the leaves wet, or it will spread. When watering, try to avoid wetting the leaves with the spray or allowing muddy water to splash up from the soil to the leaves. A soaker hose around the base of the plants might be best, to gently add water to the soil without splashing onto the leaves. Water early in the morning so that any extra wetness can dry throughout the day; if you water in the evening, the leaves will remain wet longer through the night and encourage fungal growth.

This page gives some instructions for dealing with black spot:

http://www.wikihow.com/Deal-with-Black-Spot-Leaf-D...

Roses do not like having their leaves wet. They do like a mulch cover over the dirt.

Rose plants should be watered very thouroughly. Apply it everywhere! ( Yes, including the leaves. ) Give it as much water as it can absorb. When it starts to absorb the water more slowly, don't give it anymore.