From the McDonald Garden Center Blog

Rose Gardening 101

Water - At planting, and in the warmer months, you may need to establish the root system by watering more often. Watering deeply and less frequently is preferred over frequent, light waterings. Make sure you are also not watering the tops of your plants where the foliage and the blooms are, only the soil, as water on the foliage can cause powdery mildew and black spot.