Diseases in rose plants are caused by a variety of fungi. Any part of the plants can be affected, and in some cases the whole plant can be lost. Rose plants need good air circulation, which can go a long way in preventing the growth of fungi.
Black spot is a fungus that can completely defoliate rose plants. It weakens the entire bush and makes it susceptible to cane dieback, stem canker and winter injury. It will appear as black spots on the top side of the leaves. The spots may become surrounded by a yellow ring, and the leaves will turn completely yellow. The infection happens after a period of heavy rain when the leaves are wet for several hours.
Powdery mildew is a fungus that causes young leaves to curl up and turn purple. It appears as a white powder on leaves, buds and stems. It can also affect young canes by making them become misshapen and stunted. If the infection is serious, they can die and infected buds will not open. The white powder is made up of thousands of fungus spores, which are carried by the wind to young leaves. Powdery mildew affects many types of plants, but the infection from other plants will not affect roses.
Botrytis blight is a fungus that occurs when water stays on the buds or leaves that causes the flower buds to turn brown, decay, droop and never open. Partially opened buds are also susceptible, and the whole flower can be covered in a gray fungus. Look for a smooth, sunken, gray/black lesion to appear just below the flower head, and the bud will be drooped over near the lesion. It can also appear on the stem where a flower has been cut off.
Brown canker is a fungus that can attack any part of the rose plant that is above ground and it can cause the entire stem to die. Look for small red or purple spots on the canes from the current year's growth. Eventually, they will become grey/white lesions. There may not be much damage done the first year, but the lesions will continue to grow and large brown cankers will appear encircling the stem and death will soon follow. If the canker get all the way down to the crown of the plant, the entire plant could die.