A rose bush is susceptible to several fungi. The damage can range from the loss of some leaves to the death of the whole bush. Fungus infections can be prevented by making sure the rose bush has good air circulation. Plant the bushes so you can stand sideways between them, and keep them pruned so the leaves do not overlap.
Black spot is a fungus that attacks mostly in humid areas during the summer. It appears on the leaves of the rose bush as round black or purple spots with fuzz around the edges. It will also appear on the canes as raised dark red or black spots. The fungus reproduces in water and will spread over the entire leaf until the leaf turns yellow, dies and drops off. A black spot infection can easily spread to the whole tree, completely defoliating it. Black spot can kill the entire rose bush.
Powdery mildew is a fungus that appears as a blister on young leaves. Soon after, a white powder will appear on the top surface of the leaves, the buds and in some instances the stems. The leaves look crumpled and distorted. In most cases it will not kill the bush, but it will affect the appearance of both the leaves and the flowers. It will strike when the days are warm and the nights cool or if there is so much moisture around the rose bush that it does not all evaporate during the daylight hours. Powdery mildew cannot reproduce in water like black spot can. It needs high humidity, but it will only form on dry leaves that have a soft or thin matte finish, as opposed to leaves that have a hard or thick, waxy, shiny finish.
Rust is a fungus that can be fatal. It appears as a bright reddish brown or bright orange powdery spot. It will appear as a group of spots on the underside of the leaves and the wood as well. As the disease progresses, it can spread to the top of the leaves and the spots transform first to a dark brown color and then to black pustules. The leaves will die, wilt and fall off. The individual groups of spots can grow, come together and cover the whole bush. It will appear in the spring when the temperatures are below 85 degrees. It is not that much of a problem in regions where the summers are hot and the winters cold.
Downy mildew is also called black mildew, and it is one of the most serious of the mildew fungus infections. Leaves and young shoots will suddenly wilt and die back. It will also attack the calyxes of the bud--the leaves on the bud that protect it--causing the bud to become distorted. It will appear as black, dark red or burgundy stains on the top surfaces of leaves and stems. It can also be accompanied by a downy like fungus on the bottom of the leaves, which will turn yellow and fall off. Downy mildew occurs during moist, hot and humid summers.