Roses are delicate beauties. The bushes that they grow on are susceptible to a number of pests and diseases. Even expert growers are sure to battle some form of leaf disease at one point or another. While leaf disease is a fact of life for rose bushes, plants that continually fall prey to leaf disease should be replaced with more resistant varieties or provided with more suitable growing conditions.
Powdery mildew crops up wherever there are roses growing in hot, humid conditions. The initial infection creates bumpy blister-like growths on the top surface of rosebush leaves. As the disease progresses, the blisters develop a white powder-like substance that is easily brushed off by hand. Powdery mildew will not kill rose bushes, but it will distort their leaves and eventually cause them to drop.
Black Spot Disease
Black spot disease often crops up in mid-summer after unseasonably rainy weather, or repeated overhead watering. The fungus responsible for black spot needs leaves that have been wet for at least eight hours to germinate. Then, one to two weeks later, rose bush leaves develop brown or black spots with yellowing borders. If left untreated, affected leaves will eventually drop. Black spot can also spread to rose bush canes where they manifest as raised, dark red or black splotches.
Rust is caused by a fungus that causes rust-colored, wart-like spots to develop in colonies on the underside of leaves. As the disease progresses, the topsides of the leaves also turn yellow and then rust-colored. Then the entire affected area becomes a dark brown or black pustules and the leaf drops. Rose rust generally pops up in spring and largely affects roses grown on the Pacific Coast. Rose rust is a fairly serious rose bush leaf disease and can kill young rose bushes if not treated quickly.
Downy mildew is a fungal infection that causes leaves to develop black or dark red stains. Some leaves may also develop a downy fungus on their undersides. As the disease progresses, the leaves eventually turn yellow, wilt and drop. Like many fungal diseases, downy mildew is brought on by humid growing conditions.
Purple Spotting Anthracnose
Anthracnose is a fungal disease that usually sets in after a period of heavy rain in the middle or latter part of the growing season. The disease begins by forming small red, purple, pink or brown spots with yellow rings on the leaves and occasionally the stems of rose bushes. As the disease progresses, the spots turn white and may fall away, leaving a hole in the leaf behind them. If left untreated, all affected leaves will eventually drop.
Leaf scorch is caused by a fungus that causes irregularly-shaped black spots with red or purple edges to develop on rose bush leaves.