Rose Diseases

Blackspot is the most common disease found on roses, epsecially in warm humid climates such as in the southeastern US. It is caused by a fungus called Diplocarpon rosae which spreads by water and germinates in warm temperatures. The fungus causes black spots to appear on the leaves. The spots have irregular or feathery edges. The leaf eventually turns yellow around the spots and may fall off. Leaf loss due to blackspot can defoliate a bush; however, blackspot rarely kill a plant. Some plants are more naturally resistant to blackspot, so the first line of defense is to choose varieties that are resistant. Watering by irrigation rather than overhead, and encouraging good air flow around bushes by adequate spacing and selective pruning may reduce the time the foliage is wet and reduce blackspot. Fungicides may be used as a preventative to control blackspot, but they must be used properly and regularly. Alternating fungicides is encouraged to avoid resistant strains of the fungus from forming.

Rust is a fungal disease caused by various forms of the fungus Phragmidium disciflorum, and is most common in the western US. Symptoms usually appear in the cool moist weather of spring, beginning as small rust-colored spots on the undersides of leaves and eventually becoming visible on the upper sides as well. Spores overwinter in infected plants so removal of infected material can help prevent reinfection the following year. Preventative fungicides are available, and reducing the moisture around plants by as for blackspot control also helps control rust.

Powdery mildew tends to appear during times of warm dry days followed by humid cool nights. A grayish white fuzzy coating appears on the new growth of roses, including the buds. The disease is caused by an airborne fungus called Sphaerotheca pannosa var. rosa, and preventative sprays are available. As with blackspot, some rose varieties are more naturally resistant that others. Removal and disposal of infected materials also helps control powdery mildew.

Downy mildew is a rapid and destructive disease caused by the fungus Peronospora sparsa. The fungus occurs on the flowers, leaves, and stems of roses, appearing as dark purple, purplish-red, or brown irregular blotches. Yellow areas and necrotic spots appear on the leaves as the disease advances. Fungus formation is favored by high humidity (> 90%) and cool temperatures. Downy mildew advances rapidly so action must be taken immediately when found. Removal and disposal of all infected material is required because the fungus can remain dormant and cause reinfection. If a plant becomes infected with downy mildew, spraying is recommended since severe cases can kill plants. Preventative sprays are available.

Botrytis, called botrytis blight, is caused by the fungus Botrytis cinerea and is most prevalent in humid or wet weather. This fungus appears as gray mold on flower buds, which often fail to open properly. When open, the petals may have many small dark pink spots.

Canker and dieback are caused by many different fungi. Canker usually appears as a brown or gray area on a stem, turning nectotic. They often occur due to injury or cold damage. Cut out the infected area back to healthy tissue.

Anthracnose is caused by the fungus Sphaceloma rosarum, and results in dark red, brown, or purple spots on the upper sides of leaves. The spots are small (appx. 1/8 inch), circular, and develop a gray or white dried center that may eventually drop out.

Rose mosaic virus is a general term for several viruses found in roses that cause reduced vigor, distorted leaves, and reduced flowering. The only way to know if a rose has virus is to have it tested, or if it shows one of the typical yellow streak patterns on the leaves. For images of virused leaves, see Baldo's Bugs and Roses Page given in the link below. The virus is not curable by any means availble to the home gardener. It is not generally fatal, and it is not transmitted by means other than grafting.

A virus transmitted by microscopic mites. The virus is contagious and usually fatal to the rose. Symptoms are abberant or excessive growth, excessive thorniness on new growth, and witch's brooms. Infected growth must be removed as soon as possible. Sprays for mites can help control further spread. If you suspect you may have an infected bush, please use the link below to access Ann Peck's informative ebook.