Rose Diseases

Rose plants can be severely affected by several diseases. To maintain healthy rose plants roses, these diseases must be identified and controlled.

Powdery Mildew

Powdery mildew, caused by the fungus Sphaerotheca pannosa, appears as a white, powdery growth on rose leaves, stems, buds, or flowers. It usually first appears on new growth in periods of warm, dry days followed by cool, damp nights. The new leaves may become curled or twisted and the shoots may look badly deformed. The fungus may also infect older leaves. The upper surface of the leaves often appears normal despite extensive fungus growth on the underside of the leaf.

When planting new roses, look for disease and insect resistant varieties; more  varieties are available every year. The planting site can be made less conducive to powdery mildew development by not planting roses in shaded spots, especially in areas that  tend to dry slowly in the mornings. Surrounding hedges or shrubs should be pruned or thinned to allow for more air movement over the roses.

Black Spot

The fungus Diplocarpon rosae produces round black spots with fringed margins on leaves or stems. On some varieties, yellowing may occur around the spots. These symptoms are often first evident on the lower leaves. Infected leaves will drop off, and the plant may be almost completely defoliated. Such plants are badly weakened and may die over the winter.

As with powdery mildew, some varieties of rose are less susceptible, so select and plant resistant varieties whenever possible. The fungus survives the winter in fallen  leaves; raking and removing these leaves each fall may provide some control. Avoid watering the plants by splashing water on or about the leaves; water at ground level or use soaker hoses. Plant in sunny locations where plants will dry quickly after watering, rain, or humid nights. Water in early morning. Begin a fungicide spray program as soon as new leaves appear in early spring.

Stem or Cane Cankers

Several fungi cause stem cankers on roses, each type causing slightly distinct  cankers, which are usually brown, oval shaped, sunken, or shriveled areas anywhere on  the cane. When the canker completely surrounds or girdles the cane, the cane dies and the leaves wilt from the canker outward. Small black specks of fungus spore forming structures can sometimes be seen erupting on the cane surface within the cankered area.

Always plant disease-free material. Prune out and destroy all diseased canes every year, making sure to cut well below the obviously cankered areas. Protect plants from cold injury in winter with mulch or another kind of cover. Keep the plants vigorous by fertilizing and watering properly.

Mosaic

Rose mosaic is caused by a virus. Bright yellow, wavy patterns appear on the leaves of some rose varieties, while other varieties become stunted and weak.

Infected plants cannot be cured.

Plant virus-resistant roses if possible. Control aphid populations, which will spread the virus. Do not prune healthy plants after pruning infected plants without first disinfecting pruners by dipping in a 10% solution of chlorine bleach in water. Severely infected plants should be removed and destroyed.

Rosette and Witches Broom

Rapid stem elongation may be an early symptom of this disease. Branches of the plant may eventually develop  thickened, thorny stems. Many short, deformed shoots will form, often with red pigmentation and tiny misshapen leaves. Plants die within one to two years as symptoms spread from branch to branch. This problem is uncommon in the region.

The exact cause of this disease is unknown. Infected plants cannot be cured. Try to control insects, particularly leaf hoppers and plant hoppers. To protect other plants against the possibility of an infectious agent, infected plants should be removed and discarded as soon as the disease is evident.

Crown Gall

Irregularly shaped, bulbous masses of tissue (galls) appear on stems near the soil line. Galls can appear as small swellings or reach several inches in diameter. Severely infected plants become stunted and fail to produce acceptable flowers.

Avoid buying infected material with suspicious swellings or galls on lower stems or crowns. Protect plants from injuries on the stems. Maintain plant vigor with appropriate fertilization and watering. Pull and destroy badly infected plants. There is no chemical control for this disease.

Adapted from Stephen Nameth and Jim Chatfield, Ohio State University Extension, 2000