of Rhode Island GreenShare Factsheets
Diseases of Roses
For many years, roses have been a favorite
of home gardeners and professional growers alike. Unfortunately,
there are several diseases that can severely affect rose plants;
to produce top quality roses, these diseases must first be
identified and controlled.
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.
Several rose varieties are more resistant to
powdery mildew. 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.
The fungus Diplocarpon rosae produces round black spots with fringed
margins on leaves or stems. On some varieties, yellowing may occur arround
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. 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. Plant in sunny locations where plants will dry quickly
after watering, rains or damp nights. Begin a fungicide spray program as
soon as new leaves appear in the early spring.
Several fungi cause stem cankers on roses. The different fungi cause 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 the plants from
cold injury in the winter with mulch or another kind of cover.
Keep the plants vigorous by fertilizing and watering properly.
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
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.
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.
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
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
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information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program