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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.
Identifying
Symptoms:
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.
Control:
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.
Identifying Symptoms:
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.
Control:
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.
Identifying
Symptoms:
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.
Control:
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.
Identifying Symptoms:
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.
Control:
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.
Identifying Symptoms:
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.
Control:
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.
Identifying Symptoms:
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.
Control:
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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