Rose
Diseases
Blackspot
This fungal disease can cause almost complete defoliatiation of bushes by
early fall, resulting in a weakened bush on which cane die-back and cankers
become severe. Blackspot is identified as circular black spots that appear on the
upper surface of the leaves, starting at the bottom of the plant and moving
upward. Infected leaves turn yellow and fall off prematurely. The fringed
margin and black color distinguish this leaf spot from others. Infections on
canes are identified as reddish-purple spots. Splashing water spreads
blackspot. Infection occurs after leaves are wet for several hours, making it
more serious during rainy periods. Some roses are less susceptible than others,
so cultivar selection is important. The fungus overwinters in fallen leaves and
stem cankers. Raking and removing these leaves as well as pruning out affected
canes by spring before the buds swell may help provide some control. Avoid
wetting the leaves when watering and locate plants where there is good air circulation.
Fungicide spray programs need to be started as soon as new leaves appear in the
spring.
Fungicides generally recommended for blackspot control include:
- Daconil 2787 or fungicides
containing Daconil
- Phyton 27
- Mancozeb
- Funginex
- Orthenex.
Powdery Mildew
Powdery mildew is a fungus disease that affects young leaves, causing them
to curl and twist and develop a purple coloration. As the disease progresses,
leaves become covered with white powdery fuzz. Whereas blackspot is usually
most severe on the lower part of the plant, mildew affects the top part of the
plant. Mature leaves are less likely to be affected. Mildew is spread by wind
and develops rapidly during periods of warm, dry days followed by cool, humid
nights. Infections of mildew are actually discouraged by the presence of water
on the leaves. However, keeping plants wet all night to avoid mildew provides
an environment that allows other diseases to develop. Infection can be reduced
through sanitation and fungicide spray programs. Prune out all dead and
diseased canes to reduce initial fungus infection. Because new growth is
especially susceptible, thorough coverage of new growth with fungicide is
important. Plant roses in areas where they receive good air circulation and
where the foliage can dry off rapidly in the early morning to prevent many
types of diseases.
Frequently used fungicides for control of powdery mildew include:
- Captan
- Daconil 2787
- Phyton 27
- Funginex
- Orthenex
Stem Cankers
There are several fungi that cause cankers on roses. The different fungi can
cause different-looking cankers, but they usually produce brown, oval-shaped,
sunken, or shriveled areas anywhere on the cane. The cane dies, and leaves wilt
from that point outward. Sometimes small black specks can be seen on the cane
surface within the borders of the canker. These are fungal spore-forming
structures. Cankers should be pruned out each year. Make the cut well below the
affected tissue. Protect the plant from cold or freeze injury by providing
adequate cover over the winter. Do not cover roses too early in the fall. When
roses are mulched before the soil freezes, moisture can be trapped around the
canes and this can increase the damage caused by canker disease. Keep plants
vigorous with proper culture and disease control. Canker is a disease of
stress. If plants are kept actively growing, they stand a better chance of
avoiding cankers. There are no effective chemical controls for canker disease.
Botrytis Blight
Botrytis blight is a fungal disease that generally attacks
dying tissue. It is frequently found on older flowers and other plant parts.
Under certain conditions it may also attack healthy tissue. Botrytis favors
moist, wet conditions, often causing the disease to attack entire flowers and
produce a gray fuzzy mold. This disease is often called gray mold. Good garden
sanitation and removing spent flowers often result in good control of this
disease. When this is insufficient in providing adequate control, a
preventative spray program may be necessary.
Materials used in limiting botrytis are:
- Captan
- Daconil 278
- Mancozeb
Mosaic
Rose mosaic is caused by a virus. Bright yellow patterns made up of wavy
lines may appear on the leaves of some varieties. Other varieties may show no
yellow lines, but may be stunted and weak due to virus infection.
Virus-infected plants cannot be cured. Plant virus-resistant roses if possible.
Try to control insects, especially aphids, since they help spread the virus. If
you are pruning virus-infected plants, don't prune healthy plants unless you
first disinfest your pruners. Dipping the blades in a 10 percent solution of
chlorine bleach and water for 60 seconds can do this. A 25 percent
concentration reduces the time needed to about 10 seconds. All infected plants
should be removed and destroyed to reduce the spread of the virus to other
plants.
Crown Gall
Crown gall is a bacterial disease that can survive 15-20 years in the soil.
It causes irregularly shaped, rough, dark-colored masses (galls) to appear on
stems near the soil line. These galls can appear as small swellings or be
several inches in diameter. Severely infected plants become stunted and fail to
grow properly. There are no effective controls for crown gall. Severely
infected plants should be dug up and discarded and roses should not be planted
in that area for at least 5 years. Avoid buying plants with suspicious
swellings or gall on lower stems or crowns. However, do not confuse crown gall
with normal swellings that you see as a result of the budding process. Protect
plants from injury on stems during cultivation. Maintain vigor with
fertilization and watering. Crown gall is not specific to roses and can affect
apples, raspberries, honeysuckle, euonymus, and many vegetables. For this
reason, roses should not be planted where plants susceptible to crown gall have
been removed because of the disease. Galltrol-A, a non-pathogenic bacteria, has
been used to prevent crown gall. It is often used as a dip on cane root roses
prior to planting.
Rose Rosette
Rose rosette is becoming more common and can result in significant damage.
This pathogen (not yet positively identified) is spread by an eriophid mite.
Symptoms include rapid growth of shoots, development of "witches'
broom," development of tufts of small, deformed reddish leaves and excessive
thorniness. Plants decline over time. Because affected plants can't be cured,
it is best to dig out the affected plant and destroy it. Controlling the mite
has been labeled as an option in protecting the rose from getting the disease.
It is very difficult to apply sprays in a timely and satisfactory way.