[Cankers] [Botrytis Blight] [Nematodes] [Crown Gall] [Rose Mosaic]
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Roses are one of the most popular flowering shrubs grown in North Carolina. To grow roses successfully, one must become familiar with and carry out certain cultural practices which include an effective disease, insect and mite control program. Poor disease control is one of the most common causes of failure with roses. This publication is to aid in the identification of the most common rose diseases in North Carolina and to suggest guidelines for their control. The most important diseases are black spot and powdery mildew.
Black spot, a fungal disease,
is the most serious disease of roses in North Carolina. Circular black
spots with a frayed margin on the upper leaf surface are characteristic.
As the spots enlarge or increase in number, infected leaves turn yellow
resulting in premature defoliation. If the disease is not controlled,
defoliation continues, which weakens the plant and reduces flower production.
Such weakened plants become much more susceptible to other diseases and
to winter injury. The fungus spreads from leaf to leaf during wet periods
and new spots develop in 5 to 10 days.
Black spot must be controlled to grow good roses. Black spot cannot be adequately controlled without a good spray program. A complete uniform spray deposit on both sides of leaves is necessary. Spray applications must begin as new growth starts in the spring and continued at 7-10 day intervals and after heavy rains for the entire growing season. Do not let the disease build up before startinq a spray program. If the disease occurs, immediately remove infected leaves from the rose garden as they appear and rake up and/or discard old fallen leaves during winter months. At some time during the winter months, remove all leaves from the plants and discard or compost. In fall prune hybrid tea roses to about 18 inches and destroy the prunings. In spring prune again to about 10-18 inches and destroy the prunings.
Powdery mildew is a very common fungus disease. The characteristic
white mold can occur on the surface of young leaves, shoots and flower
buds. The disease causes leaf distortion but less leaf drop than black
spot. Powdery mildew is usually more severe in shady areas and during
cool periods. The fungus is windborne and can increase during periods
of heavy dew. Remove and destroy diseased foliage and canes during the
growing season and follow a rigorous fungicide program.
Excellent control of black spot and powdery mildew can be obtained by spraying chlorothalonil (Daconil) and alternating with one of the following three fungicides: triforine (Funginex), propiconazole (Banner Maxx), or myclonbutinol (Systhane, Eagle, or Immunox). Spray applications should be made every 7-10 days. Spray thoroughly to completely cover all plant surfaces. Follow all directions and precautions provided by the manufacturer on the label.
Cankers are brown to black discolored areas on canes. Cankers
frequently start at wounds or where a young shoot comes out of a cane.
These dead areas enlarge until they surround the cane and kill all growth
above the canker or elongate until they reach the crown of the plant and
kill the entire plant. Canker-affected canes should be pruned back to
healthy tissue. The fungicide spray program for black spot will also help
control canker, especially applications immediately after pruning in the
spring. Make pruning cuts clean and just above a bud.
Botrytis flower and cane blight is a fungal disease that affects
young succulent shoots, freshly pruned canes in spring and flowers. The
fungus often enters new shoots killed by early fall or late spring freezes.
It can spread 2 to 3 feet down a succulent shoot very rapidly, killing
the entire shoot. Infected flowers turn brown and are often covered with
a light tan to gray mold whereas infected canes usually turn tan to black.
Botrytis also causes small water-soaked spots with red borders on flower
petals of white roses.
Botrytis damage can be reduced
by removing flowers as soon as they begin to die and by promptly pruning
out discolored canes down to healthy tissues. The black spot spray program
will reduce Botrytis during the growing season.
Nematodes are small soil-inhabiting worms. Several different types of nematodes damage rose roots. One type, the root-knot nematode, causes small galls or swellings on rose roots. Nematode-damaged roots cannot take up water or fertilizer as well as healthy plants. Nematode-affected plants may be stunted, weak, lack normal green color and do not flower as profusely, and have a shorter life span.
Nematodes can be controlled by planting nematode-free plants in areas where nematodes have not been a problem. Before planting, nematodes can be eliminated by fumigating the soil with SMDC (Vapam).
Crown gall is a bacterial
disease than causes large galls or swellings (1 to 2 inches in diameter)
on roots or lower parts of the main stem. Affected plants are stunted,
weak, and lack normal green color. Crown gall can be avoided by purchasing
healthy plants. Do not plant roses in areas where this disease has been
a problem. If swellings do occur, prune them out promptly. Also, avoid
wounding roots, lower stem and graft area of rose plants. mulching plants
to control weeds will help reduce wounding.
Rose mosaic is a viral disease
that causes irregular yellow mosaic patterns or lightgreen to yellow line
patterns in otherwise green leaves. Leaf symptoms are apparent during
periods of rapid shoot growth and may disappear. Mosaic weakens rose plants
and may increase severity of other diseases. Mosaic spreads slowly, if
at all, in established rose plantings through root grafts. Infected plants
should be removed from highly prized plantings. Avoid purchasing plants
showing mosaic symptoms.
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service Office.
Outside North Carolina, look for your state extension service partners.
of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data. All recommendations for pesticide
use were legal at the time of publication, but the status of registration
and use patterns are subject to change by actions of state and federal