Rose Diseases and Their Control in the Home Garden
Ornamental Disease Information Note 2
R.K. Jones, Extension Plant Pathologist
D.M. Benson, Plant Pathologist

General Information

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

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

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.

Black Spot and Powdery Mildew Fungicides

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

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

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

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

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

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.

Other Resources

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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.

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Recommendations 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 regulatory agencies.

Published by the North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last update to information: January 1999
Last checked by author: January 1999
Web page last updated on 16 December 1999 by A.V. Lemay