Contributors: J.R. Baker, Extension Entomology Specialist; D.M. Benson, Plant Pathology, Professor; L.F. Grand, Plant Pathology, Professor; R.K. Jones, Extension Plant Pathology Specialist; M.A. Powell, Extension Horticulture Specialist; D.L. Stephan, Extension Entomology Specialist; J. Scott, North Carolina Department of Agriculture; H. Singletary, North Carolina Department of Agriculture

ROSE DISEASES

Successful rose gardens adhere to certain cultural practices and include an effective disease, insect and mite control program. Poor disease control is one of the most common causes of failure with roses. This section will aid in the identification of the most common rose diseases in North Carolina and suggest guidelines for their control. Of the many diseases attacking roses, black spot, powdery mildew, rust, crown gall, and the cankers are the most serious.

BLACK SPOT OF ROSE
Photo courtesy of the Dept. of Plant Pathology


Black Spot (Diplocarpon rosae) is the most serious fungal disease of roses in North Carolina. Circular black spots surrounded by a yellow halo and a frayed margin are characteristic of infected leaves. As the spots enlarge or increase in number, infected leaves turn yellow, resulting in premature defoliation. If the disease is not controlled, defoliation continues, and severely infected plants may be almost completely defoliated by mid summer. If left untreated, the plant will become weakened, which will result in a reduction of flowers produced as well as increasing the potential for winter injury, dieback, and stem cankers.

Black spot is spread from leaf to leaf by water, which must remain on leaves for at least 6 hours before the infection takes place. New spots can develop in 5 to 10 days. Black spot must be controlled to grow good roses.

In fall, prune hybrid tea roses to about 18 inches and destroy the prunings. At some time during the winter months, remove all leaves from the plants and discard or compost. In spring, prune again to about 10 inches and destroy the prunings. This will eliminate some infected canes on which the disease overwinters.

Black spot cannot be adequately controlled without a good spray program. A complete uniform spray deposit on both sides of leaves is necessary. Spraying or dusting with a labeled fungicide must begin in the spring when leaves are half grown and continue at 7- to 10-day intervals and after heavy rains for the entire growing season. Do not let the disease build up before starting a spray program. If the disease occurs, immediately remove infected leaves as they appear and rake up and/or discard old fallen leaves during winter months.

POWDERY MILDEW OF ROSE
Photo courtesy of the Dept. of Plant Pathology


Powdery Mildew, caused by Sphaerotheca pannosa,is a very common fungus disease. White powdery masses of spores appear on the surface of young leaves, shoots and flower buds, as well as unopened flower buds. The disease may cause young shoots to be swollen or distorted and foliage to be stunted. Less leaf drop occurs with powdery mildew than with 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. It overwinters on fallen leaves and in infected bud scales and flower stems. Remove and destroy diseased foliage and canes and apply labeled fungicides during the growing season.

ROSE CANKER
Photo courtesy of the Dept. of Plant Pathology


Canker, caused by the fungal pathogen Crytosporella umbrina,appears as brown-to-black discolored areas on canes. Cankers commonly occur in plants that have been weakened by black spot, winter injury, or poor nutrition. Cankers frequently start at wounds or where a young shoot comes out of a cane. They first appear as small reddish spots on the stem. These spots enlarge and eventually encircle the stem, causing the cane and all growth above the canker to die, or they elongate until the crown of the plant is reached and the entire plant is killed.

For control, bushes should be kept free of black spot, provided with proper winter protection, and careful pruning practices. All canker-affected canes should be pruned back to healthy tissue by making clean cuts near a bud. Pruning tools should be disinfected with alcohol after each use on a cankered shoot. A fungicide spray program for black spot will also help control canker, especially applications immediately after pruning in the spring.

BOTRYTIS ON ROSE BUD
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University


Botrytis Flower and Cane Blight, caused by the fungus Botrytis cinerea, affects young succulent shoots, freshly pruned canes, 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.

Botrytis also causes small water-soaked spots with red borders on flower petals of white roses. Infected flowers turn brown and are often covered with a light tan to gray mold. Infected canes usually turn tan to black.

Botrytis damage can be reduced by removing flowers as soon as they begin to die and by promptly pruning out discolored canes back to healthy tissues. The black spot spray program will also reduce Botrytis during the growing season.

Rust,caused by Phragmidium sp., is a fungal disease that produces yellow or orange pustules on leaves. Plants may be defoliated. Disease may also attack young stems.

Rust overwinters in fallen leaves. It is spread by wind. Rust is favored by cool, humid summers and mild winters. It is troublesome primarily along the Pacific coast.

Nematodes.Rose roots are susceptible to Stunt, Lesion, and Root-knot nematodes. Root-knot nematode, (Meloidogyne sp.), 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, and lacking normal green color; they 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.

CROWN GALL ON ROOTS OF ROSE
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University


Crown Gall,caused by the bacteria Agrobacterium tumefaciens, begins as small swellings, usually at ground level but sometimes on the upper part of stems or on the roots. The swellings slowly increase in size, causing large galls or swellings (1 to 2 inches in diameter). Affected plants are stunted, weak, and lack normal green color. Plant death can result.

Crown gall can be avoided by purchasing healthy, disease-free plants and plant them in soil that has been free of crown gall-infected plants for at least two years. If swellings do occur, prune them out promptly and burn them. Avoid wounding roots, lower stem, and graft areas of plants. Mulching plants to control weeds will also help reduce wounding.

ROSE MOSIAC
Photo courtesy of the Dept. of Plant Pathology, North Carolina State University


Rose Mosaic is caused by a virus. Symptoms include irregular yellow mosaic patterns, light green-to-yellow lined patterns, small, angular, colorless spots on the foliage, and ring, oakleaf, and watermark patterns on otherwise green leaves. Infected plants can appear unaffected or slightly to severely dwarfed. Leaf symptoms are apparent during periods of rapid shoot growth, but may disappear. Mosaic weakens rose plants and may increase severity of other diseases.

Rose viruses are spread by propagation of infected plants. The diseases do not seem to be spread by insects or by handling. Mosaic spreads slowly, if at all, in established rose plantings through root grafts. Infected plants should be removed from highly prized plantings.

The only control for viruses is prevention; avoid purchasing plants showing mosaic symptoms.

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Last Modified: 07/10/96