Rose Black Spot
4 May 2005
Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580 fax: 434.872.4578
If you grow roses you are probably familiar with the black spots that are commonly seen on these plants. Spring is a good time to be on the lookout for this disease. Depending on the susceptibility of your rose cultivars periodic fungicide use may be wise.
Rose black spot, caused by the fungus Diplocarpon rosae, is the most common cause of defoliation of landscape roses in Virginia. The disease is less of a problem in greenhouses where relative humidity can be carefully controlled. Susceptible landscape roses must be sprayed frequently with fungicides to keep the disease under control. Fortunately for rose growers, some of the newer cultivars and hybrids have resistance to this disease.
The symptoms start with small, round spots, ranging in size from 1/16" to 1/2" in diameter, appearing on the upper sides of leaves. Leaf tissue adjacent to spots turns yellow. Whole leaves eventually turn yellow and fall prematurely. Black spot can be distinguished from other leaf spot diseases of rose by the generally fringed margins and the darker and consistently black color of the leaf spots. Similar spots may appear on petioles and fruit. Raised, reddish-purple spots may also appear on canes. If black spot is left uncontrolled and early defoliation occurs, bushes are weakened and cane dieback the following spring may be severe. Weakened plants may continue to die even after the plants leaf out.
During dormancy the fungus survives in infected canes and fallen leaves. Spores are spread to the highly susceptible, young, unfolding leaves in spring by splashing water. Infection takes place only when water remains on the leaves for seven or more hours. Therefore, the disease is most serious in regions of high rainfall and high humidity. Because the fungus tolerates a wide range of temperatures, symptoms can continue to develop all season long if moisture is adequate.
The integrated pest management of black spot starts with choosing resistant cultivars. The most effective way to prevent black spot is to plant roses that have resistance to the disease. Most roses get black spot to some degree, but roses that have been bred for resistance to this and another common disease of roses, powdery mildew, will require less maintenance than those that are known to be susceptible to these diseases. Some hybrids and cultivars that showed good to excellent resistance to both black spot and powdery mildew in a 1990 survey in Maryland by R. C. Lambe are listed in the following table. Note that the degree of resistance exhibited by these roses in a given landscape may vary somewhat depending on local environmental conditions.
|Roses with good to excellent resistance to black spot and powdery mildew|
Canadian White Star
Maid of Honor
Miss All American Beauty
Always A Lady
Blanc Double de Coubert
Frau Dagmar Hartopp
Roseraie de l'Hay
The second line of defense against black spot is the use of cultural control tactics. A preventative program for black spot should begin in the fall with a thorough sanitation program. Diseased leaves on the ground should be raked and burned or removed. All diseased canes should be pruned back to healthy wood. These practices will reduce the amount of overwintering fungus. During the growing season, overhead irrigation, which prolongs leaf wetness, should be avoided. If plants are overhead irrigated, watering should be done in the morning rather than the afternoon so that leaves dry quickly.
Finally, the last resort is the use of chemicals. Unlike most insect problems, which are treated therapeutically, diseases are difficult to cure once diagnosed. Fungicides registered for black spot control should be applied preventatively to susceptible roses starting in spring before the new leaves become spotted. From this time through frost, the plants should never pass through a rainy period without a protective coating of fungicide on the leaves. Fungicides registered for black spot control include the following (trade names in parentheses where different from active ingredient). Propiconazole (Infuse, Lawn Disease Control, Liquid Systemic Fungicide), Chlorothalonil (Broad Spectrum Liquid Fungicide, Daconil 2787, Fungicide Disease Control, Fung-Onil Multi-Purpose Fungicide, Garden Disease Control, Multipurpose Fungicide), Mancozeb, Maneb, Captan, Copper hydroxide or copper salts (Copper Fungicide, Liqui-cop), Thiophanate-methyl (Halt Systemic, Lawn Fungus Control, Systemic Fungicide, Systemic Fungicide 3336WP). Most of these fungicides can be sprayed at 7-10 day intervals when rains are infrequent but during rainy weather, it may be necessary to spray the plants more frequently. It is always good to read the label before purchasing or applying any pesticide.
For more information about landscape topics contact your local Virginia Cooperative Extension office. Many county Extension offices have a help desk that is staffed by Master Gardener volunteers. These volunteers are trained to answer questions about home and landscape pest problems. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.