G1060

Black Spot of Roses

Causes, symptoms, distribution, disease cycle, cultural and chemical management and available fungicides for control of black spot, Nebraska’s most troublesome disease of roses.

• Cause and Distribution
• Symptoms
• Disease Cycle
• Favorable Conditions
• Management
• Acknowledgment

The rose is the national flower of the United States. Roses are one of the most versatile ornamentals for landscaping, with cultivars adapted for any garden site and landscape purpose. They offer many positive attributes to a landscape, including flower color, form, texture, winter color and interest. Growing roses can be more challenging than growing other ornamentals because roses often require more intensive management. As with any group of plants, some cultivars are easy to care for and others are quite problematic. One of the greatest challenges to successfully growing garden roses is disease management.

Cause and Distribution

Black spot is caused by the fungus Diplocarpon rosae (perfect or sexual stage: Marssonina rosae). It is the most important disease of roses worldwide. It was first recorded in the United States in 1830 and is now found throughout North America. In Nebraska, it is the most serious disease of roses and, if not effectively managed, it can severely weaken plants and lead to increased susceptibility to winter injury or dieback due to other causes.

Symptoms

Black, nearly circular spots ranging in diameter from just under one-tenth of an inch to about one-half inch occur on the upper leaf surfaces (Figure 1A). They have characteristic feathery margins. The spots can coalesce (Figure 2), but often remain distinct. Infected leaflets usually turn yellow (Figures 3 and 4) and drop from the plant. Lower leaves are usually infected first, followed by middle and upper leaves (Figure 4). Excessive defoliation reduces stem length and size and the number and quality of leaves and blossoms. It also weakens plants and increases the risk of winter injury from cold temperatures. In resistant cultivars or during dry weather, only small spots may form without defoliation.

Symptoms also can occur on canes (Figure 1B). They usually occur in the form of raised purple blotches on immature wood of first-year canes, which later become blackened and blistered. Branches are rarely killed by lesions on canes; however, the pathogen survives the winter in these lesions. Inconspicuous, reddish-purple spots may result from infection of petioles, stipules, peduncles, fruit and sepals.

Figure 1. Black spots with feathery margins on leaflets (A) and a cane exhibiting symptoms of black spot fungus (B).
Figure 2. In severe cases or when environmental conditions favor disease development, black spots may coalesce into larger blotches.	Figure 3. Yellowing of a leaflet infected by black spot.
Figure 4. Lower leaves usually are the first to become infected by black spot, followed by middle and upper leaves. Infected leaves turn yellow and drop.

Disease Cycle

The black spot fungus overwinters as mycelia or spores in infected canes and leaves. In the spring, overwintering mycelia or spores cause primary (initial) infections on new shoots. Within about two weeks after primary infections, fruiting structures form within lesions and produce spores which cause secondary infections throughout the growing season.

Favorable Conditions

Rose leaves are most susceptible to infection when they are young and actively expanding. At least seven hours of continuous wetness is required for spores to cause infection. Infection occurs directly through the cuticle on both sides of the leaf. Temperatures ranging from 72°F to 86°F favor symptom development with 75°F being the optimum temperature for disease development.

Wet weather favors disease development and spread. Black spot usually is not a problem in greenhouses where humidity is carefully regulated and measures are taken to avoid prolonged wetting of foliage. Spores are spread mainly by splashing water but may become windborne. The pathogen also can be disseminated locally by windblown leaves.

Disease development can be influenced by plant architecture. Compact roses or those that develop leaves close to the ground are more prone to infection than roses with an open canopy. Crowded plantings generally have higher humidity within the canopy which favors disease development.

Management

An integrated disease management approach can be used to minimize damage caused by black spot. The first step is to select disease resistant cultivars. Most garden catalogs will identify rose cultivars resistant to black spot and other diseases. Growing resistant cultivars will save time and money spent on buying and spraying fungicides.

Rose cultivars vary widely in their resistance to black spot. Hybrid teas, grandifloras and miniature roses are more susceptible. Floribundas, shrub roses and climbers are more resistant or tolerant. Table I lists nationally known rose cultivars that have been demonstrated to be disease resistant and winter hardy over a period of four or more years through evaluations by Nebraska rosarians and extension Master Gardeners. Because of the occurrence of different pathogenic races of the black spot fungus, resistance of a given cultivar may vary depending on the environment.

Sanitation and cultural practices that do not favor disease development are an essential part of an integrated black spot management program. Removing and destroying infected canes and fallen leaves in the fall will reduce the amount of over-wintering inoculum which could cause primary infections during the following growing season. Planting roses in sunny locations and spacing plants adequately to allow good air circulation will promote quick drying of foliage. If possible, avoid sprinkler irrigation. If sprinkler irrigation is used, water in the morning and not in the evening. A 3-inch layer of mulch around the drip line of plants will reduce splashing of spores from fallen leaves. Maintain good plant health by proper watering and fertilization.

It is often necessary to include fungicide sprays in an integrated disease management program when roses susceptible to black spot are grown and environmental conditions favor disease development. In Nebraska, conditions favorable to disease development occur from mid-May to mid-September. Several fungicide products are available for control of black spot (Table II). Repeated applications at intervals specified on the label may be necessary to protect newly emerging foliage. To reduce the chances of fungicide resistance build-up in the black spot fungus, tank-mix or alternate fungicides with different modes of action. To maximize efficacy of a fungicide application, ensure thorough coverage of the foliage. Because rose foliage is waxy, adding a commercial spreader sticker (surfactant) or household detergent (about half a teaspoon per gallon) to the fungicide spray mixture will improve coverage. Always read and follow label instructions for mixing and applying fungicides. Do not apply a fungicide immediately before rainfall or sprinkler irrigation as these can wash off some or all of the fungicide, reducing efficacy.

Acknowledgment

The previous edition of this NebGuide was written by John E. Watkins, retired UNL Extensin plant pathologist.

Visit the University of Nebraska–Lincoln Extension Publications Web site for more publications.
Index: Horticulture
Ornamentals
1991, Revised January 2007