Black spot of rose was observed in Europe and in the United States around 1830. Because it was observed by people of various countries around the same time, the nomenclature for the causal fungus varied with about 25 different names. The asexual stage is now known to be Marssonina rosae while the sexual and most common stage is known as Diplocarpon rosae. Because of the high demand for blemish-free roses and because D. rosae requires a humid, moist environment which can be found in many places throughout the world, black spot is considered the most important disease of roses.
Diplocarpon rosae over seasons as mycelia, ascospores, and conidia in infected leaves and canes. In the spring during moist, humid conditions, ascospores and conidia are wind-borne and rain splashed to newly emerging leaf tissue. Upon infection, disease progresses from the lowest leaves upward causing defoliation. Defoliation reduces photosynthate so the plant uses reserves in the root stalk to compensate for this. However, plants are often weakened because they are not as winter-hardy.
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Black lesions with feathery margins surrounded by yellow tissue are found on the leaves. Infected leaves drop prematurely. Purple/red bumpy areas on first year canes may be evident. Plants may be weakened due to defoliation and reduced flower production may be observed.
The best way for managing black spot of rose is to integrate several pest management strategies.
Resistant Varieties-This is the most effective management for black spot. Many varieties are now available with resistance to black spot. However, many of the older 'tea roses' are susceptible.
Cultural-Roses should be planted where the sun can quickly dry the night's dew such as the east or south side of the house. Space roses far enough apart for good air circulation to allow for quicker drying. Avoid overhead watering and keep foliage as dry as possible.
Sanitation-Remove infected canes and burn diseased leaves.
Fungicides-Mancozeb, chlorothalonil, benomyl, or a copper-sulfur dust can be used every 7-10 days upon new leaf emergence or first appearance of black spot. However, several applications are needed throughout a growing season, since black spot may develop at any time during moist, humid conditions
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This page was drafted by: Jennifer Fleer, Department of Plant Pathology, University of Nebraska-Lincoln
Images from: Dr. John Watkins, Department of Plant Pathology, University of Nebraska-Lincoln
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Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln
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