In the Garden: Black Spot on Roses

The fungus Diplocarpon rosae or "black spot" can infect first-year
rose canes with the right wet conditions, even during 
North Carolina winters. Photo by J.S. Corser, Durham Co. EMG. 

Black spot is a common and serious rose disease often reaching epidemic proportions in a season. The disease is caused by the fungus, Diplocarpon rosae. It is most severe after long wet, warm periods in the spring. Symptoms occur on rose leaves as circular, black spots surrounded by a yellow area. Infected leaves often drop from the plant. Infection continues throughout the summer months. The immature wood of first year canes develops raised, purple-red irregular blotches. Plants become stunted and produce fewer, paler flowers. By mid-summer severely infected plants may have lost all of their leaves.

Prevention & Treatment: The spread of black spot can be reduced and future infections minimized by following these cultural practices:

• Plant Resistant Varieties: (See the following list)
• Maintain Good Sanitation: Sanitation practices are critical in reducing future disease development. In the fall or winter remove all old leaves on the ground along with any mulch that has been contaminated with infected leaves. Replace with a fresh layer of mulch before new rose growth begins in the spring.
• Remove & Destroy Infected Canes: Canes affected by black spot have dark or reddish areas (lesions). Severely infected plants should be pruned back in the winter or early spring to within 1 to 2 inches of the bud union, according to variety and cultivar. During the growing season, remove and dispose of infected leaves as they appear.
• Keep Leaves Dry: It is best not to syringe plants with water, and do not use overhead irrigation, especially not in the late afternoon or early evening. Soaker hoses are an excellent way to water roses and to conserve water. Promote rapid drying of leaves by planting roses in the full sun. Space new plants far enough apart to allow for good air circulation.

Use fungicide sprays to control black spot effectively, even on resistant varieties. A rigorous fungicide program must be followed during conditions that favor disease development for susceptible cultivars. Select one of the following fungicide sprays, if disease is severe enough to warrant control: chlorothalonil, mancozeb, myclobutanil, propiconazole, or copper fungicides. Apply all chemicals according to directions on the label.

Black Spot - Resistant Roses:

• Hybrid tea: ‘Pride N Joy’
• Floribunda: ‘Sexy Rexy’
• Grandiflora: ‘Prima Donna’

http://www.clemson.edu/extension/hgic/pests/plant_pests/flowers/hgic2106.html