What is black spot? Black spot is a common and potentially serious leaf spot disease affecting many types of roses. Black spot is found wherever roses are grown.
What does black spot look like? Black spot lesions are roughly circular and may be up to 1∕2 inch in diameter. Lesions often have feathery margins, and are dark brown to black in color. Black spot first appears during periods of wet weather when rose leaves are first emerging. Lower leaves become infected first, but the disease will spread to the entire plant. Severe black spot leads to yellowing of leaves and defoliation. Black spot can also develop on one-year old canes leading to raised, purplish-red blotches that blacken and blister.
Where does black spot come from? Black spot is caused by the fungus Diplocarpon rosae, which survives in rose leaf litter and in infected rose canes. Spores of the fungus are easily spread to newly emerging leaves by wind, or splashing rain.
How do I save a rose with black spot? If your rose is lightly affected with little or no defoliation and dry weather prevails, then no treatment is necessary. If your roses have a history of severe black spot, and the weather is wet, then consider applying fungicide treatments. Mancozeb, chlorothalonil, triforine, thiophanate-methyl, myclobutanil, ziram, sulfur, azoxytrobin, propiconazole, copper-containing fungicides and neem oil are available for black spot control. Baking soda (1 1/2 tablespoons) and horticultural oil (3 tablespoons) in water (1 gallon) has also been shown to be effective for black spot control. For most products, you will need to treat every seven to 14 days from bud break until wet weather subsides. Be sure to read and follow all label instructions of the fungicide that you select to insure that you use the fungicide in the safest and most effective manner possible.
How do I prevent problems with black spot in the future? Select rose varieties with a high level of black spot resistance. The hybrid shrub rose Rosa ‘Meipotal’ (Carefree Delight®), rugosa rose varieties such as ‘Blanc Double De Coubert’ and ‘Fru Dagmar Hastrup’, and climbing roses such as Rosa × Kordesii ‘William Baffin’ and Rosa × Kordesii ‘Henry Kelsey’ all have excellent resistance to black spot. When planting roses, select a high light environment, and space shrubs far apart to promote good airflow and quick drying of foliage. Promptly remove diseased leaf litter. Prune diseased branches six to eight inches below any obvious infections. Prune only in dry weather. Disinfest pruning tools between cuts by dipping them for at least 30 seconds in a 10% bleach solution or alcohol (spray disinfectants that contain at least 70% alcohol can also be used). Water roses at the base using a soaker hose. This method of watering will minimize wetting of leaves and reduce movement of spores of the black spot pathogen.
For more information on black spot of rose: See UW-Extension Bulletin A2531 or contact your county Extension agent.