Overview of black spots
Black spot of rose, or rose black spot, is caused by the fungus Diplocarpon rosae. As one of the most common and important diseases affecting roses, it is found wherever they are planted. The fungus infects leaves in the spring and reduces the plant’s vigor throughout the growing season, causing them to be more susceptible to winter damage and other stressors. If left unchecked, bloom numbers will decrease each year.
Signs and symptoms of black spots
Symptoms vary depending on the rose variety as well as the strain of the fungus. Typical symptoms are rapidly expanding black or purple spots on the upper leaf surface and may possess yellow halos. Black lesions may also appear on young stems, and leaves may not drop despite the presence of black or purple lesions.
Signs of the disease are small black lesions on the leaf. These lesions contain fruiting bodies (acervuli), that carry the spores of the disease that are used for infection of nearby plants.
Disease cycle of black spots
Diplocarpon rosae, overwinters in fallen leaves and lesions on canes before producing spores in the spring. Spores are splashed onto the leaves and cause infection when provided with seven hours of moisture, and optimal temperatures ranging between 75-85° F with high relative humidity. Within two weeks of infection and the necessary moisture and temperatures, more spores are produced, allowing for rapid infection of nearby healthy rose plants during the growing season.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of black spots
To manage black spot planting highly susceptible cultivars should be avoided, which include most yellow and copper colored roses, most hybrid teas. Reported highly resistant cultivars are: ‘Coronado’, ‘Simplicity’, ‘Grand Opera’, ‘Carefree Beauty’, ‘Bonica’, and ‘Fortyniner’. For information on other cultivars inquire with your nursery or mail order source. ‘Carefree Beauty’ is one of the Griffith Buck roses. While at Iowa State, Dr. Buck introduced more than 85 rose cultivars. ‘Carefree Beauty’ is his most popular and widely grown cultivar.
The ‘Forty Niner’ rose is a hybrid tea rose. The ‘Coronado’ rose is a hybrid tea rose. There are white, pink, and red ‘Simplicity’ roses. The ‘Simplicity’ roses are floribunda roses. Hybrid tea and floribunda roses can be grown in Iowa, but should be given winter protection. ‘Bonica’ is a shrub rose. It was a 1987 All America Rose Selection. It is more winter hardy than hybrid tea roses. Some winter protection would be helpful.
Maintaining plant health
Since the fungus enjoys a wet and humid environment, plant roses in open sunny locations with ample spacing between plants to promote drying of leaves and decreased humidity. Overhead watering should be avoided or done early in the morning to ensure that the leaves are dry by nighttime. If plants are starting to grow close to one another, prune to allow for maintained air flow and drying of foliage.
Inspecting (Scouting) for disease
Early in the season, look for fringed margins on leaves as well irregular black spots. Late in the season, yellow hallows will develop and leaf drop off will begin to be apparent.
Rake and discard all fallen leaves and make sure to prune and discard all infected canes and leaves throughout the growing season. If composting, do not place composted infected leaves into an area where roses are or will be grown.
Fungicides can be applied regularly to prevent the spread of infection to new leaves. They should be applied every 7-14 days and reapplied after ¼ inch of rain or overhead watering. Labels should be checked for control of black spot, but products with copper and/or chlorothalonil as active ingredient may help.