Problem type: Disease
Name of problem: Black Spot
Plant name(s): Rose
Symptoms / Characteristics:
Circular to irregular black spots or blotches, 2-15 mm in diameter. Spots may coalesce to form larger blotches throughout the season. Spots are usually found on the upper leaf surface and have characteristic fringed margins. Leaf tissue surrounding the spots will begin to yellow until the entire leaf turns yellow and falls off the branch. One-year old rose canes can also become infected. Raised purplish red lesions may develop on the canes that will later turn black and appear blistered. Lesions on canes do not normally do much harm to the plant, but do serve as a survival site for the fungus over the winter.
Black spot is caused by the fungus Diplocarpon rosae, which infects only rose species. The fungus thrives under warm, wet conditions with high humidity and temperatures around 20-24°C. Spring offers a good environment for the disease to develop as weather is wet and leaves are still expanding. Dispersal of fungal spores is usually through splashing water, but can be spread through infected fallen leaves. The fungus does not live in the soil, but survives in fallen debris and infected canes.
Control / Preventions:
Reducing leaf wetness is the most important factor in helping control black spot. Choose an appropriate planting site when planting roses. The site should receive lots of sunlight and good air circulation. This will aid in drying the foliage after a watering or rain. Leaves should not remain wet for longer than 7 hours, as this is the length of time it takes for infection to occur. Avoid watering in the evening and overhead watering during humid weather.
Remove fallen leaves and prune out infected canes to reduce overwintering sites for the fungus and any chance of spreading the disease. Fungicide sprays can be applied throughout the growing season to control black spot. Chemical control is a preventative means of control and will not eliminate the disease once already present. Spray programs can begin as soon as the foliage emerges in the spring, continuing throughout the summer at 7-14 day intervals, or depending on favorable conditions for black spot development. Products that can be used to control black spot include Banner and Captan, or products containing copper or sulfur. Fungicides will not be successful if cultural and sanitation practices are not followed.
Black spot resistance roses are available, but may be hard to find. Some resistant cultivars include Fortyniner, Grand Opera, Carefree Beauty, Simplicity, Coronando and Dave Thompson.