Black spot disease is a fungal disease that affects roses. The fungus, called Diplocarpon rosae, develops on rose leaves and stems, creating round black spots. Though it is called black spot disease, this fungus can also produce brown and red spots. The disease causes discoloration of the leaves and leaf drop, leading to unsightly appearances and weakened plants. Diplocarpon rosae develops in warm, humid weather and tends to attack in late summer.
This plant disease can lay dormant on rose plants through cold or dry weather. When conditions turn favorable, the fungus begins to proliferate. Diplocarpon rosae begins to develop when the temperature is between 65 and 75°F (about 18 to 23°C) and conditions are damp. Late summer, when the weather is often hot and the air is humid, is the most common time for black spot disease to develop in the rose garden.
Once the fungus is activated by the combination of the right temperature and moisture levels, it takes seven hours to begin its cycle. During this time, the rose leaves on which the developing fungus resides must be damp. The resulting black spots usually appear within seven to 14 days. The temperature window is fairly strict — when the weather rises above 85°F (about 29°C), the fungus cannot continue to develop.
Black spot disease causes discoloration of the leaves. The spots can grow up to 1/2 an inch (about 1.3 cm) in diameter. Yellowing of the leaves occurs around the black spots and eventually spreads over the entire leaf. Ultimately, this fungal infection can cause complete defoliation.
It also is important to note that watering an infected plant can cause the disease to spread. As the water splashes off the leaves, spores get caught in the drops and can transfer to nearby plants. Roses should be watered at the base to prevent splashing and to keep the leaves dry. Additionally, in hot weather, roses should be watered in the morning so that any moisture on the leaves will dry during the heat of the day.
Fungicidal sprays work well to combat black spot disease. Many products are available that specifically target Diplocarpon rosae. Depending on the concentration, most fungicidal sprays are designed to be used once a week or once every two weeks until the fungus is eradicated. Removing dead wood from the plant as well as infected plant material from the area helps to control the spread of black spot disease. As the fungus can remain dormant in cool and cold weather, all plant material should be disposed of in an incinerator to prevent it from spreading.