Black Rot on Roses

History

• In the 1830s, rose growers in both Europe and the United States discovered black rot growing on roses. At least 25 names exist for black rot due to the discovery of the fungus in many countries during the same time period. Rose growers face a high demand for blemish-free roses, making black rot one of the most important rose diseases.

Symptoms

• Black rot appears in the form of circular black spots on the upper leaf surface of roses. The spots can reach up to 1/2 inch in diameter and often feature fringed edges. Once infected, the leaves turn yellow and drop off the plant. The symptoms may also occur on other parts of the plant, including the petioles and the rose hips. The canes often develop lesions that become black and raised. The weakening of the plant often leaves it vulnerable to other infectious diseases.

Favorable Conditions

• Diplocarpon rosae requires constant moisture to infect the rose leaf. In most cases, the first cycle of black rot starts with the splashing of water containing spores onto the leaves of the rose plant. The spores must be wet for at least seven hours to germinate. High humidity, wet leaf surfaces and temperatures between 68 and 75 degrees Fahrenheit present ideal conditions for black rot infestation. Spore germination and infection generally occur within one day, with symptoms appearing four to five days later. New spores germinate in 10 to 11 days.

Management

• Management of black rot is possible as the spores do not survive longer than one month and will not survive in the soil at all. To save a rose infected by black rot, prune and discard any leaves or canes affected by the fungus. Properly clean the area of all fallen leaves and take care to not splash water onto the plant when watering. Sunny locations with good air circulation allow for rapid drying of the foliage. Use fungicides in multiple applications, especially when new leaves appear on the rose.