Rose Leaf Blight

Cause

• Rose leaf blight is also referred to as leaf spot or black spot of rose. Leaf blight is a common disease of roses that is likely to affect the plants growing in all geographical locations. The fungal disorder is caused by an infection from Diplocarpon rosae and is the main cause of defoliation in roses.

Damage

• The disease starts with the appearance of small-sized, dark spots on foliage upper sides during late spring. With disease progression, the spots gradually expand in size up to ½ inch in diameter and also develop feathered borders. Occasionally, there are yellow halos around the spots. In cases of severe infection, purple-colored spots are also seen on the canes. The foliage starts to turn yellow before dropping. Black fruiting bodies of the fungus are visible within the spots.

Favorable Conditions

• Rose leaf blight is frequently seen on rose plants on which the foliage is allowed to remain consistently moist for extended periods of up to 7 or more hours. Plants that are located in humid areas with poor air ventilation are also more prone to the infection. Miniature rose varieties are more susceptible to the disease than other roses.

Control

• Try to choose varieties and hybrids that are resistant to leaf blight and other foliage diseases such as rust and anthracnose. Avoid the use of overhead sprinklers in order to keep foliage drier. Water early in the day so that the plants get a chance to dry well during the day. Remove all affected foliage and destroy by burning. Chemical control options include the use of fungicides containing chlorothalonil or triforine. The use of neem oil is also effective in controlling rose leaf blight.