Black spot of rose, also known as leaf blotch, and leaf spot, is a disease caused by a fungus called Diplocarpon rosae. The optimal conditions for disease development are 75-85 F and high relative humidity. Infection may be greatest on leaves that remain wet for six hours or longer. Leaves and canes can become infected.
Leaf symptoms are roundish, black spots with fringed margins that can be up to 1/2 inch in diameter. The spots form on the upper sides of leaves. The tissue surrounding the spots turns yellow. Infected leaves may prematurely drop from the plant. Usually lower leaves are infected first. Excessive leaf drop weakens the plant, predisposing it to other forms of injury such as those caused by temperature extremes.
Cane symptoms are blister-like, purple blotches that later turn black. The fungus probably will not kill the branches, but cane infections can be important in the pathogen's survival through the winter.
Management of black spot includes sound cultural practices such as raking fallen leaves and removing infected canes. Avoid wetting leaves when watering and maintain air circulation around the plants to promote drying. Several rose cultivars have good resistance against black spot. If a cultivar is not resistant, fungicide applications may be necessary for disease control. Check with your local garden center for products labeled for black spot. Some varieties may require spraying every 7-10 days throughout the growing season, especially if the weather is cloudy, warm, and humid. Cover both sides of the leaves when spraying. If possible, alternate between different fungicides. It is important to read the label and follow application recommendations. Fungicide treatments are most effective when used in conjunction with good cultural practices.
This article originally appeared in the May 4, 2001 issue, p. 47.