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 ½" 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, reddish-purple blotches that later turn black.
Black spot is caused by the fungus Diplocarpon rosae. The fungus overwinters on diseased canes and in infected leaves on the ground. In the spring, the fungal spores spread to emerging leaves and stems by wind or splashing rain.
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 and within the plants to promote drying.
Several rose cultivars have good resistance against black spot. Growing resistant or tolerant roses means that you will get better disease-control results with less frustration and less fungicide spraying. 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.