Roses are beautiful, but they can be quite difficult to grow. These flowers are susceptible to a number of diseases, including black spot or Diplocarpon rosae. This fungal disease most often attacks young leaves and, once established, black spot fungus spreads quickly. A seasoned rose gardener should learn to recognize black spot early and treat it promptly by pruning affected tissue or applying fungicidal soap and increasing air circulation within the bush. If black spot continues to be a problem in your garden, consider replacing your bushes with disease-resistant cultivars.
Know when to look for the first signs of black fungus. This disease most often crops up after warm (around 75 degrees Fahrenheit), wet and humid conditions that last longer than seven hours. Once germinated under these conditions, black spots will develop in anywhere from three to 10 days.
Look for early signs of black spot. Black spot fungus first manifests as lighter, yellow half-inch spots on the upper or lower surface of the leaf. These yellow spots distinguish themselves from other leaf diseases by their fringed edges. However, you need a magnifying glass to see them. A magnifying glass will also reveal the tiny black fungal spores developing in the center of the yellow spots.
Look for mature black spots. As the yellow spots grow in size, they will develop black centers. Their edges may also be blackened as the surrounding leaf tissue dies. Spreading spots may combine to encompass the entire leaf.
Examine the leaf litter at the base of defoliated rose bushes. Black spot is not the only cause of rose bush defoliation. However, if the leaves at the base of the bush are afflicted with the spots described in step 1 or 2, black spot is likely the cause.
Examine the rose bush's petals, petioles and canes. Black spot will occasionally manifest itself as small red dots or patches on other plant tissues. If examined with a magnifying glass, black spot fungus spores can be spotted in their centers.