How to Prevent and Treat Black Spots on Rose Leaves

Black spots on roses are symptoms of the fungal disease Diplocarpon rosae. The spots appear as dark circular blotches or lesions with feathery edges, usually starting on the upper leaf surface of the lower leaves and stems of rose bushes. If left unchecked, black spots can cause yellowing, premature leaf drop, and eventually the death of the plant. Other rose bush diseases include powdery mildew, canker, and rust.

Similar to other roses diseases, black spots on roses (also known as the fungus Diplocarpon rosae) are the direct result of prolonged moisture on leaves and stems. Many conditions can increase the chances of black spots on rose bushes:

• At least seven hours of moisture: Extended periods of moisture are the primary culprit of black spots. When moisture like leftover water or morning dew sits on rose leaves and stems for longer than seven hours, the plants become highly susceptible to the disease.
• Lack of airflow: Areas with minimal airflow can trap moisture and dry slowly, increasing the chances of rose bushes becoming infected.
• Early spring temperatures: Black spot thrives in temperatures between seventy and eighty-five degrees Fahrenheit, which usually occur early in the growing season. It can easily spread when these temperatures occur at the same time as wet conditions.
• Clustered leaves: When landscapers and home gardeners plant rose bushes too close together, or cultivars that grow dense leaves low to the ground, excess moisture can get trapped in the dense canopy and create conditions where black spots can flourish.
• Young plants: Young rose bushes and new leaves don’t have the defenses to fight fungal diseases, so they’re especially susceptible to conditions like black spot.

To prevent black spot on rose bushes:

• Choose disease-resistant cultivars: A key way to prevent black spot on rose bushes is to grow disease-resistant cultivars. Floribundas, climbers, and shrub roses offer disease-resistant rose varieties, and any rose bush without densely packed leaves will naturally be resistant to black spot. Learn how to grow and care for roses.
• Give individual bushes ample space: Give your rose bushes ample space to grow when planting to encourage airflow and reduce the chances of a dense canopy that traps in moisture. Each cultivar will have different spacing needs—the minimum for miniature roses is at least two feet.
• Plant in a sunny location with good airflow: When planting, choose a spot that gets plenty of sunlight and is open on at least two sides—these conditions will help the plant dry faster and prevent excess moisture from collecting on leaves and stems.
• Water through a drip system: Where possible, avoid using an overhead watering system to water your rose bushes, as it can spread moisture around the plant. Instead, use a drip system or bottom irrigation system to apply water directly to the soil and roots.
• Water in the morning: If you water your plants with a watering can or sprinkler system, be sure to water in the morning (rather than the afternoon or evening) so that they have time to dry out before the temperatures drop at night.
• Clear old branches and leaves: Old branches and fallen leaves can trap extra moisture on and around rose bushes—periodic pruning can help refresh your bushes and offer them good air circulation.

If you notice black spots in your garden, here are a few ways that you can treat your plants:

• Immediately prune infected leaves and canes: As soon as you see the first signs of black spot on a rose bush, prune away and dispose of any diseased leaves and infected canes to reduce the chances of spreading to the entire plant. Sanitize your pruners afterward to prevent spreading the fungal spores to other rose plants.
• Monitor and reduce moisture levels: If you notice black spots on your rose bushes, take stock of the moisture on the leaves and see if there are ways to reduce it. If you’re currently watering with a top-down sprinkler system, consider switching to a drip irrigation system. If the bushes are in a spot with low airflow or little sunlight, consider pruning the surrounding plants to give them more light and air or transplanting them to a better area.
  
• Apply fungicide: If the black spots on your roses still linger even after you’ve reduced the moisture levels, consider using an organic fungicide spray or neem oil solution that you can apply directly to the leaves according to the package instructions. Some gardeners make a solution of baking soda and water as a homemade treatment for mild cases of black spot.

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