Black Spot

Black spots on leaves and petals is a sign of disease.

Black spot on rose leaves (LiquidGhoul) Public Domain

Spring and summer fogs and dew can leave behind just enough moisture to create breeding grounds for several different bacteria and fungi. While there is a specific disease called black spot, there are other bacterial and fungal diseases that can cause black spots, including citrus blast, common leaf spot, bacterial spot, anthracnose, bacterial speck, and entosporium leaf spot. Black spot mostly attacks roses, but its presence can indicate potential other problems in the garden or landscape.

What are the black spots on my leaves?

Black spots on leaves, fruit, canes, stems, and twigs are areas where a pathogen is breeding and feeding on plant tissue. These black spots are generally round because the infection begins at one point and spreads out equally in all directions. The spots have perforated edges and can reach one-half inch in diameter. As the area of dead tissue expands even larger, it can take on many different shapes. There may be a yellow halo around these leaf spots and yellowing in the surrounding plant tissue.

​Black spot: the disease

The black spot disease is caused by a fungus called Diplocarpon rosae. These fungi spread through rain and sprinkler splash, wind, ants, and aphids, to tender new growth. This disease usually spreads from lower leaves, moving upward. Garden tools can also spread the disease to other plants, so be sure to sanitize your tools with a household cleaner, such as Lysol, after each cut when working with a potentially infected plant.

This disease is not to be confused with blackspot, or Ascochyta blight, caused by Ascochyta rabiei. That particular fungal disease causes brown lesions with black dots, most commonly seen on chickpeas.

Black spot treatment

Once a leaf is infected, there isn’t anything you can do for it except remove it from the plant and throw it in the trash. Antifungal sprays, such as Bordeaux mixture, can be used to prevent future infection and at the first sign of the disease. Sulfur or diluted neem oil can also be used. Treatments will need to be repeated every 7 to 10 days for as long as temperatures are between 75 to 90 °F.

Preventing black spots
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As usual, prevention is a lot easier (and more effective) than treatment. These tips can go a long way toward preventing the problem of black spot in the first place:

• Remove fallen leaves
• Avoid overhead watering
• Spray new growth with fixed copper or Bordeaux mixture
• Irrigate in early morning to give plant surfaces time to dry out (spores need 7 hours of moisture to reproduce)
• Provide ample space between susceptible plants
• Prune for better airflow
• Apply sticky barriers to block ants from protecting aphids
• Spray aphids off plants early in the morning, using a garden hose, or knock them off with a soft-bristled brush

Next time you are out in the garden, see if you can spot any telltale signs of black spot on your plants.