Black spots on leaves and petals is a sign of disease.
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, and wind, to tender new growth. This disease usually spreads from lower leaves, moving upward. Tools used on an infected plant can also spread the disease to other plants, so be sure to sterilize tools with a 1 part bleach, 9 parts water solution, or spray tools with bathroom cleaner after each cut when working with a potentially infected plant.
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
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:
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