A casual stroll from your garden may take a concerned turn if you notice that some of your prized plants are mottled with black spots on their leaves. Black spots on plant leaves are signs of an unhealthy plant that has likely fallen victim to a range of fungal diseases. Black spots on leaves are more common on some plants than others, and swift identification followed by appropriate remedies are the only ways to save your plants.
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Before you treat your plants for the causes of black spots, properly identify the symptoms of your plant. Black spots that occur solely on leaves are treated differently than black spots that extend to other parts of the plant like the blossoms, buds, roots and stems. Black spots may also be accompanied by red or brown spots, but they are rarely accompanied by grey spots as well. If you notice other symptoms on the leaves of the plant, like powder-like mildew or mould, you may have a more complicated infestation that requires additional treatments.
The most common cause of black spots on plant leaves is a fungal infection, and several types of fungi can cause the spots. Rose plants with black spots are infected by a fungus called Diplocarpon rosae; the fungus attacks the plant and eats away at the leave structures, causing black spots that eventually yellow and cause the leaf to die. Other fungal infections may be caused by fungi in the Alternaria, Coniothyrium, Microstroma and Rhytisma genera. Over 1,000 fungal species are currently known to cause black spots on plant leaves.
Bacteria causes are much less common than fungal causes, but certain plants are known to be susceptible to bacterial infections that cause black spotting on leaves. In particular, fruit trees like peach and apricot are prone to bacterial infections caused by Xanthomonas pruni. The aggressive bacteria hibernate in infected twigs throughout the winter, and take over nearby leaves when the spring growing season approaches. The result is leaves and fruit that are dotted with distinctive black spots.
Some plants are more prone to black spot infections than others. Black spot is one of the most common afflictions for rose plants. Tomato plants are also prone to cases of fungal black spot. Non-flowering plants and trees like maple and elm are also susceptible to black spots caused by various fungal infections. Ornamental garden plants, particularly those that sit low to the ground in moist areas, are susceptible to black spot problems.
Fungal black spot is treatable with a variety of fungicides, and sulphur-based chemical treatments are particularly effective if your plants are already infested. Copper-based fungicides are also effective treatments for black spots, especially in tomatoes. If your black spot is caused by a bacterial agent, fertilisers to replenish the soil and plant of health nutrients is a preferred method. No bactericide spray has demonstrated full effectiveness in curing bacterial black spot, according to horticultural expert Michael A. Ellis of Ohio State University. (See Reference 2) Instead, opt for regular pruning to remove infected areas and control the spread.
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- University of Illinois Extension: Fungal Leaf Spot Diseases of Shade and Ornamental Trees in the Midwest
- University of Rhode Island Landscape Horticultural Program: Peact Bacterial Spot
- University of Maine Cooperative Extension: Black Spot of Rose
- Cornell University: Tar Spots Factsheet
- University of Illinois Extension: Controlling Black Spot Disease of Roses
- Washington State University Whatcom County Extension: The Top 10 Most Unwanted Pests Black Spot