Plants enhance indoor and outdoor spaces with natural colour, beauty and fragrance. However, sometimes plant leaves turn black, causing alarm and raising concern. Black leaves are caused by a variety of factors such as improper care and diseases. Healthy plants require a delicate balance of different factors. Understanding the reason behind the blackening of foliage helps restore natural colouring, thus minimising plant damage.
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A serious foliage disease, early symptoms of bacterial leaf spot (Pseudomonas spp.) includes yellow flecks on the undersides of leaves that develop into red streaks if untreated. The streaks grow long and turn black, causing leaf tissues around them to turn yellow. Leaf spots are accompanied with stem cankers that cause stems to turn black and shrivel up. To prevent bacterial leaf spot, keep the plant in a well-ventilated area exposed to the amount of sunlight it needs. Avoid overwatering the plant or wetting the foliage. Also, ensure plants are evenly spaced to prevent overcrowding. Prune an infested stem, as it cannot be saved, and discard.
Symptoms of fungal leaf spot include tiny black spots on the foliage of infected plants that increase in size and merge with other spots, forming irregular blotches and patches. Because fungal leaf spots are more common on newly purchased plants, isolate them for the first few months to establish whether they carry the fungal spores or not. Avoid overhead irrigation and keep the plant's foliage dry to prevent the disease. Prune infected foliage from the plant and discard it, and use a registered fungicide to control spread.
Improper watering causes plants leaves to turn black and drop prematurely. While overwatering causes margins or tips of leaves to turn black and roots to rot, underwatering causes them to dry up and appear scorched, especially if exposed to direct sunlight. While different plants have different irrigation requirements, ensure the soil around the plant remains evenly moist at all times or water when the top 1 to 1 1/2 inches of the soil feels dry. If container growing plants, use pots with adequate drainage holes at the bottom. Avoid causing puddles or pools of water at the base of the plants. If possible, use a drip irrigation system or soaker hose that provides a controlled amount of water at soil level instead of a garden hose.
Excessive fertiliser leads to salt build-up in the soil that causes plant leaves or only tips to turn black. Reduce the amount and frequency of fertiliser so the plant foliage regains its natural colour. Douse potted plants with water every one to two months so excess water seeps out of the drainage holes and leeches accumulated fertiliser salts as well.
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- University of Minnesota Extension; Bacterial Leaf Diseases of Foliage Plants; F. L. Pfleger, et al.; 2009
- Purdue University Extension; Houseplant Problems; Paul C. Pecknold; 1996
- City of Austin Watershed Protection: Fungal Leaf Spot
- Missouri Botanical Garden: Fungal Spots, Blights and Blotches
- Pan Germany: Bacterial Leaf Spot