Tending plants can be an enjoyable way to spend your spare time, but if your green thumb isn't so green, you may find the garden of your dreams isn't quite as delightful as you'd hoped. Plant leaves can turn black for a number of different reasons, from something as simple as watering too much – or not enough.
Early symptoms of bacterial leaf spot (Pseudomonas spp.), a serious foliage disease, include yellow flecks on the undersides of leaves that develop into red streaks if untreated. Ultimately, these streaks grow long and turn black, causing leaf tissues around them to turn yellow. Leaf spots are accompanied by unsightly 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, and 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 this unappealing plant 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, but this can happen if you water too much, and can also happen if you water too little. 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, there are a few steps you can take to make sure your plant is properly watered. Ensure the soil around the plant remains evenly moist at all times, and when the top 1 to 1 1/2 inches of the soil feels dry, you know you're good to water. If you're growing plants in a container, use pots with adequate drainage holes at the bottom, and definitely 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 fertilizer leads to salt buildup in the soil that causes plant leaves (either the whole leaf, or only the tips) to turn black. Reduce the amount and frequency of fertilizer so the plant foliage regains its natural color. Also, douse potted plants with water every one to two months so excess water seeps out of the drainage holes and leeches accumulated fertilizer salts as well.