Providing your houseplant with the environment it needs for optimum growth is the best way to avoid problems. However, if problems do occur, it's good to know that there are actions you can take once you understand the cause of the problem. Black leaves on your plants can occur for several reasons, including bacterial or fungal infections and problems with watering or fertilization.
Bacterial Leaf Spot
Bacterial leaf spot is one of the most serious foliage plant diseases. Symptoms can include the appearance of brownish-black tissue injuries on the leaves. A yellow halo may surround the lesions. Leaf spot can be accompanied by the appearance of stem cankers, and the stems may also turn black and shrivel up. To prevent bacterial leaf spot, ensure that plants receive the lighting they require for best growth. Avoid high humidity, overcrowding or poor air circulation. Avoid getting the leaves wet, and don't overwater your plants. Isolate infected plants and prune diseased leaves. If the stem is infected, the plant can't be saved and should be destroyed.
Fungal Leaf Spot
Symptoms of fungal leaf spot include the appearance of black spots on the leaves. The spots can grow bigger and merge with other spots to form irregularly shaped blotches. Leaves may turn yellow and die. This fungus is commonly seen on newly purchased plants, so you should keep them away from your other plants for a few months after bringing them home. Prevent fungal leaf spot by keeping the leaves dry. Avoid overhead watering. Don't crowd plants. If the infection isn't too serious, you can pick the infected leaves off your plant and destroy them.
Overwatering can cause the tips or the margins of your houseplants to turn black. Prevent overwatering problems by using pots with holes in the bottom. This helps ensure good drainage and avoids waterlogged soil. Water plants until you see water coming out of the drainage holes. Don't water again until the first inch of soil is dry to the touch. Providing your plants with a thorough watering also helps wash away any salts in the soil. High levels of salt can turn the leaves black.
Overfertilizing your plants can turn the tips of the leaves black because salts build up in the soil when too much fertilizer is used. One indication that your problem might be caused by too much fertilizer is if the color of your leaves is good except for the black tips. In addition to reducing the amount of fertilizer you use, you need to leach the extra fertilizer out of the potting soil. Place your plant in the sink and water it until large amounts of water pour through the holes in the bottom of the pot. Repeat this process half a dozen times over several hours. Allow the soil to dry thoroughly before watering your plant again to avoid any overwatering problems.