Rosemary plants will turn black due to a variety of reasons. Humidity and insects can cause mold to form, overwatering the plant can lead to a fungal disease and not caring for the rosemary plant properly can cause it to shed its foliage. When you notice a black discoloration on the rosemary plant, treat it properly or the plant may eventually die. Fungal diseases and mold will continue to spread on the plant and insects can even transmit the diseases to other plants in your home or garden.
Insects can cause mold or mildew to grow on the rosemary plant. The insects often leave behind a honeydew substance that causes mold to grow. Once mold forms on the rosemary leaves, it will turn them black. Mold can form on any part of the plant and it may even thrive on the soil if the conditions are right. Too much humidity can lead to mold growth. Remove infected leaves from the plant with clean gardening shears. Cut the leaves off and disinfect the shears after each cut to prevent spreading the mold.
Fungal diseases often occur on rosemary plants when they are overwatered. Fungal diseases such as leaf spot will discolor the leaves and cause them to turn yellow, brown or black. Small black dots may appear on the leaves as well, which indicates that fungal spores have formed. Fungus will continue to spread unless you remove it from the plant by treating it with a pesticide. Fertilize the rosemary plants to enrich the nutrients after you remove the fungal disease.
Rosemary plants need full sunlight to thrive. Shady conditions can cause the leaves to die on the plant. The leaves will turn black or brown as they shed from the plant due to the lack of sunlight. The plant will continue to drop foliage until it receives an adequate amount of sunlight. Find a new location to place the rosemary plant where it will receive direct sunlight.
Water rosemary plants from the bottom to allow the plant to absorb the water. Place the water in the plant saucer to prevent getting water on the stems or foliage of the plant. Remove dead and diseased foliage from the plant to prevent it from spreading. Spray the rosemary with an insecticidal soap if you notice insects on the plant. Place the rosemary in a location where it can receive at least 8 hour of direct sunlight a day. Transplant the rosemary to a new location if you are growing it outdoors. Avoid placing rosemary in a humid location because the plants thrive in dry air.