Citrus trees are susceptible to the development of sooty mould, which is the presence of a black fungal coating on the leaves, stems and fruit of trees. This condition can cause lemon trees in particular to produce less fruit, have a light fruit set and experience growth stunting.
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Sooty mould is a condition that is often seen on fruit trees, such as lemon, orange and grapefruit. Sooty mould fungi do not invade the tissues of the tree, so it is a condition instead of a true disease. A black or dark grey coating of crusty fungus often coats lemon trees with sooty mould. If severe, sooty mould can cause less fruit yield and reduced vigour. Mild cases of sooty mould may not damage tree health or fruit yield, but can result in a mess.
Infestations of aphids, whiteflies, mealybugs, leafhoppers or scale insects are the main causes of sooty mould on lemon trees. These insects feed on the leaves of lemon trees, using their sharp mouthparts to extract juices and plant sap from the leaves. Heavy consumption of plant sap, causes these insects to excrete a substance known as honeydew. This is a very sticky substance; sooty mould fungi adhere to it and grow on its surface. Sooty mould can also develop on anything honeydew sticks to, such as cars or driveways, near the insect-infested tree.
Preventing insect infestations is the best way to prevent sooty mould from developing on your citrus trees. Washing your lemon tree twice each week with a hard stream of water from the garden hose often knocks honeydew-producing pests from the tree. Another way to avoid sooty mould, is to choose a tree that is not a preferred host of sucking insects. According to Ohio State University, citrus trees such as lemon and orange are at an increased risk of developing sooty mould.
Spraying your lemon tree with a liquid dish soap solution can help reduce infestation of honeydew-producing insects. Combine 1 tbsp of liquid dish soap with 1 quart of water and spray your tree down with the soapy solution. Some trees and plants are susceptible to the effects of soap solutions, so test a small area on your lemon tree before applying the solution to the entire tree. If no adverse symptoms develop within seven days, it is safe to apply the soap solution to the entire tree. Horticultural oil products are also a common chemical control method for sucking insects. These oils work by coating the leaves and insects on the tree, causing insects to suffocate. Horticultural oils work on contact. Oil products also help loosen and remove sooty mould fungi from the surface of the leaves and fruit of lemon trees.
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