Sooty mould affects citrus trees and it is at this time of year when we are wanting to pick our lovely lemons for hot lemon and ginger to ward off colds and flu.
Sooty mould is a black fungal disease that develops on the fruit, stems and leaves. It is a secondary infection as it grows on the secretions of scale insects like aphids, mealybugs and whitefly.
To treat it - wash off the soot by wiping with a strong detergent / soap solution over the tree in the cooler months, like now! It is important to wipe off the soot as it can block the sun's light to the plant and reduce photosynthesis, which is essential for plant growth.
To prevent it - you need to deal with the pests that created the secretions - so spray the plant with a insect control spray, such as Yates Success Ultra at the first sign of bugs being present. If you dont like spraying, then an open shaped tree discourages insects, so give your tree a good prune this winter to reduce the density of its branches.