For most lemon-owning gardeners the first sight of black mould on any plant is enough to bring on certain amount of chest pain, and at the very least a churned stomach. However any disease observed on citrus plants should be a cause of concern, and not just because of the considerable cost of replacing them.
Better known as sooty mould - Ascomycete species, you will be pleased to learn that this rather disgusting infestation looks a lot worse than it actually is. While the mould is indeed a fungus it is not actually an infection of the plant itself, but is in fact a benign growth than appears on the sugary solution expelled by aphids known as honeydew.
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You can remove sooty mould from the leaves by gently washing it off with a soft cloth and some luke-warm water. Of course this does not address the underlying cause of aphid or scale infestation. A more effective method is to use an insecticidal soap, dish soap, or detergent dissolved in water and sprayed onto the plant. Leave for ten minutes or so and then use a hose on a soft setting to wash off the sooty mould. One tablespoon of soap to a gallon of water is the usual recommendation.
If you intent to eat the lemons fruit then treat the pests organically. Neem oil, which is an organic broad spectrum pesticide,and can be used on fruit indoors and outdoors. It is biodegradable and has not been shown to be toxic to mammals, birds, bees, earthworms, or other beneficial insects.
Using systemic insecticides such as orthene, malathion, or diazinon will also be effective but will also harm beneficial insects and pollinators as well as entering the food chain. Always follow all instructions on such garden chemicals, and never use these on plants which produce fruit you intend to eat.