From the photo you supplied, it looks like you have a fungus problem - black sooty mould is a common issue on citrus trees that indicates the presence of sap-sucking pests like scale insects. 

These pests excrete a sticky substance called honeydew, which provides a perfect surface for the mould to grow. Here are some steps you can take to address the problem:

• Inspect the tree carefully, especially the undersides of the leaves, for small insects.
• Treat the infestation by spraying the tree with a good quality horticultural oil, ensuring you cover both the tops and undersides of the leaves.
• Once the insects are eliminated, the honeydew will stop being produced, and the mould should being to disappear.
• Wash any affected lemons with soapy water before eating them.
• Maintain regular care with proper watering, feeding, and attention to help keep your tree healthy and less prone to future infestations.
• Citrus are ‘gross feeders’ so apply Daltons Garden Time™ Fruit & Citrus Fertiliser just outside the tree’s ‘drip line’ (directly under the outer circumference of the tree’s branches), at six-weekly intervals from late October to early December, recommencing late February through to early April.