Lemon trees need a high nitrogen fertiliser added through the
winter to spring. You will find this at hardware stores and most
nurseries.
Pests – Pests like mealy bugs, spider mites, aphids and scales
occasionally attack lemon trees. Use a solution of 1teaspoon of Neem oil, a little dishwashing liquid in 1 Litre of water. Spray all fruit trees every second week to prevent insects attacking your trees. This solution cannot be kept for longer than 8hrs. pour the left over onto the soil as it is good for the soil too.
Most lemon trees get bumps on the leaves at some point and it
is necessary to treat this with the above and if this does not work then it will be necessary to use a systemic pesticide.