It is not uncommon for citrus trees to naturally drop a small number of leaves especially in winter. Where a large proportion of leaves fall it is a sign that the plant is under stress and there are a number of possible causes.
Low light levels area common reason for leaf drop and sometimes shoots may die back. The plant should be placed in a position where it receives the brightest light. Any shoots that have died should be pruned back to healthy wood.
Sudden temperature changes or low temperatures can result in the loss of leaves. This is more likely to occur in winter.
Care should be taken to avoid over or under watering as this will lead to leaf loss. An over wet compost will also result in the roots rotting. In winter water the plants well and allow the compost to dry out before watering again. As temperatures start to rise in spring and new growth appears increase watering to keep the compost moist.
Indoors a dry atmosphere can result in leaf drop, misting with tepid water and standing the pot in a saucer containing gravel and water will increase the humidity around the plant. Ensure the pot is not standing in water as this will cause the roots to rot.
Citrus trees should never be stood near a radiator or other heat source. The trees like good air movement and when conditions are favourable and temperatures start to rise ventilate the greenhouse or conservatory on warm sunny days.
Potting the plant in a container that is too large can put the plant under stress and will also reduce the number of fruit produced.
Scale insect are a common pest of citrus trees and can be found on stems and both upper and lower leaf surfaces often next to leaf veins. Scale insects can be flat or domed and brown or greyish white in colour. Severe attacks can result in leaf drop. Spray the plant with an appropriate insecticide.
A citrus tree can loose all its foliage but will recover and under ideal conditions will start to produce new shoots within a few weeks. While the tree is leafless take extra special care to avoid overwatering the plant.