From the picture, it looks like it was overwatered, and now that it's winter, the water is slow to evaporate.

Hello. Your lemon tree has a blight. Need sterilization, available trichlorfon solution with nitrogen, phosphorus and potassium compound fertilizer for root irrigation treatment, or lime water solution spray sterilization.

Lemon tree this look, the general situation has the following reasons caused by overwatering: overwatering will lead to water in the potting soil, so that the plant's root malabsorption leads to yellow leaves. At this time, it is necessary to take off the pot in time, cut out the rotten roots and then replant. Improper fertilization: too much fertilizer will burn its roots and lead to yellow leaves.

Occurring on the fruit is called Tee Rot, which is caused by a fungus, and is severe in plants of weak, dead and diseased trees.

The first observation of which position of the leaf yellow, if it is the veins of the leaf yellow is iron deficiency, leaf flesh yellow is nitrogen deficiency, Shi corresponding supplement iron, nitrogen fertilizer can be; the second is the soil fertility is not enough to lead to lemon tree nutrients can not keep up with the cause of leaf yellow.