Citrus (lemons, limes, oranges and grapefruit) are evergreen. They retain foliage throughout the year, although some of the oldest leaves continually drop. Unusually heavy leaf fall can occur when a tree is stressed, which may be caused by a variety of factors, including improper irrigation (which keeps soil too dry or too wet), conditions that increase water loss from foliage (like high temperature, wind and low humidity), high soil salinity (from over fertilization or high salts in irrigation water), and root damage due to poor drainage, disease and pests, like gophers.
On mature citrus, water and fertilizer should be applied in a broad doughnut shaped area around the trunk extending a little beyond the edge of the canopy. Apply enough water to wet the soil at least 2 feet deep and do not irrigate again until soil in the top few inches begins to dry. Apply soluble fertilizer evenly on soil around the tree starting at least a foot from the trunk. Sufficient irrigation should follow to dissolve fertilizer and carry it into the root zone.
Hot, dry, Santa Ana winds rapidly increase water loss from citrus leaves. Green leaves may be damaged and drop if the root system cannot replace water fast enough. If this occurs and the soil is dry, thoroughly irrigate the tree. Do not keep the soil constantly wet as this can damage roots. If a tree is severely defoliated, paint a whitewash on the upper side of horizontal limbs and on the trunk to protect the bark from sunburn damage. You can buy whitewash at some garden centers or make your own by mixing interior flat white latex paint with an equal volume of water.
For more information see the growing guide “Citrus for the Home Garden”: http://mastergardenerssandiego.org/downloads/citrus%20for%20the%20home%20garden.pdf
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, October 2012