It is normal for a lemon tree to lose an occasional leaf, however excessive leaf drop indicates the tree is stressed. Common stresses causing leaf drop include nutrient deficiency, water stress and pest infestation.
Nitrogen deficient lemon trees exhibit yellowing leaves and stunted growth before leaf drop. Young leaves are smaller, thinner and lighter green than usual. Older leaves change slowly from green to mottled yellow. Fertilise young lemon trees with nitrogen every month. Trees of fruit-bearing age should be fertilised just prior to flowering and again at the time of fruit-set.
Iron deficiency occurs in lemon trees grown in soils high in calcium, in wet soils with poor drainage, and in trees grown in cool temperatures. Symptoms of advanced iron deficiency, iron chlorosis, in lemon trees include pale to white, thin leaves and leaf drop. Add compost and peat to improve drainage in soils with heavy clay content. Treat trees suffering from iron chlorosis with iron chelates. Do not use a fertiliser containing phosphorous as this will exacerbate iron chlorosis. Read the fertiliser label carefully to ensure all the contents are chelated. Apply at rate indicated on fertiliser label.
Lemon trees are moisture sensitive and will drop leaves when over- or under-watered. Initially, leaves on drought-stressed trees curl and turn dry. Leaves on over-watered trees flatten and droop. To establish young trees, water thoroughly three times the first week. Water two times a week for the following two weeks. After four weeks, water only when the soil is dry 1 inch below the soil surface. Water lemon trees from the trunk to the edge of the tree's drip line until the soil is wet 2 feet below the soil line.
Citrus Rust Mite
Severe infestations of rust mite may cause leaf drop on lemon trees, especially in midsummer. Other symptoms of citrus rust mite include darkened, small fruit. Sunburst, Fallglo, and Ambersweet cultivars are particularly vulnerable to rust mite infestation. Treat citrus rust mite with neem or another horticultural oil according to the directions on the label.