Leaf drop on a lemon tree is a common symptom indicating the tree is under environmental or physiological stress. Citrus trees are sensitive and often shed leaves to conserve resources when stressed. Identifying the specific cause is the first step toward restoring the tree’s health and preventing further defoliation.
Cultural Care: Hydration and Soil Issues
Improper watering is the most frequent reason a lemon tree drops its leaves, creating stress that the tree manages by shedding foliage. This problem stems from two extremes: chronically waterlogged soil or prolonged dry-out periods.
Overwatering suffocates the roots, which require air pockets to function properly. This leads to the yellowing and dropping of older leaves as the tree suffers root damage.
Conversely, underwatering causes drought stress, characterized by leaves that curl inward or appear dull before turning brown and dropping prematurely. Lemon trees prefer deep, infrequent watering, allowing the top inch or two of soil to dry out between applications. If the tree is in a container, ensure the pot has ample drainage holes and the soil is well-aerated to prevent water from pooling.
Environmental Shock: Temperature and Light Stress
Lemon trees are highly susceptible to sudden changes in their surroundings, and these environmental shifts can trigger a defensive leaf drop. Exposure to cold is a common culprit, as citrus trees are sensitive to temperatures below 55°F (13°C). They will shed leaves when subjected to cold drafts or rapid temperature drops. This is often seen when trees are moved near drafty windows or brought indoors for the winter.
The amount and quality of light also significantly impact a lemon tree’s health. Moving a tree from a bright outdoor setting to a dimly lit indoor location causes a major shock, leading to the defoliation of leaves that cannot photosynthesize efficiently. Furthermore, intense heat or moving a tree from low light to direct sunlight too quickly can cause leaf burn and subsequent leaf drop.
Soil Health: Nutrient Deficiencies
A lack of essential nutrients causes leaf yellowing, known as chlorosis, and subsequent leaf drop, which presents with distinct visual patterns. Nitrogen deficiency is a common issue that causes the entire leaf to become uniformly pale green or yellow, starting with the older leaves. This occurs because nitrogen is a mobile nutrient that the tree moves to new growth. Correcting this requires a nitrogen-rich fertilizer formulated specifically for citrus.
Deficiencies in micronutrients produce more patterned chlorosis. Magnesium deficiency typically appears on older leaves as a distinctive yellow blotch near the base, leaving an inverted V-shaped green area near the leaf tip before the leaf drops. Zinc deficiency shows up as small, mottled yellow areas between the green veins of new growth, often resulting in smaller, narrower leaves. These imbalances require targeted fertilization, such as a foliar application of a chelated nutrient.
Biological Threats: Pests and Pathogens
Certain pests and diseases inflict damage that directly results in leaf drop. Sap-sucking insects, such as scale, mealybugs, and aphids, feed on the tree’s vascular system, weakening it and causing leaves to yellow and fall. These pests often excrete honeydew, a sticky substance that leads to the growth of black sooty mold, further impeding photosynthesis and contributing to leaf loss. Scale insects appear as small bumps on the stems, while mealybugs are recognized by their cottony, white masses. Aphids can cause new growth to twist and distort.
The most serious biological threat linked to leaf drop is the soil-borne pathogen Phytophthora, which causes root rot. This fungal infection thrives in consistently waterlogged, poorly draining soil, attacking the roots and preventing them from absorbing water and nutrients. Above-ground symptoms include the sudden yellowing of leaves, followed by rapid defoliation that mimics severe drought stress. Unlike pests, root rot requires examining the root ball for soft, dark, and decaying roots to confirm the diagnosis.