Expert gardener’s tip: For clues to what’s causing them, observe the location and patterning of the yellow leaves.
Sunburn and Frost Damage
Leaves with bleached, yellow patches on the south or west side of your tree’s canopy indicate sunburn. A strictly cosmetic problem, it typically affects leaves newly exposed to the sun by pruning.
Lemon trees grow in USDA hardiness zones 9 through 11 and suffer frost damage below 29°F (-1.7°C). On mature trees, light frost typically yellows the outer leaves. On young ones, the inner leaves may also turn. Frost-damaged leaves eventually drop.
As soon as frost is forecast, water your tree thoroughly and drape it with a frost blanket.
Nutrient Deficiencies
Yellow areas on lemon tree leaves often indicate nutrient deficiency:
- Magnesium deficiency: A spreading, yellow-green blotch at the base of the leaves.
- Nitrogen deficiency: An entire canopy of pale-green to yellow leaves.
- Zinc, manganese or iron deficiency: Yellow leaves with green veins.
- Potassium deficiency: Yellow leaves with tips bent downward.
Regular applications of a 6-3-3 citrus fertilizer fortified with essential trace minerals prevents most nutrient deficiencies. Follow the label’s directions on timing and amount.
Disease
Phytophthora root rot targets lemon trees in poorly draining soil, turning their leaves pale green or yellow. To prevent it, plant on a well draining site or raised bed and water only when the top 6 inches of soil are dry.
Sap-sucking Asian citrus psyllids transmit lethal citrus greening disease. The infection causes asymmetrical yellowing on one area of the canopy. Leaf and fruit drop soon follow. Trees diagnosed with greening should be removed and destroyed.
Soil Toxicity
Boron and chlorine are essential to your lemon tree’s health. In excessive amounts, however, they cause leaf yellowing, early leaf drop and other problems. Boron toxicity is most severe in late summer and fall, Chlorine injury starts at the leave’s tips and moves down toward their bases.
To avoid them, irrigate your tree with water low in boron (or borax) and salt (sodium chloride). Always apply your fertilizer at the manufacturer’s recommended rate.
