Why Is My Lemon Tree Dying from Root Rot?

Root rot is the single most common cause of death in container lemon trees and a significant problem in poorly drained garden soils. The condition develops when roots are deprived of oxygen by waterlogged growing media for extended periods, allowing opportunistic water mould pathogens — primarily Phytophthora and Pythium species — to colonise and destroy the root tissue. Once established, root rot spreads rapidly through the root system and the tree deteriorates quickly, often showing visible symptoms only when the damage is already severe. Early intervention and a change in watering practice are the most important factors in recovery.

Symptoms of Root Rot

Above ground, root rot presents as wilting that does not respond to watering, yellowing and loss of leaves beginning at the base or throughout the canopy, and a general collapse in vigour. The tree may drop both old and new leaves simultaneously. Stems can develop a dark water-soaked discolouration at the base. To confirm, remove the pot and examine the root ball. Healthy citrus roots are white or pale tan and firm. Roots affected by Phytophthora are brown or black, soft, and pulpy, with a characteristic musty smell. The rot often extends from fine feeder roots up toward the crown.

The Role of Drainage and Watering

Citrus are highly sensitive to waterlogging. Even a few days of saturated growing media deprives roots of oxygen and creates the anaerobic conditions in which Phytophthora thrives. The commonest causes are: pots without drainage holes or drainage holes blocked by compacted compost; standing water in the saucer not emptied after watering; potting into heavy general-purpose compost rather than a free-draining citrus or ericaceous mix; and watering on a fixed schedule regardless of whether the compost has dried. Always allow the top two to three centimetres of compost to dry before watering again, and never let the pot sit in standing water.

Emergency Treatment — Repotting

If the tree is still alive and the rot is not fully systemic, repotting offers a genuine chance of recovery. Remove the tree from its pot, shake off as much old compost as possible, and inspect the root system carefully. Cut away all visibly rotted roots cleanly with sterilised secateurs or scissors, cutting back to firm white tissue. Dust the cut ends with sulphur powder or a fungicide containing copper oxychloride. Repot into fresh, dry, free-draining citrus compost in a clean pot with excellent drainage. Do not water heavily for at least two weeks — allow the cut root ends to callus in a damp rather than wet medium.

Fungicide Drenches

Phosphonate-based fungicides (fosetyl-aluminium or phosphorous acid products) applied as a root drench are effective against Phytophthora and are systemic — meaning they are taken up by the plant and provide protection within the remaining tissue as well as in the root zone. Follow the dilution rate on the label carefully, as these products are pH-sensitive. A second application four weeks later provides insurance. Copper-based fungicides applied as a soil drench have some activity against Phytophthora but are less reliably systemic and should be considered a supplement rather than a replacement for phosphonate treatment.

Long-Term Prevention

The single most important preventative measure is correct potting mix. Use a citrus-specific compost or blend general compost with 20–30% perlite or sharp grit to ensure rapid drainage. Pot into containers with multiple large drainage holes and elevate them slightly on pot feet so drainage is never impeded. Water only when the compost is partially dry, and during winter reduce watering frequency dramatically — a cool lemon tree in winter may need watering only once every two to three weeks. Avoid pots that are disproportionately large for the root ball, as excess wet compost surrounds the roots and cannot dry between waterings.