Saving Potted Citrus from Stem Rot and Leaf Drop

Diagnosing Blackening Stems and Yellowing Leaves on Indoor Citrus

Three young potted citrus trees (two limes, one Meyer lemon) in Alberta, Canada, brought indoors for winter, developed yellowing leaves, leaf drop, and progressive blackening of the stem from the top down after seven healthy months. The soil retains moisture too long.

Root Rot: The Core Problem

Blackening stem tissue moving downward is classic Phytophthora or Pythium root/crown rot, favored by waterlogged soil. Citrus requires excellent drainage. The "heavy" soil mix and possible standing water in a saucer created anaerobic conditions, killing roots. The plant cannot uptake water, causing yellowing and drop. The pathogen then moves up the stem.

Immediate Rescue Steps

1. Repot immediately: Do not wait. Gently remove the tree, wash all old soil from roots.
2. Trim rot: Cut away all black, mushy roots and stem tissue back to firm, white/healthy wood. Sterilize pruners between cuts.
3. Use proper mix: 50% coarse orchid bark or perlite, 30% peat/coco coir, 20% compost. Add a handful of horticultural charcoal.
4. Pot correctly: Use a container with ample drainage holes. No saucer holding water.
5. Water wisely: Water only when the top 2 inches of mix are dry. Then water thoroughly.
6. Light and air: Bright south window or grow lights. Good air circulation.

If the rot reaches the graft union or main trunk base, the tree may be lost. Act fast. Meyer lemon is more cold-tolerant but equally drainage-sensitive.