Why Are My Lettuce Leaves Slimy and Rotting?

Few things are more disheartening than reaching a maturing lettuce only to find the leaves gone slimy, brown and rotting, often from the inside or the base. This is bacterial or fungal rot, and it takes hold where lettuce leaves stay wet and air cannot circulate. Lettuce, with its dense leaves close to the soil, is especially vulnerable. The good news is that simple changes to how you grow and water it make a big difference. Let me explain.

Bacterial soft rot

The classic cause of slimy lettuce is bacterial soft rot. Bacteria enter through wounds, tip-burned leaf edges, or damaged tissue and turn the leaves into a soft, slimy, foul-smelling brown mush, often starting in the heart or the outer leaves touching the soil. It spreads fast in warm, wet, humid conditions and on crowded plants where leaves stay damp. Once soft rot is well into a head, that head is lost. It thrives on the moisture trapped in dense lettuce, especially after rain or overhead watering.

Grey mould and other fungal rots

Grey mould (botrytis) causes a soft brown rot covered in fuzzy grey spores, often starting at the base of the plant or on damaged and ageing leaves, especially in cool, damp, humid weather. Other fungi cause bottom rot and drop, attacking the lower leaves and crown where they meet the soil, collapsing the plant. All of these fungal and bacterial rots share the same loves: moisture, poor airflow, crowding, and contact with wet soil. So although the organisms differ, the prevention is the same.

How to prevent it

Keep lettuce dry and airy. Space plants properly so air circulates and leaves dry quickly — crowding is a major cause. Water at the base in the morning, never overhead in the evening, so the leaves are not left wet overnight. Grow in well-drained soil or raised beds so the crown is not sitting in water, and avoid splashing soil onto the leaves by mulching. Keeping the lower leaves from resting on wet soil helps; some growers space plants so heads sit slightly proud. Prevent tip burn (through steady watering and moderate feeding), since the damaged leaf edges it creates are a common entry point for soft rot.

Manage it and harvest cleanly

Remove and bin any rotting leaves or plants promptly — do not leave them in the bed or compost diseased material, as they spread spores and bacteria. Harvest heads promptly when mature rather than leaving them to sit in warm, wet conditions where rot sets in. After rain, a lettuce bed is at high risk, so check it and harvest vulnerable heads. There is no spray that cures soft rot, so prevention through spacing, dry-leaf watering, good drainage and prompt harvest is the whole game. Do those, and your lettuce will reach the kitchen crisp and sound rather than slimy.