Why Is the Base of My Lettuce Rotting?

When a lettuce collapses from the bottom — the lower leaves and the crown at the soil line going brown, soft and rotten while the plant wilts — you are dealing with a base or crown rot. These fungal diseases attack where the lettuce meets the wet soil, and they can take a near-mature plant in days. Lettuce, with its low leaves resting on the ground, is especially exposed. The good news is that better drainage, spacing and planting depth prevent most of it. Let me explain.

Bottom rot and lettuce drop

A few fungi cause base rot in lettuce. Bottom rot attacks the lower leaves touching the soil, causing brown, slimy decay that spreads up into the head. Lettuce drop (sclerotinia) attacks the crown at the soil line, causing the whole plant to wilt and collapse with a watery, soft rot, often with white cottony fungal growth and small black resting bodies at the base. Both thrive in wet, humid conditions and where leaves and the crown stay damp against the soil. Once these rots girdle the base, the plant is lost.

Why it happens

These rots need moisture and soil contact. The classic risk factors are wet, poorly drained soil keeping the crown damp; planting too deeply so soil and moisture sit against the stem; lower leaves resting on wet ground; overhead watering that keeps everything wet; and crowded plants with poor airflow. The fungi live in the soil and on debris, so they return to beds where infected lettuce grew before. Cool, wet weather and heavy soils make outbreaks worse.

How to prevent it

Keep the crown dry and off the wet soil. Grow lettuce in well-drained soil or raised beds so water does not sit around the base. Do not plant too deeply — keep the crown at the soil surface, not buried. Space plants well so air circulates and the base dries. Water at the base in the morning rather than overhead in the evening, so plants are not wet overnight. Mulch can help keep leaves off bare wet soil, but keep it pulled back slightly from the crown itself. Remove lower leaves that rest on the soil if rot threatens.

Manage it and rotate

Remove and destroy any rotting plants promptly — do not leave them in the bed or compost them, since they spread the fungus and its resting bodies into the soil. Rotate where you grow lettuce so the disease does not build up in one spot, and clear away all debris at the end of the season. There is no easy cure once base rot has taken a plant, so prevention through drainage, spacing, shallow planting and dry-base watering is the whole strategy. Do those, and your lettuce will stand sound and crisp to harvest.