Bottom Rot

Rhizoctonia solani

Fungalalso: Rhizoctonia bottom rot

A soilborne fungal disease of lettuce that rots the lower leaves and the base of the head where they touch the ground. Bottom rot is most damaging as lettuce nears maturity, when the fungus spreads from the soil into the wrapper leaves and can collapse the whole head, often just before harvest.

🔎 How to spot it

It starts as small rust-colored to brown, sunken spots on the underside of the lower leaf midribs and stems that touch the soil. The spots enlarge and run together into a slimy brown rot at the base of the plant, sometimes with a brown ooze and, in damp weather, a sparse brown web of fungal threads. The damage is most obvious at heading, when lower leaves brown and rot.

🥀 Damage it causes

The fungus rots the bottom leaves and the base of the head, so growers must strip off spoiled wrapper leaves, cutting quality and yield, and badly affected heads collapse into a brown, slimy mass and are lost. Because it strikes near maturity, the loss often comes right at harvest. Wet conditions can rot heads quickly once it starts.

🔬 What causes it

Bottom rot is caused by the common soil fungus Rhizoctonia solani, which survives in the soil and in plant debris and attacks the parts of the plant resting on or near the ground. It is favored by warm, wet conditions, dense plantings, and soil splashed up onto the lower leaves, and by fields with a lot of undecomposed crop residue.

🛡️ Prevent it

Grow lettuce in well-drained soil or on raised beds so the lower leaves stay drier and off the wet ground, and space plants for good air movement. Avoid planting into beds full of undecomposed residue, rotate with non-host crops such as sweet corn or onions, and water at the base in the morning so foliage dries quickly. Choose more upright varieties that hold their leaves off the soil.

🧯 If it is already here

There is no cure for a rotted head; remove and destroy affected plants and strip spoiled outer leaves, and harvest promptly once heads mature so they do not sit in the field and rot. Improve drainage and airflow for the next crop and lengthen the rotation. Preventive fungicides exist for serious or repeated problems, applied early around thinning, but cultural steps are the foundation.

💡 Good to know

Because the fungus comes up from the soil, anything that keeps the lower leaves dry and off the ground, such as raised beds, wider spacing, and upright varieties, prevents much of the damage. The disease is most dangerous right at harvest, so timely cutting limits losses. Rhizoctonia also causes damping-off of seedlings in the same soil.

For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.