Bottom Rot of Lettuce

Although Indiana is not known for lettuce production, an increasing number of growers find that augmenting retail sales with leaf lettuce can be profitable. Since lettuce is a cool season crop, field planted leaf lettuce around Indiana may be reaching harvest stage. Leaf lettuce can generally be grown with few pests. However, bottom rot of lettuce was recently observed in a field of leaf lettuce.

Lettuce affected with bottom rot often first appears stunted. A closer inspection may reveal that outer leaves are wilted and necrotic (Figure 1).  Eventually, the entire plant may die. If the plant is removed from the ground and observed, the leaf ribs that touch the ground may exhibit a dark, rotten appearing lesion (Figure 2). Fortunately, bottom rot does not spread from plant to plant. The plants that are diseased will most likely die without spreading the disease to other plants. This fact limits the seriousness of the disease.

Figure 1: Bottom rot of lettuce often causes stunting and wiling.

Figure 1. Bottom rot of lettuce often causes stunting and wilting.

Figure 2: Symptoms of bottom rot of lettuce may include brown, rotten lesions on leaf mid-ribs. In addition, the stem of the lettuce plant may appear rotten.

Figure 2. Symptoms of bottom rot of lettuce may include brown, rotten lesions on leaf mid-ribs. In addition, the stem of the lettuce plant may appear rotten.

The reason that bottom rot does not spread from plant to plant is that it is caused by a fungus which lives in the soil. The fungus that causes bottom rot is Rhizoctonia solani. Some growers may recognize this name. Rhizoctonia species may cause disease on many different hosts. For example, this fungus may cause damping-off of many seedlings. Plus, this fungus survives well in the soil.

There are several cultural methods that can help to reduce the damage caused by this disease. Deep plowing of residue buries the fungus that causes bottom rot. Crop rotation also helps. Anything that increases drainage, such as raised beds, should also reduce disease severity. Finally, although most growers do not find the disease severity sufficient to require fungicides, several are listed in the Midwest Vegetable Production Guide for Commercial Growers. These include: Endura 70WG®, Luna Sensation®, Nevado 4F® and Quadris®.  The timing and application methods of these fungicides vary widely.

Bottom rot of lettuce is usually not a serious disease. However, it makes sense to make oneself familiar with the symptoms and management of this disease.

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