What to do when fungus attacks lettuce in the garden

The fungus Sclerotinia can infect hundreds of types of plants. Unfortunately, there are few fungicides available to control it.

For The PressQuestion: I am growing several types of lettuce. They were looking beautiful one day, then one morning all the buttercrunch was lying flat on the ground. What happened?

Answer: The death of your lettuce was caused by an infection by a fungus called Sclerotinia. This fungus can infect more than 360 different host plant species and has more than 61 different common names. It is called white mold in beans, watery soft rot in cabbage, stem rot in potato and tomato plants, and drop in lettuce.

Cool, moist weather promotes the development of the symptoms, which causes wet rotting or watery decay at the base of your plant. The outer leaves will wilt first and then the entire plant will collapse. The best way to identify this fungus on lettuce is to look for small black structures called sclerotium at the base of the plant. They are flat spherical structures that are 3 to 10 mm long and 3 to 7 mm wide. These structures are a source of inoculum and enable the fungus to survive from season to season. It can persist in the soil in this stage for many years depending on soil conditions.

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Sclerotinia in lettuce begins when the outer leaves still attached to the plant naturally fall to the soil surface where they come in contact with the sclerotium. The fungus grows through the leaf and into the base of the lettuce. Once there it begins to destroy the stem causing the plant to collapse or drop. The entire lettuce can be invaded by the fungus. When the plant is examined there will be a cottony white fungus growing on the plant surfaces and then followed by the growth of the black structured sclerotia when conditions are favorable.

This fungus causes the greatest damage when temperatures range from 60 to 70 degrees. High humidity with dew promotes the spread and increases the intensity. When the environmental conditions change suddenly the spores are injected into the air and carried to healthy plants where they germinate.

Unfortunately for the homeowner there are few fungicides available for control. Specific cultural practices have been shown to help control infection from this fungus.

The plants should be removed as soon as symptoms are observed and disposed of properly. Do not compost. When planting lettuce make sure the soil is well draining. Plants should be spaced widely and avoid overhead irrigation.

In areas where there was an active infestation lettuce plants should not be grown there in subsequent years.

Mona Bawgus is a certified master gardener and consumer horticulturist with Rutgers Cooperative Extension of Atlantic County. Write to her c/o Rutgers Cooperative Extension, 6260 Old Harding Highway, Mays Landing, NJ 08330. Email: bawgus_mona@aclink.org

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