White Mold

White mold, also known as lettuce drop, is a disease that affects far more than just lettuce in your garden.

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White mold causes outer leaves to wilt (UCANR)

White mold (Sclerotinia spp.) can remain dormant in the soil for a very long time. It takes a significant amount of cool moisture to wake one of these fungi up from its dormant sleep, but the underside of a head of lettuce, or a cabbage, provides just the sort of humidity needed to trigger an awakening and the ensuing infection. This disease is also known as Sclerotinia stem and crown rot and it is caused by two different fungi, depending on the host plant.

White mold on lima bean pod (UCANR)

White mycelium on bean plant (UCANR)

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Symptoms of white mold

White mold is seen on outer leaves, lower stems, and pods, in the case of beans. Starting at the base, the mold spreads, causing outer leaves to wilt and fall away from the plant, while remaining attached. Garbanzo beans are particularly likely to become infected in the crown area. Affected plant tissue develops watery lesions as cottony white mycelium form on the surface. Mycelium are the vegetative part of a fungus, made up of threadlike hypha.

White mold sclerotia on bleached stem (UCANR)

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​Stems may become girdled by the decay. As damaged tissue dies and dries up, it will turn white and looked bleached. Tiny (0.25–0.5 inch), irregularly shaped black flecks, called sclerotia, can be seen on the surface and inside of dead stems. Sclerotia are the resting body of the fungi, made of of a cluster of hyphal threads, and able to remain dormant for a surprisingly long time.

Lesions and cottony mycelium of white mold on potato (UCANR)

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​White mold host plants

Along with lettuce and escarole, several members of the nightshade family and the cabbage family are susceptible to white mold. This means that your tomatoes, potatoes, and eggplants, along with Brussels sprouts, mustard greens, broccoli, cauliflower, turnips, radishes, rutabagas, and kale can all become infected. Dried bean plants, alfalfa, other broad-leafed plants, and many weeds can also become infected.

Tomato stems bleached by white mold infection (UCANR)

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How to avoid white mold in the garden

Since prolonged moisture is needed for this fungus to come among us [sorry, I couldn’t resist], keeping things dry is a good defensive plan. These specific steps can help you avoid a white mold problem in your garden:

• use raised beds
• irrigate to the side of plants, rather than directly on them
• space plants in such a way as to provide good air flow
• apply only as much nitrogen as is needed
• inspect senescent flowers and leaves for signs of infection
• remove and destroy infected plants immediately
• monitor plants closely after 60° to 70°F days with high relative humidity (95–100%)
• inspect nearby ornamental plants and weeds for signs of infection
• employ crop rotation with non-host plants
• in really bad cases, fungicides and soil solarization may be needed

As prevalent as white mold is, it’s a good idea to know what to look for ahead of time.