Timber rot is a sporadic but devastating disease of tomato that can cause significant plant and yield loss if cool weather and high humidity prevail for a long duration. The increasing number of greenhouses and high tunnels that grow tomatoes year-after-year has made the disease much more prevalent today, compared to decades ago.

Timber rot (sometime called "white mold") is a stem rot disease that is caused by the fungus Sclerotinia sclerotiorum. The fungus has a wide host range of over 300 crops and frequently vegetables such as beans, cabbage, lettuce, sunflower, carrots, cucumbers, peas, pumpkins and squash. A number of common weeds such as lambsquarters, pigweed, Canada thistle, and wild mustard also are susceptible and can serve as a source of infection.

Symptoms of timber rot mostly begin as water-soaked areas from the stem axils or in stem joints, either at or above the soil level. The fungus enters from the plant at the soil level if senescent tissue is present. Once established, the disease progresses from these areas. In time, the stem becomes covered with white "cotton-like" fluffy mycelium and girdled; later, the water-soaked area becomes dry, discolored (bleached appearance) and hard; and the plant (eventually) wilts, collapses, and dies.

As the disease progresses, hard, grayish-black sclerotia about the size of a plump grain of rice develop inside the stem. Sclerotia are hardened masses of mycelium containing food reserves. Their role in the life cycle of the disease organism is to detach from the host and remain dormant (in the soil) until environmental conditions favorable for infection occur. Slicing the diseased stem longitudinally reveals these embedded structures, making positive identification fairly easy.

Typical symptoms and signs of timber rot disease on tomato plant (right), and close-up pictures of the infected stem showing mycelia (middle) and sclerotia (right). (Photo credit: Z. Mersha, LUCE)

Timber rot infection usually does not occur until after flowering has begun. At this time soil moisture usually is high and soil temperature low because of the shading provided by leaves. Under this combination of cool, moist conditions, the above-mentioned sclerotia produce mycelium that infect the stem of the plant.

These sclerotia also produce sexual spores called ascospores from structures called apothecia (cup or little mushroom like structures). Ascospores of timber rot are wind-borne and can cause infection within several days after landing on the leaf of a plant. Again, cool (e.g. 60-70°F), moist conditions favor infection.

Timber rot in greenhouses and high tunnels that produce tomatoes year-after-year is problematic because of the buildup of sclerotia in the soil from diseased plants of previous crops. Crop rotation along with less favorable environmental conditions makes timber rot on outdoor tomato plantings much less of a problem.