It’s no wonder that tomatoes are the favorite plant of American vegetable gardener; their sweet, juicy fruits appear in a huge range of colors, sizes and shapes with flavor profiles to please nearly everyone’s palate. Tomatoes are also hugely popular with fungus, including those responsible for tomato timber rot.
What is Timber Rot?
Tomato timber rot, also known as sclerotinia stem rot, is a fungal disease caused by the organism known as Sclerotinia sclerotiorum. It appears sporadically around the time tomatoes start to flower due to the favorable conditions that heavy tomato foliage cover creates. Timber rot of tomatoes is encouraged by prolonged periods of cool, wet conditions caused by rain, dew or sprinklers and the high humidity that builds between the ground and the lowest tomato leaves.
Tomatoes with sclerotinia stem rot develop water-soaked areas near the main stem base, at lower branch crotches or in areas where there has been serious injury, allowing the fungus access to internal tissues. The fungal growth that begins in these areas progresses outward, girdling tissues and developing white, fuzzy mycelium as it grows. Black, pea-like structures about ¼-inch long may appear along infected sections of stems, inside and out.
Control of Sclerotinia
Timber rot of tomatoes is a serious, difficult to control problem in the home garden. Because the disease-causing organisms can live in the soil for up to 10 years, breaking the lifecycle of the fungus is the aim of most control efforts. Tomatoes with sclerotinia stem rot should be promptly removed from the garden – their death is inevitable, pulling them at the first signs of infection can protect unaffected plants.
You should aim at controlling the conditions that allow this fungus to germinate, amending your tomato bed as needed to increase drainage and watering only when the top 2 inches of soil are completely dry. Spacing tomatoes further apart and training them on trellises or tomato cages may also help, since dense plantings tend to hold in more humidity.
The spread of sclerotinia during the growing season may be halted by removing affected plants along with the soil in an 8 inch radius around each one, to a depth of about 6 inches. Bury the soil deeply in an area where non-susceptible plants are growing. Adding a plastic mulch barrier to remaining plants can also prevent the spread of spores originating from the soil.
At the end of each season, make sure to remove spent plants promptly and completely remove any leaf debris before plowing your garden. Do not add spent plants or plant parts to compost piles; instead burn or double bag your debris in plastic for diposal. Applying the commercial biocontrol fungus Coniothyrium minitans to the soil during your fall clean-up can destroy many of the infectious sclerotia before planting in the spring.