The MU Extension Commercial Horticulture team hosted a high tunnel field day at a small farm in northeast Missouri a couple of weeks ago. The grower had a very well managed high tunnel with rows of tomatoes interplanted with lettuce. The tomatoes were in the row center, and the lettuce was planted on the shoulder of the beds. The tomatoes were growing vigorously and looked very healthy overall. The lettuce had sized up nicely and most was ready or nearly ready for harvest. Most of the lettuce looked healthy, but a portion of the heads had collapsed completely or were in the process of collapsing.
Photo c/o Justin Keay
Photo c/o Peng Tian
When plants are wilting or collapsing or wilting, whether greens or fruiting vegetables, it can be helpful to dig a plant out of the soil and inspect the roots. Are the roots thick, healthy, creamy white? Are the roots woody, corky, brown, rotting, or few and far between? It can also be helpful to cut the stem of the plant to look for signs of vascular discoloration in the xylem (tissue transporting water up the plant) or phloem tissue (tissue transporting sugars down the plant). When the vascular tissues of a plant are compromised or infected, plants can go downhill quickly.
We cut open several heads of lettuce that were in the middle stages of decline. The vascular tissue looked healthy everywhere except the base of the plants, where healthy tissue would abruptly transition into brown mush.
Photo c/o Peng Tian
The lettuce heads had a fuzzy white mold at the base of the crown of the plants. This type of fuzzy mold is always indicative of a fungal pathogen.
Photo c/o Peng Tian
Scraping off the moldy white material and soil from the top of the root crown revealed many small brown/blackish structures tucked around the base of the plants. The structures are known as sclerotia, an overwintering structure for fungi. This complex of symptoms are tell-tale signs of a disease called white mold, Sclerotinia sclerotiorum. In lettuce this disease is commonly called lettuce drop, the same disease is called timber rot in tomatoes.
Photo c/o Justin Keay
The sclerotia overwintering structures will reside in the soil until the following spring when they infect another round of plants. In fact, these structures can survive in the soil for up to 10 years. This disease is a problem during cool, moist growing conditions, present during spring lettuce production.
This disease has a very wide host range and over time can build up to damaging levels in high tunnels. The image below shows a tomato stem with signs of a timber rot infection. Sclerotia can also be found in bisected tomato stems of plants infected with timber rot.
Photo c/o Peng Tian
Photo c/o Z Mersha
The overwintering structures of this disease make it uniquely challenging because they can persist for so long in the soil. This disease is also problematic because it has an extremely broad host range, infecting virtually every vegetable crop besides sweet corn and crops in the allium family. Fungicide control options are extremely limited. Contans* is an OMRI listed bio-fungicide that has proven to be very effective for control of white mold. However, it is costly, and while labeled for use on many vegetables, it is not labeled for tomatoes.
In high value growing areas such as high tunnels, it is critical to take steps to reduce the buildup of this disease. Sanitation is a foundational practice for long term control. Removing infected plant material and the accompanying sclerotia can slow the buildup of this disease. Deep plowing can help bury the sclerotia and reduce disease pressure, but this isn't a surefire solution and may be impractical in a high tunnel. Further tillage could bring buried sclerotia back to the surface. In addition to the overwintering structures, this disease can spread through aerial windborne spores, called ascospores. Weeds can serve as a host for this disease, so keeping a weed-free perimeter around the high tunnel can help prevent ascospores from infected weeds blowing into the tunnel.
With proper identification and sanitation practices, this disease can be managed. Without following these two principles, this disease can build up to devastating levels in high tunnels. The MU Plant Diagnostic Clinic is a valuable resource for Missouri growers and can help identify diseases through digital or physical sample submissions. Check out the website for more details. https://extension.missouri.edu/programs/plant-diagnostic-clinic.
Further Reading:
- White Mold Disease - https://ipm.missouri.edu/meg/2021/5/whiteMold-PT/
- Timber Rot on Tomato - https://ipm.missouri.edu/MPG/2015/8/timber-rot-on-tomato/
*Inclusion of a pesticide tradename does not represent an endorsement of a product.