White mold, or timber rot, caused by Sclerotinia sclerotiorum can be a serious problem in tomatoes grown in greenhouses and high tunnels.
This photo shows tomato stem with the diagnostic white mycelium and black sclerotia of the white mold fungus.
If not controlled properly, losses to white mold can become very serious. Once the soil becomes infested with sclerotia (i.e., the little black bodies) found on and inside infected stem tissue, it becomes very difficult, if not impossible to grow tomatoes, or any other susceptible crop, in that high tunnel.
All infected plant material needs to be cut out and/or removed immediately, and placed in a plastic bag and thrown away. Growing other susceptible crops near the high tunnel needs to be avoided at all costs. Spores of the fungus can blow into the high tunnel in the spring and cause infections in branches.
As a note, if infections are taking place at the soil line (see photos) then infection(s) probably originated from the soil within the high tunnel. If canopy infections are taking place then source of inoculum may have come from a nearby source outside the high tunnel. Protectant fungicides prior to bloom and fruit set may be neccessary, especially if there has been a history of white mold. Please see Table E-14 for products labeled for use in greenhouses and high tunnels in the 2012 Commercial Vegetable Production Guide.
Bottom Photo: As stems or branches become infected the plant will start to show signs of wilting. Infected branches and stems will often bleach out and become a light tan color and be very dry and brittle. If sclerotia are not present of the surface of the stem, they can almost always be found if you break open the infected tissue. Sclerotia can survive in the soil for many years without a host!
