Timber Rot, White Mold, Sclerotinia Rot in Spring Greenhouses and High Tunnels

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

The fungus Sclerotinia sclerotiorum along with other Sclerotinia spp. cause disease on hundreds of plant species, including most vegetables. Diseases caused by Sclerotinia, such as timber rot or Sclerotinia rot are becoming very serious problems in vegetables grown in greenhouses and high tunnels. The diseases overwinter in the soil as large resistant sclerotia, which multiply during years of susceptible crop production. However, even when a high tunnel is moved between seasons, the disease can be severe because the fungus overwinters both in and around the greenhouses and tunnels. The primary source of inoculum may originate inside as well as outside these structures. In the spring when the soil is moist, the fungal fruiting bodies emerge and spores (ascospores) are released. These ascospores will be released continually throughout the spring and are carried throughout the house if originating inside the structure. If the ascospores originate outside the house, they are carried on wind through the doors, vents or raised sides of nearby structures. Ascospores are usually carried or dispersed less than 330 feet. Therefore, it is especially important to use sanitation within 330 feet of a greenhouse or high tunnel. No plants, leaf clippings, potting mix, or soil from the greenhouse or high tunnel should be discarded within this area.

Inside the greenhouse or high tunnel, improve air flow in and around the plants by increasing in-row spacing and trellising plants to reduce disease incidence. Conversely, the proliferation of leaves near the soil will increases disease. The biocontrol Contans has been effective in managing Sclerotinia diseases in the field. The active ingredient of Contans, Coniothyrium minitans, parasitizes the overwintering (or surviving) sclerotia. If Contans is sprayed on the area around the high tunnel and watered into the soil, it may help reduce ascospore formation in future years. Contans must sprayed long before disease development occurs (2 months) to be effective within a crop year. Because the product is living, handle it carefully prior to use. Contans would be a good choice to try in fields or areas around greenhouses and high tunnels that are used repeatedly for a susceptible crop. See the Contans label for additional information.

Other fungicide products labeled for Sclerotinia in the greenhouse are Botran and Terraclor. Maximizing fungicide coverage to plants during application is important. Apply fungicides prior to disease development for greatest efficacy. Keep in mind that the fungus becomes established on senescing tissue first and then colonizes the plant. See the Botran and Terraclor labels for information on individual vegetable crops.

The black sclerotina on the small tomato fruit will overwinter and result in ascospore formation in future years. The fruit should be either buried or discarded more that 330 feet from the high tunnel.