Solarization and Biofumigation for Organic Control of White Mold In High Tunnels

The fungus Sclerotinia sclerotiorum causes white mold in lettuce and attacks a wide range of other cool-season crops commonly grown in high tunnels. It survives summer heat by producing heat-resistant soil-borne sclerotia that germinate when conditions favor the pathogen's growth. In 2006, 2007 and 2008 we conducted field studies in commercial organic high tunnels in Kentucky to test two organically-acceptable management tactics for their ability to prevent fall germination of S. sclerotiorum sclerotia. The first was solarization, laying clear plastic over the soil surface to trap solar energy and heat the soil. The second was biofumigation, soil incorporation of Brassica green manures to release volatile isothiocyanates toxic to some soil-borne fungi. In late July plots were randomly assigned to be untreated, solarized, biofumigated with 1 kg Indian mustard (Brassica juncea cv. ‘Pacific gold') per square meter, or solarized and biofumigated. Mesh bags of sclerotia were buried 0, 5, 10 and 15 cm below the soil surface at the center and edge of each plot prior to treatment, then retrieved 2, 4 and 6 wk after treatment so that sclerotia could be incubated with moist soil at 16 ˚C for 6 wk to stimulate germination. Soil temperature was recorded hourly at 0-15 cm throughout the 6 wk treatment period. Surface temperatures in solarized plots reached an average daytime peak of 55 ˚C and stayed above 30 ˚C at night. Temperature swings were moderated with soil depth. Solarization for 4 wk completely inhibited germination of sclerotia at the soil surface and 5 cm below the surface in all years. Inhibition varied between years deeper in the soil profile. Biofumigation did not inhibit germination of sclerotia.