Control of Sclerotium cepivorum, causal agent of white rot of onion and garlic

White rot, caused by the ascomycotina fungus Sclerotium cepivorum , is the most serious disease of Allium food crops world-wide. S. cepivorum spreads and overwinters as sclerotia, which germinate only in response to Allium root exudates. It can be a very difficult disease to control because sclerotia may remain viable for more than 30 years, even in absence of a suitable host. Primary control options include sclerotia germination stimulants (SGS) and fungicides. The aim of this study was to develop control recommendations for this disease, including 1) optimize current fungicide controls in field studies, alone and combined with SGSs, 2) determine the sensitivity of S. cepivorum to tebuconazole (the most important fungicide for white rot control) and the dissipation rate of tebuconazole in soils with high cation exchange capacity (CEC), which are common in Allium production areas in California, and 3) examine potential causes of natural white rot disease suppression with the aim of developing a novel biological control in the future. Combinations of single or multiple applications of fungicides and biological controls were evaluated in field trials on onions in Tulelake, CA in naturally infested soil. The dissipation rate of tebuconazole in plant tissue and soil with high and low CEC was determined using liquid chromatography coupled with electrospray ionization mass spectrometry. Causes of natural soil disease suppression were examined by isolating bacteria and fungi that colonized S. cepivorum sclerotia, and comparing the number of isolated antagonistic bacteria and fungi in suppressive and non-suppressive sites using 454 pyrosequencing. Tebuconazole was the most effective fungicide, increasing marketable yield in many cases by over 5 metric tons per acre and reducing disease incidence by 60%. Two other fungicides, penthiopyrad and fluopyram, also significantly increased marketable yield. Boscalid and fludioxonil were not effective in reducing white rot incidence. Multiple applications of fungicides did not significantly improve disease control compared to a single application. Diallyl disulfide (DADS) reduced sclerotia levels in soil by approximately 80%, while garlic oil only reduced sclerotia by 50%. The efficacy of DADS alone was also extremely high, increasing marketable yield by 40%. Although garlic oil reduced sclerotia numbers in soil, it did not significantly reduce the incidence of white rot with or without fungicides. No significant differences in sensitivity to tebuconazole were detected among S. cepivorum isolates. Dissipation rates of tebuconazole were lower soil with high CEC (longer half-life) than in sandy loam soil in greenhouse experiments. Inoculation experiments showed that at least 5 ppm tebuconazole in garlic cloves or 9 ppm tebuconazole in soil significantly reduce the percentage of infection of garlic cloves. Two strains of Bacillus sp. and three strains of Fusarium sp. were identified as significantly antagonistic from the disease suppressive soil. There was a higher percentage of Bacillus sp in the soil from the suppressive field than the non-suppressive fields. These data provide evidence to promote the registration of new fungicides for white rot control and support an integrated control program that includes both a SGS and fungicides. They also support the recommendation of reducing applications of tebuconazole in soils with high CEC and applying tebuconazole only once at planting to control S. cepivorum. This may reduce total applications for white rot control. Several bacterial and fungal strains were also identified as potential future biological controls.