Integrated control of allium white rot by reducing sclerotia in soil before planting and suppressing infection during the growing season

Three integrated tactics were tested to control white rot (caused by Sclerotium cepivorum Berkeley) of Welsh onion (Allium fistulosum L.) in a severely infested commercial Welsh onion field: reducing the population of sclerotia in soil by soil fumigation, controlling germinated sclerotia in the rhizosphere by soil application of simeconazole granules at planting and a soil drench with penthiopyrad later, and reducing the virulence of sclerotia by hilling, with serial lime applications in the winter. To reduce the population of sclerotia, we fumigated the soil with dazomet before planting and sealed the soil by either watering it (dazomet-water) or covering it with PVC sheeting (dazomet-cover). The average number of living sclerotia was reduced from 35.4/100 g of dry soil (average survival, 75.8%) in the untreated control to 7.1/100 g dry soil (25.6%) in dazomet-water and 0.5 (0.9%) in dazomet-cover. The lowered numbers of surviving sclerotia resulted in a clear difference in disease severity at harvest (rate of diseased plants: 96.0% in control, 97.9% in dazomet-water, 24.2% in dazomet-cover). The combination of the inhibition of germinated sclerotia and hilling, with serial lime applications in the winter, efficiently reduced white rot (15.3% of plants diseased). The integration of all three tactics almost completely suppressed white rot (1.6% of plants diseased), confirming the efficiency of this strategy in a severely infested field. This strategy should be tested in different types of infested fields, and the possibility of gradually reducing labor to control white rot should be verified.