Reduction of Stromatinia cepivora inocula and control of white rot disease in onion and garlic crops by repeated soil applications with sclerotial germination stimulants

The effect of soil-applied Allium crop products on the Stromatinia cepivora viability and the incidence of white rot in subsequent onion and garlic crops were evaluated in this work. The tested products were onion powder, garlic powder, onion oil, garlic oil and Allium waste (onion and garlic) that are described as sclerotial germination stimulants. Under in vitro conditions, data revealed that more than 80% of the sclerotia died in the soil treated with sclerotial germination stimulants. Under greenhouse conditions, soil-artificially infested with sclerotia of S. cepivora and treated with sclerotial germination stimulants for 6-months before cultivation, significantly reduced the incidence of white rot on onion and garlic. Onion oil, garlic oil and Allium waste were the most effective treatments, decreasing disease incidence by 78.6% in onion and 80.0% in garlic. Under field conditions, sclerotial germination stimulants were incorporated into the soil in commercial fields naturally infested with S. cepivora. Two fields were chosen based on differential sclerotial density. Within 6 months after treatment, more than 70% of the sclerotia died in the plots treated with sclerotial germination stimulants. In subsequent onion and garlic crops planted approximately one year after soil treatment, sclerotial germination stimulants were more effective than the control in reducing white rot symptoms coupled with low inoculum density (45.9 sclerotia/kg of soil). Reduction of white rot disease was accompanied by increased of growth and bulbs yield of onion and garlic plants. Despite the efficacy of sclerotial germination stimulants to reduce populations of viable sclerotia in soil with a high inoculum density (594.7 sclerotia/kg of soil), the pathogen caused substantial white rot and yield losses in subsequent onion and garlic crops planted approximately one year after soil treatment.