The role of crop waste and soil in Pseudomonas syringae pathovar porri infection of leek (Allium porrum)

Abstract Pseudomonas syringae pv. porri , the causal agent of bacterial blight of leek, is a current threat for leek ( Allium porrum ) production in the Netherlands. The roles of post-harvest crop waste and plant invasion from soil in leek plant infection was investigated with the purpose to gain better understanding on the ecology and epidemiology of this pathogen. In a survey done over 167 leek fields, P. syringae pv. porri was present in 101 fields. About a tenfold higher P. syringae pv. porri average infection rate was recorded in leek plants from fields where post-harvest crop waste was returned than in those from fields where no waste was returned. P. syringae pv. porri could survive for at least 1 month in shredded plant material made from infected plants (mimicking crop waste in practice). Field experiments done over three successive years revealed that leek plants that were treated with crop waste at the nursery and production growth stages consistently showed higher infection rates than plants that remained untreated. Plants that were treated with crop waste only at one of both stages revealed intermediate infection rates. In a plant–soil microcosm study, it appeared that P. syringae pv. porri strain P55R colony numbers in the leek rhizosphere were approximately 4 orders in magnitude higher than in corresponding bulk soils. P. syringae pv. porri was shown to colonize leek roots and cells of the pathogen were also present in the leaves. It was concluded that post-harvest crop waste play an important role in the epidemiology of P. syringae pv. porri in leek production. The following route for primary infection of leek plants is proposed, from crop plant residues via soil to leek roots and plants.