Potential suppressiveness of different field soils to Pythium damping-off of sugar beet

A highly reproducible, low resource requiring disease severity bioassay for testing suppression of Pythium damping-off in sugar beet caused by Pythium ultimum in soil is described. Changing of soil characteristics and the native microflora are avoided and standardization of the assay is carried out by testing infested soils on evenly germinating plants growing in an inert medium. The disease suppressiveness of the test soils is compared to standard, conducive soil, and quantified by calculation of symptoms on the hypocotyls. The reliability of the assay was investigated in nine well characterised organically or conventionally managed soils. In the standard, conducive soil it was clearly demonstrated that disease expression is correlated with available nutrient supply (oatmeal). Addition of oospores to the test soil had no effect on disease severity and the role of inoculum level is discussed. There was a clear tendency for low clay content to result in higher disease suppression. Partial sterilisation of the selected soils by gamma-irradiation and steaming at 54 C indicated that suppression was of microbial origin. Although texture may play a role in suppressiveness no clear effect, related to any management practice, was found. Tests on 64 soils confirmed that sandy soils tend to be more suppressive than clayey soils. (C) 2002 Elsevier Science B.V. All rights reserved.