An Enrichment Method to Estimate Potential Seedling Disease Caused by Low Densities of Pythium ultimum Inocula in Soils. Ran Lifshitz, Department of Plant Pathology, University of California, Berkeley 94720. Joseph G. Hancock, Department of Plant Pathology, University of California, Berkeley 94720. Plant Dis. 65:828-829. Accepted for publication 13 February 1981. Copyright 1981 American Phytopathological Society. DOI: 10.1094/PD-65-828.

Inoculum densities of Pythium ultimum of about 25 propagules (sporangia and ripened oospores) or less per gram of soil can initiate significant seedling disease of alfalfa or cotton when environmental conditions are favorable. However, inoculum densities of P. ultimum are frequently too low to measure directly. Therefore, low densities of inocula were magnified by amending soils with dried cotton leaf fragments (1 mm²) and measuring population enhancement. A direct correlation was found between final inoculum density and disease severity measured in soils before amendment. This enrichment technique may be used to estimate the inoculum potential of P. ultimum in greenhouse or field soils.