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Bioassay of Pyrenochaeta terrestris Inoculum in Soil. Sidney R. Siemer, Research Assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331, Present address of senior author: Amdal Company, 6507 North Colonial St., Fresno, California 97704; Edward K. Vaughan, Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. Phytopathology 61:146-148. Accepted for publication 24 August 1970. DOI: 10.1094/Phyto-61-146.

A method is presented for assaying soil for propagules of Pyrenochaeta terrestris pathogenic on onion. Disease readings were made 6 weeks after planting susceptible Southport White Globe onion in selected soil:sand dilutions. Results obtained with this bioassay method showed that infective P. terrestris propagules were most often associated with soil particles 0.5 to 1.0 mm in diam, and that the propagules survive to a depth of 18 inches in the soil. Large variations in inoculum level existed in the same field as well as between fields.