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A Root Rot Complex of Horseradish. J. A. Percich, Associate Professor, Department of Plant Pathology, University of Minnesota, St. Paul 55108. D. R. Johnson, Research Assistant, Department of Plant Pathology, University of Minnesota, St. Paul 55108. Plant Dis. 74:391-393. Accepted for publication 28 January 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0391.

A root rot of horseradish (Armoracia rusticana) was shown to be caused by a complex of three organisms: Fusarium roseum ‘Acuminatum,’ Verticillium dahliae, and Pseudomonas fluorescens. Root infection by F. roseum ‘Acuminatum’ alone caused scattered brown lesions in the cortex and/or stele followed by dry, fibrous rot after 60 days. Infection by V. dahliae was restricted to the xylem tissue in the root, crown, and petioles, and no root rot occurred, regardless of inoculum density or incubation period in the laboratory. In the early stages of root infection, P. fluorescens was isolated from the interior of 62% of surface-sterilized roots free of both F. roseum ‘Acuminatum’ and V. dahliae. When horseradish was planted in soil artificially infested with different combinations of the three pathogens, the resulting preemergence losses and root disease severity were greater than with each pathogen separately. Optimal mycelial growth in culture and maximum disease severity in horseradish occurred at 24 C for F. roseum ‘Acuminatum’ and at 20 C for V. dahliae.