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Root Deterioration Associated with Verticillium Wilt of Horseradish. J. P. Mueller, Graduate Student, Department of Plant Pathology, University of Wisconsin, Madison 53706. J. A. Percich, Postdoctorate Fellow, and J. E. Mitchell, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 66:410-414. Accepted for publication 19 August 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-410.

A root-fragmentation plating technique was used to analyze populations of microorganisms in root samples taken at regular intervals from naturally infested soil. No organisms were detected by this method in the root tissue at planting. Within 1 mo of planting, total internal bacterial populations rose to 1.5 × 104 colony-forming units per gram of tissue without noticeable alteration in appearance of the tissue. By the sixth month, bacteria were present at or above 1 × 106 colony-forming units per gram. The major component of these populations with pectolytic activity was similar to Pseudomonas marginalis. Verticillium dahliae was first detected in the root tissue 2 mo after planting. Several Fusarium species were isolated during the fifth month of disease development. V. dahliae is believed to be primarily responsible for initiation of disease. Bacteria apparently do not incite disease in the absence of V. dahliae. The fungal and bacterial infections apparently weaken the root tissue, allowing ingress of other pathogens such as Fusarium sp., resulting in rapid and complete breakdown of root tissue.