Root Deterioration Associated with Verticillium Wilt of Horseradish

Abstract

Mueller, J. P., Percich, J. A., and Mitchell, J. E. 1982. Root deterioration associated with Verticillium wilt of horseradish. Plant Disease 66:410-414. 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 X l04 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 X 106 colonyforming 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. Verticillium wilt of horseradish (Armoracia rusticana Gaertn., Mey. & Scherb.) and the root deterioration associated with it have been known since at least the middle of the 19th century in Europe. In 1899, Sorauer (17) described the syndrome in detail but did not identify its cause. Potschke (16) was the first to show that Verticillium spp. were involved in the disease complex. Poole (15) attributed the soft rot phase of the disease to an unidentified bacterium and reported that it caused very heavy losses in storage pits. Klebahn (9) implicated bacteria, Verticillium spp., and an unidentified fungus as causal agents. Verticillium wilt of horseradish, attributed to V. albo-atrum, was first reported in the United States in Michigan in 1931, where it was said to have caused a 20% loss in yield (2). In 1937, Heald et al