Yield and Quality of Pathogen-Free Horseradish (Armoracia rusticana) Planting Stock: Implications for a Certification Program

Illinois hosts the most concentrated production of horseradish (Armoracia rusticana P. Gaertn., B. Mey. & Scherb.) and horseradish products in the United States. However, the overall quality and yield of Illinois horseradish roots has declined significantly within the past few years, and is believed to be related to the build up of soil- and insect-borne diseases, including viruses.  Horseradish is propagated asexually from root cuttings, which can lead to additional pathogen-build up issues.  Horseradish is plagued by many pathogens, but the most detrimental virus is thought to be Turnip mosaic virus (TuMV) and the most important fungal pathogen species are thought to be Verticillium dahliae and V. longisporum. To free horseradish plants of TuMV and Verticillium species, plantlets were subjected to meristem culture in vitro. Apical meristem explants, small enough to be free of leaf primordia (ca. ½ mm in length), were excised and grown in vitro. Meristem culture resulted in the regeneration of TuMV- and Verticillium-free horseradish planting stock.  To screen for the presence of these two pathogens, the meristem-derived plants were assayed using RNA reverse transcriptase and polymerase chain reaction (RT-PCR) and PCR, respectively.  Horseradish plantlets that were determined to be TuMV- and Verticillium-free were designated “pathogen-free”, or PF.  PF plants were acclimated, propagated in a greenhouse, and transplanted into the field to examine yield and quality differences in commercial production areas.  Three formerly productive, disease-susceptible horseradish cultivars were used in two geographically separated locations.  Within each location, two fields were selected based on their planting history.  One field had no history of horseradish, and the other field had been planted to horseradish within the past two years and/or had a history of disease.  Under field conditions, PF horseradish plants were more vigorous and productive than plants propagated by sets in the traditional manner (control).  PF planting material produced higher yields than the control in all fields (history and no history).  In many cases, PF plants produced two to three times the yield of the control.  Additionally, 90% of the PF plants remained symptom-free (marketable) after at least one season in a field with a history of horseradish.  Based on these data, we outline a field-based production protocol for rapid and cost effective propagation of PF horseradish sets.  Using PF plants combined with complementary cultural modifications growers can regain yields and quality of roots, even into a second growing season.