Horseradish (Armoracia rusticana Gaertn. Mey. & Scherb.), a root crop in the Brassicaceae family, is cultivated for its white, pungent, fleshy root (Courter and Rhodes, 1969; Kadow and Anderson, 1940). Approximately half of the total commercial horseradish supply of the United States is grown in the Mississippi River Valley, near East St. Louis, IL (Babadoost et al., 2001; Rhodes, 1977). Eau Claire, WI, Clearbrook, MN, and Tule Lake, CA, are the other three major horseradish production areas, each producing ≈15% of the total commercial horseradish in the United States (Babadoost et al., 2004). Horseradish also is grown on small scales in Connecticut, Massachusetts, Missouri, Mississippi, New Jersey, Ohio, Pennsylvania, and Washington states. Horseradish is a cash crop with the value of processed products exceeding $15,000 per hectare (Babadoost, 2006).
Internal discoloration of horseradish roots occurs widely in the world and is the main limiting factor in horseradish production (Babadoost et al., 2004; Eastburn and Chang, 1994; Gerber et al., 1983; Potschke, 1923; Stark, 1961). The disease has been known since 1920 in Europe (Potschke, 1923). Internal discoloration of horseradish roots in the United States was first described in 1931 in Michigan, where 20% of the yield was lost as a result of root infection (Boning, 1938). The discoloration of horseradish roots was reported in western Washington in 1937 by Heald et al. (1937) The disease was diagnosed in Wisconsin in 1973 (Mueller et al., 1982). Since the early 1980s, horseradish producers in Illinois have experienced substantial reductions in marketable yield of horseradish as a result of the internal discoloration of roots (Atibalentja and Eastburn, 1996; Babadoost, 2006; Eastburn and Chang, 1994; Gerber et al., 1983). Yield losses of up to 100%, caused by the internal root discoloration, have frequently occurred in Illinois (Babadoost, 2006; Babadoost et al., 2004).
Potschke (1923) was the first to report a Verticillium species as the causal agent of the internal discoloration of horseradish. Heald et al. (1937) identified Verticillium dahliae in the internally discolored roots of horseradish in Washington. Eastburn and Chang (1994) reported V. dahliae as the primary causal agent of internal discoloration of horseradish roots in Illinois. Percich and Johnson (1990) and Babadoost et al. (2004) reported that internal discoloration of horseradish roots is a disease complex. Three fungal species, V. dahliae, V. longisporum, and Fusarium solani, were identified as the causal agents of the internal root discoloration (Babadoost et al., 2004).
Horseradish producers save their horseradish sets from their harvest to plant in the next season. Most of the sets that the producers save are apparently healthy (asymptomatic), but they are often infected with Verticillium and Fusarium spp. (Babadoost, 2006). Set transmission of these pathogens is important because infected set usually gives rise to severely discolored root, which is unmarketable (Babadoost, 2006; Babadoost et al., 2004). Thus, starting horseradish production from pathogen-free sets is essential for managing the internal discoloration of roots (Babadoost and Islam, 2002).
Pathogen-free sets of horseradish can be generated by tissue-culturing of horseradish leaves (Meyer and Milbrath, 1977; Norton et al., 2001; Uchanski et al., 2004). However, tissue-culture generation of horseradish sets with the existing technologies is not economically feasible (Aitken-Christie et al., 1995; Uchanski et al., 2007). In addition, the sets can become infected before used by producers because the final stage of tissue-culture set production involves increasing sets in the field at least for one full growing season. Thus, alternative methods of producing pathogen-free horseradish sets are needed.
Hot water treatments of seed and nursery stocks have been commonly used in plant disease management (Herder and Turechek, 2006; Lear and Lider, 1959; Nega et al., 2003; Walker, 1923; Wilson, 1974). As early as the end of the 19th century, hot water treatment was applied to control loose smut caused by Ustilago nuda in cereals (Jensen, 1888). In the 1920s, hot water treatment of cabbage seed to control black leg disease was a standard method in the United States (Walker, 1923). McGee (1995) reported that hot water treatment was the only treatment available to eradicate deep-seated pathogens in the seed without significant losses of seed germination or plant vigor. Lear and Lider (1959) recommended eradication of root-knot nematodes from grapevine rootings by hot water treatment. Herder and Turechek (2006) reported eradication of Xanthomonas fragariae in strawberry nursery stock by hot water treatment. Neri et al. (2009) reported eradication of Neofabraea alba in apple by treating fruits in water at 50 °C.
This study was conducted to determine the effectiveness of hot water treatment of horseradish sets to control set-borne fungal pathogens without negatively affecting set germination or plant vigor.
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Trierweiler, B. Schirmer, H. Gräf, V. Tauscher, B. 2006 Hot water treatment—A possible method to reduce Verticillium infection of horseradish (black discoloration) International Symposium on Vegetable Safety and Human Health Beijing, China 21 23 Aug. 2006 Abstracts.
Uchanski, M. , Norton, M.A. & Skirvin, R.M. 2007 The logistics and economics of producing pathogen-free (PF), tissue culture derived horseradish sets 14 19 Horseradish Res. Rev. & Proc Horseradish Growers School, Univ. Ill. Extension
Uchanski, M. , Skirvin, R.M. & Norton, M.A. 2004 The use of in vitro thermo-therapy to obtain Turnip mosaic virus-free horseradish plants Acta Hort. 631 175 179