Developing an Effective Strategy for Management of Internal Discoloration of Horseradish Root

• 2007 annual report
• 2008 annual report
• 2009 annual report

This study was conducted to develop an effective method for management of internal discoloration of horseradish roots. Internal discoloration of horseradish is a disease complex caused by at least three fungi, Verticillium dahliae, V. longisporum, and Fusarium solani. These fungi are carried in the propagating roots (set-borne inoculum) and also survive in the soil (soil-borne inoculum). The first step for management of the disease was to develop a reliable method to eradicate set-borne inoculum of the pathogen. This was achieved by treating the sets in hot water. The most effective treatment for eradication of set-borne inoculum, without adversely effecting set germination or plant vigor, was determined to be hot-water treatment of horseradish sets at 47ºC for 20-30 min. The Second step for management of the disease was protecting plants against soil-borne inoculum in the fields. This goal was fulfilled by set treatment with the fungicide fludioxonil (Maxim 4FS or Maxim Potato WP)or biocontrol agent Trichoderma virens (SoilGard 12G or G-41) prior to planting sets. By combining the hot-water treatment of the sets and application of the fungicide or biocontrol agent onto the sets the internal discoloration of horseradish roots was effectively managed.

Introduction:

Horseradish (Armoracia rusticana Gaertn. Mey. & Scherb.), a root crop in the Brassicaceae family, is cultivated for its white, pungent, fleshy root. Approximately half of the total commercial horseradish supply of the United States (US) is from the Mississippi River Valley, near East St. Louis, Illinois. Eau Claire, Wisconsin, and Tule Lake, California, are the other two major production areas of horseradish with each producing about 20% of the total horseradish supply in the US. Horseradish is a high-value crop with the value of processed-products exceeding $ 6,000 per acre. Internal discoloration of roots, a complex disease of horseradish, is a major limiting factor in horseradish production. Verticillium wilt of horseradish and the associated root deterioration have been known since the middle of the 19th century in Europe. Verticillium wilt was first reported in the US in 1931 in Michigan where it was reported to have reduced yields by 20%. Verticillium wilt and root deterioration resulted in a substantial yield loss in Wisconsin in 1973. Over the past 20 years, the growers in Illinois and other parts of North America have experienced substantial reductions in marketable yield of horseradish due to the internal discoloration of roots. Yield losses of up to 100%, caused by the internal root discoloration, have frequently occurred in Illinois. There is no effective strategy to control this disease complex. Potschke (1923) was the first to determine that Verticillium spp. causing internal discoloration of horseradish roots. Eastburn and Chang (1994) reported Verticillium 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 complex disease. At least 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). It is also likely that additional bacterial and/or fungal pathogens are involved in causing internal discoloration horseradish roots. Gerber et al. (1983) related the internal discoloration problem to soil and suggested that soil sterilization would help alleviate the problem. There has been, however, little success in controlling the disease through application of fungicides or soil fumigation. Attempts to control the disease using plant resistance also met with practically no success, as no resistant cultivars are commercially available and are not expected to be available in the near future. The practice of crop rotation alone is not effective due to set-borne nature of the disease and pathogen’s wide host range and long-term survival in the soil. The growers save their horseradish sets from previous harvest to plant in the next season. Most of the sets the growers save are apparently clean (asymptomatic), but infected. Set-borne inoculum of Verticillium and Fusarium species is considered important because infected sets usually give rise to severely infected root, which is unmarketable. Thus, starting horseradish production from pathogen-free sets is essential. Pathogen-free sets of horseradish can be generated by tissue culturing of horseradish leaves. However, there have been major constraints to large scale deployment of tissue culture. Tissue culture is frequently more expensive than other forms of propagation, it is more labor intensive, and requires more specialized environmental control throughout the numerous stages of development. Pathogen-free sets may also be produced by treating the sets in hot water. However, an effective temperature and treatment time have to be established in order to achieve eradication of set-borne pathogens without reducing germination of sets or vigor of resultant plants. All three pathogens (V. dahliae, V. longisporum and Fusarium solani) causing the internal discoloration of horseradish roots survive in soil for several years. Crop rotations have not had significant effect on controlling the disease, as the pathogens have wide host range. Attempts for controlling the discoloration problem by soil fumigation and fungicide treatment of plant stock have resulted in failure. Our preliminary trials had showed that fungicide fludioxonil (Maxim 4FS or Maxim Potato WP) and biocontrol agent Trichoderma virens (SoilGard 12G or G-41) were effective to protect plants in the field against soil-borne inoculum of the pathogens, when they were applied onto pathogen-free sets. The goal of this study was to develop effective strategy for management of the internal discoloration of horseradish roots.

This was a two-year project to develop an effective strategy for management of the internal discoloration of horseradish roots. The specific objectives of this research project were:
(i) to evaluate and demonstrate the effectiveness of thermo-therapy for control of set-borne inoculum of the internal root discoloration;
(ii) to demonstrate the effectiveness of the biofungicides for control of the internal root discoloration;
(iii) to demonstrate effectiveness of an IPM approach to solve the complex internal discoloration disease of horseradish root; and
(iv) to establish a sustainable horseradish production system.