Battling disease in this unique product

Huntsinger Farms in Wisconsin grows more horseradish than anybody else in the world, and a crop rotation program is what keeps disease out of their roots.

As hot as the chemicals in horseradish are, they are no match for diseases caused by Verticillium and Fusarium. Field infestations by these fungi can cause root discoloration and loss of market. Growers in Illinois, the horseradish headwaters of the world, are particularly feeling the bite.

“Disease problems are a major limiting factor to horseradish production,” says Mohammad Babadoost, plant pathologist at the University of Illinois at Urbana-Champaign. The state’s approximately 2,500 acres grown annually amounts to over 50 percent of the nation’s output, with Wisconsin, Minnesota and northern California growing a significant amount of the balance. This is a stable crop overall, but there is a lot of uncertainty among growers because of the disease issue.

Horseradish is not a radish at all, but instead a member of the mustard family, with a large root that can be utilized for processing for condiments. Babadoost says it can be grown as a permanent crop, with one farm in California not replanting for more than 30 years. However, the Verticillium and Fusarium in the soils of the Midwest cause internal root discoloration, a disease complex of several fungal species. Growers have been known to grow a crop and discard the entire harvest because it is discolored and unmarketable.

Once the pathogens are in the field they basically cannot be eliminated, Babadoost points out. The way horseradish is planted is not conducive to control. Planting sets are generally the lateral roots, up to an inch in diameter and a foot long, that come off the main root. The sets are saved after the harvest, placed in cold storage and used for the next year’s crop. If those sets are infected, which they will be if the main root is infected, then it perpetuates the spread of the disease complex. With planting in April and May, the disease shows up in July or August. Once it is identified, there isn’t time to switch crops. This can lead to large crop losses; he says it isn’t uncommon for 30 percent of yield to be lost to discoloration.

Clean planting sets are the key to disease prevention, and new technologies may provide them.

Verticillium has a very broad host range,” Babadoost says, and includes weeds and other crops grown in the area such as potatoes and tomatoes. This makes it difficult to avoid the disease even if a grower rotates away from horseradish for a couple of years. Thus, horseradish is an annual crop in Illinois and the rest of the Midwest, and growers are always looking for clean ground. Management measures include crop rotation and the use of clean sets for planting.

Babadoost says that through utilization of some fungicides and biocontrols, internal root discoloration of horseradish can be controlled. A preplant treatment to the sets will keep the roots pathogen-free for up to 12 weeks, but the sets have to be pathogen-free first, and if they are not, the results can be disastrous. If the method is successful, the soil infestation does not have time to get into the roots and discolor them after that 12-week period. What growers aim for is an acceptable level of discoloration. If less than 10 percent of the field’s yield is discolored, that may be acceptable to processors.

However, chemical controls are expensive. He points out that it is growers with large acreages that have the best chance of avoiding the disease complex, because they have more opportunity for rotating away from the pathogens. Rotation gives the best chance of success in the long term.

Huntsinger Farms in Eau Claire, Wis., is the largest horseradish producer in the world, and its subsidiaries, Silver Spring and Bookbinder’s Foods, process and market almost all of the farm’s production. Ken Traaseth, farm manager for Huntsinger, says that long-period rotations are the backbone of his success in growing the crop. That is possible because the farm owns 4,500 acres of cropland and leases another 2,500 acres annually.

“We can have a six or seven-year rotation with horseradish,” Traaseth says. The 400 acres of horseradish grown in Wisconsin is spread out over a five-county area, and the 300 acres grown in Minnesota is isolated to the northern part of the state. In general, the rotation in Wisconsin is two years of horseradish (it is planted one year and harvested the second year), a year in sweet corn, then successive years in soybeans, more field corn, snap beans and possibly some oats or rye thrown in. After the horseradish is harvested that second year, the volunteer plants are killed off with 2,4-D herbicide so that no horseradish roots will come back and harbor the disease as the rotation is playing out.

A successful horseradish harvest can take place under a six or seven-year crop rotation.

The rotation in Minnesota is entirely different. The crop again is in for two years, then harvested. After that, a neighbor plants wild rice on those acres for two years. After each rice harvest, the fields are flooded with water for 60 days. This is based on an old University of Minnesota recommendation for soilborne disease control, and flooding is facilitated by the bordered fields on that farm. The next year, soybeans are planted for one year, with another horseradish crop to follow that.

Traaseth acknowledges that he hasn’t carried out any large-scale experiments to verify that either of these methods is the best way to achieve disease control, but he says that as a result of this program, the disease complex does not persist and their fields are clean. In fact, as long as this rotation is in effect, he does not consider that the farm has a disease problem, even though they use their own lateral roots for sets just as everybody else does. In fact, their clean sets are in demand from other horseradish growers, and they sell some.

He does have one 50-acre field in Wisconsin where he tried fumigation for Verticillium and cut the rotation short, but it was expensive and Traaseth doesn’t consider the treatment a success. He has another 1,000 acres that had once been in horseradish, then was in pasture for 25 years. When that land was again planted to horseradish, the disease complex was still present.

“Once the land has Verticillium it’s tough to get rid of,” Traaseth notes. So, even without independent research verification he is certain that his persistent long-term rotation is the best methodology for growing disease-free horseradish. He currently is conducting an experiment on one field to see if a permanent crop starting out on clean ground will yield clean roots year after year. He is only two years into that test and cannot report results yet.

Careful handwritten records are kept of the 1,000-pound harvest totes from each Huntsinger Farms field so that any disease can be traced back from the processing plant, which notifies Traaseth if any discoloration is found in the roots; if any is found, he will not use laterals from that field for sets the next year. Other factors may play a part in avoiding infestations. For example, careful cleaning of the harvested roots—using soap and water and a disinfectant—is carried out prior to processing or storage for use as planting sets. Cold storage is kept at 34 degrees and the warehouses are kept humidified. Big Top Western is the variety the farm uses, though it isn’t considered resistant and is selected primarily for its yield potential. Irrigation, including center pivot sprinklers, is used to control quality and yield in dry years. He isn’t certain whether any of this helps prevent disease, he just keeps doing what works.

Horseradish sets are taken from lateral roots of the plant, and if they are infected with disease, the next crop will be infected.

Traaseth feels that possibly the colder growing regions of Wisconsin and Minnesota enable him to avoid disease better than growers in Illinois. He knows an Ontario grower who rotates extensively with success. As long as he can grow the 8 million pounds of horseradish used by the company, and get a premium price for it as is done now, Traaseth will stay with his highly successful rotation program.

Babadoost notes that current research may offer alternatives for disease control other than long-term rotation, including the development of disease-resistant varieties, currently ongoing. Even with new varieties, however, there is risk of adaptation by the pests. Another promising biotechnology development is in the area of plant tissue culture from horseradish, which would yield clean sets for planting. This technology is promising, but very expensive.

A third area of interest, which has been tested at the University of Illinois, is to take growers’ sets and kill the pathogens without killing the sets with a hot water bath.

“You pretty much eradicate the pathogens without harming the sets,” Babadoost says. After hot water treatments, either a fungicide or a biocontrol can be applied to the sets prior to planting. This integrated approach will effectively minimize the occurrence of internal root discoloration. The drawback is that some 10,000 sets are needed to plant an acre of land, and the task of placing that much tonnage in a hot water bath would require a massive, scaled-up technology, which has not been adopted yet.

All in all, Babadoost likes Huntsinger Farms’ method of using long-term crop rotations over thousands of acres for minimizing the losses caused by internal root discoloration. The problem is that land is expensive in Illinois and this isn’t often an option. He hopes technology becomes available for hot water treatment, which could save the valuable horseradish industry of Illinois and other growing regions of the nation.

Don Dale is a freelance writer and a frequent contributor. He resides in Altadena, Calif.