We constantly get reports of significant root lesion nematode problems in ginseng, mostly based on the appearance of rusty symptoms and/or constrictions on roots. However, when ginseng fields are tested for nematodes beyond ginseng germination, we rarely can find any nematodes present. The purpose of this article is not to say that root lesion nematodes are not an issue in ginseng, because they are. However, they are probably not an issue once the symptoms are present in the field, and many growers do not have an issue with them at all. Before you waste money on unnecessary treatments, consider the following:
- Root lesion nematodes do not like ginseng. Research in Ontario and elsewhere has consistently shown that root lesion nematodes may feed on ginseng when they have nothing else to feed on but cannot survive and reproduce solely on ginseng. That initial feeding can cause significant damage to ginseng when the roots are first emerging after germination, causing characteristic constrictions containing vertical rusty stripes (Figure 1). Soon after germination, those nematodes gradually die off unless the field is full of weeds that are alternative hosts. In hundreds of soil tests for nematodes taken in Ontario in the seedling year beyond June 1, less than 5% have shown the presence of root lesion nematodes, and even those were very low. By the third year of the crop we have only had one sample out of several hundred that showed the presence of root lesion nematodes and that sample had only 2 nematodes per kilogram of soil.
- Rusty root is probably not caused by root lesion nematodes. Almost every ginseng field in Ontario has some amount of rusty root. It appears as a rusty discolouration of the skin of the root, sometimes accompanied by constrictions (Figure 2). The rusty root photos included below were from greenhouse and field studies on sites that definitely did not have root lesion nematodes. While the causes of rusty root are not fully known, it is thought that it is a response by the plant to attack by soil fungi and may be worse under certain soil nutrient and moisture conditions. The symptoms can look very similar to root lesion nematode damage. Most growers automatically assume that rusty root is caused by nematodes, but that is probably not the case.
- Don’t believe me? Prove me wrong – take soil and/or samples! There is a simple and fairly inexpensive way to determine if you have root lesion nematodes in your field – take a soil sample. There are many guides to taking nematode soil samples for nematodes. Click the following link for an article on taking soil samples for nematode analysis (http://www.omafra.gov.on.ca/english/crops/facts/06-099.htm). A single soil test for nematodes costs about $50. Multiple samples are recommended for large fields. Root samples can also be taken for nematode extraction and may be better than soil when nematodes are actively feeding. However, a sample would require a significant number of seedling roots. Nematode samples are certainly worth the cost if they can help you avoid expensive treatment options that will have no effect on the problem.
- A dead nematode cannot get any deader. If I am correct and the nematodes are already dead in your field, no treatment option is going to make them deader. Their damage is already done, and it is too late to do anything about it. We don’t know as much about rusty root, but we have not found any fungicides or nematicides that have had any effect on rusty root severity. Good control of both root lesion nematodes and rusty root depends on good fumigation practices. Nematode controls after fumigation would need to be applied before or during ginseng germination. Once symptoms are present in the field, there is no evidence that any product will help get it under control. The roots can sometimes grow out of some of the damage by harvest, but scarring often remains.
Root knot nematodes, which cause distinctive nodules/galls on ginseng roots, are an issue in ginseng, and ginseng is definitely a host. However, they are often patchy in the field and do not cause as much marketable damage to ginseng unless populations are really high. Controls for these nematodes, even if they were available, would not work in mid-summer because the nematodes are not active in the soil at this time of year. The only time of year they could be managed would be in spring and fall when they are moving through the soil.
It is important for growers to conduct soil samples and think about what can and cannot be done to control nematodes before money is wasted on ineffective treatment options. It is the next crop of ginseng that you need to focus on for control of root lesion nematode issues. Take nematode samples before and after fumigation, fumigate effectively, and if post-fumigation treatment options are available, time them for when nematodes are active in the field.