There has been a lot of confusion on what is causing rusty symptoms and constrictions on ginseng roots. Some growers assume it is all root lesion nematode damage, but some damage seems to occur where no root lesion nematodes can be found in soil tests.
Over the past six years, OMAFRA and the OGGA have been collaborating on numerous research trials with root assessments being conducted both after the seedling year and at final harvest. Many of these trials involved extensive testing of the soil ahead of time to determine the populations of plant parasitic nematodes. We have also conducted experiments in pots where there was more control over what was present in the soil. From these trials we now have a much clearer picture on what the damage from root lesion nematodes looks like compared to rusty roots.
Rusty symptoms on ginseng roots can be caused by many different factors. A similar symptom in humans would be hives, a rash on the skin caused by the body’s reaction to something in the environment. In the case of both hives and rusty root, it isn’t a pathogen that causes the symptom directly, but the body’s/root’s reaction to the pathogen. Once the rusty symptom develops on the skin, often the pathogen is long gone. The rust colouration is an accumulation of chemicals the ginseng produces to defend itself against invasion or in response to another environmental stress. The symptoms may be different depending on what is invading the root. Root lesion nematode damage contains rusty symptoms, and therefore, the damage can be considered a type of rusty root. The purpose of this article is really to help growers differentiate rusty root caused by root lesion nematodes from that of other unknown causes.
Root lesion nematodes have been shown previously to cause constrictions on the tap root with rusty vertical streaks within the constriction. Our research confirms that this is the case. We recently harvested seedling roots from a site with high populations of root lesion nematode and very few other diseases (Fig. 1 and 2).
The symptoms appear as very distinct and narrow constrictions that resemble an hourglass shape with rusty colouration clearly confined to the constrictions. These symptoms match those identified in our previous fumigation experiment where root lesion nematodes were confirmed to be present (Fig. 3 and 4).
As the root continues to grow, the constrictions eventually disappear but the narrow band of vertical streaks is still often present (Fig. 5). Root lesion nematodes should not be confused with root knot nematodes, a different parasitic species that definitely attacks ginseng throughout the life of the garden, but results in distinct nodules on the roots (Fig. 6).
Figure 5. A band of vertical rusty streaks on a 3-yr old root left over from root lesion nematode damage in the seedling year. Note the lack of additional damage since the root lesion nematodes died off.
We also have roots from several experiments where we are confident there were no root lesion nematodes. This includes a pot experiment in commercial growing mix where the mix was tested for parasitic nematodes and confirmed not to be present. These symptoms can look very similar to root lesion nematode damage (Fig. 7-9). There do appear to be a few distinct differences:
- Rusty symptoms occur outside of the constrictions on many roots and on some they occur on the whole root surface.
- The rust colour is more orange and often streaks within the rust are not all vertical and not always present. Deeper cracks can develop on the surface of the root.
- Constrictions that do occur are often a wider band and not the typical hourglass appearance.
Figure 7. A selection of rusty seedling roots from a field site where no root lesion nematodes were found in numerous soil tests throughout the life of the garden. Note that constrictions occur, but rusty symptoms are elsewhere on the roots and the constrictions are not as distinct.
In these cases the cause of the symptoms is not entirely known but they are likely caused by a reaction of the root to invasion by a fungus or bacteria in the soil. The symptoms are often also related to adverse weather conditions. We are still working on determining causes of this disease. Older roots with rusty root symptoms can look similar to seedlings with the same issue. This is because rusty root can continue to develop over time, whereas root lesion nematodes generally die off during the seedling year. The narrow bands present after root lesion nematode damage are absent in roots with general rusty root or they are accompanied by other rusty root symptoms (Fig. 10 and 11).
Figure 10. A 3-year old root with severe rusty root symptoms from a site with no root lesion nematodes. Note that there are vertical streaks present but they are accompanied by other rusty symptoms over the whole root.
Trying to differentiate the symptoms based on one root would be very difficult because some roots without nematodes present have distinct constrictions containing rust. Several roots are required to be more confident in diagnosing the cause. It is even more difficult to differentiate the symptoms in older roots, since roots that have nematode damage can develop other rusty root symptoms over time. Having nematode tests conducted prior to establishment of the garden provides much more confidence in the diagnosis. Our research is showing that most of the damage from nematodes occurs early in the seedling year. By the time the damage occurs, it is too late to do anything about it.
Having the nematode tests conducted prior to fumigation can help in determining the best way to control them including choice of fumigant, cover crops for nematode suppression and the type of straw and nurse crops you apply after seeding. Rye is a known host of root lesion nematodes, and growing a rye nurse crop in the fall of the seeding year could increase nematode populations prior to ginseng germination.