Reports of nematode damage on ginseng seem to be increasing lately. As I wrote on this blog last year, some of the damage thought to be nematode damage is actually due to other causes. When there have been reports of damage, in most cases the grower never took a nematode sample, so whether plant parasitic nematodes were actually present in the field and at what levels are unknown. Given the differences in fumigants in their ability to control nematodes in the soil, it is very important to know before fumigation whether nematodes are a problem on a site. Metam sodium products are known to control plant parasitic nematodes better than chloropicrin, although this depends on whether the proper application procedures are followed. It makes the most sense to sample for nematodes even before choosing the land for ginseng production, because some sites may not be suitable for ginseng production due to excessively high nematode populations.

Sampling soil for nematodes is very similar to sampling for soil nutrient tests. A number of soil cores are collected from around the field, mixed together and sent to a laboratory for analysis. There are only slight differences:

1. Nematodes can be very variable in the soil, so more cores may be needed per field and more samples collected across the field
2. The top 2.5 cm (1 in.) of each core should be discarded because nematodes do not like the variable conditions near the surface of the soil.
3. Nematodes are living organisms and will die if the sample is not handled properly. Samples should be immediately placed in a cooler with an ice pack and delivered to the lab within a day for best results. It is also important not to bring them to the lab late in the week, so the lab can process them immediately.
4. Nematodes do not like hot or cold conditions. Optimal sampling times will be when soils are moderate in temperature in the spring and fall. Under hot or cold conditions the nematodes may move deeper in the soil profile.

For optimal sampling procedures consult the following OMAFRA factsheet:

Sampling Soil and Roots for Plant Parasitic Nematodes

It will take about 10 days for the lab to process the samples. Nematode samples are relatively inexpensive, similar to the costs of a soil nutrient test.

There are two main parasitic nematode species that are known to affect ginseng: root knot nematode and root lesion nematode.

Root knot nematodes are very patchy in the field but can cause damage throughout the life of the ginseng garden. Ginseng is a suitable host for this species and numbers will build over the life of the garden. The presence of root knot nematodes in a soil sample should be a red flag to potential problems. There are no specific thresholds for root knot nematode. If populations are low (e.g. <500 nematodes/kg soil), then careful attention to proper fumigation techniques may result in little economic damage, since a few knots on fibrous roots will not significantly affect the marketability of the root. Fumigants are only partially effective in controlling root knot nematodes because the nematodes are protected by the root nodules of previous crops. If numbers are high, the field or portions of it may not be suitable for ginseng production until numbers can be reduced through multiple management techniques (e.g. crop rotation with non-hosts, fumigation etc.).

Root lesion nematodes can be more widespread in the field. Keep in mind that fumigation does not control all of the root lesion nematodes in the soil, just knocks down populations significantly. The surviving populations can build back up on rye or weeds in the field after fumigation. While we are still learning about the impacts of this species on ginseng, it appears ginseng is not a suitable host for root lesion nematodes. Instead, the nematodes attack ginseng seedlings when they have nothing else to feed on and then mostly die off by the second year. They can cause considerable damage at fairly low populations in the soil (e.g. <100 nematodes/kg). If starting populations are low, they can be reduced effectively to non-damaging levels by proper fumigation. If levels are higher, a year of pearl millet can be used to reduce populations to lower levels prior to fumigation. In fields with high root lesion nematode populations, consider using wheat straw and avoid seeding rye as a cover crop in the field, since rye is a better host than wheat for the nematode and populations can re-build on the rye.

Other species such as pin, stunt, cyst and dagger nematodes may show up in soil tests. While these are not known to affect ginseng, little is known about their potential effect on the crop. Some species may wound the roots enough to allow other pathogens to get into the root. Determining the effects of different nematode species on ginseng is one of the focuses of a proposed research project at the University of Guelph over the next few years.