Throughout the summer growers often come across various unknown diseases and disorders that require diagnosis. In addition, growers often want to have new land tested to know if it is suitable for growing ginseng. I thought it would be good to summarize the various tests available at diagnostic labs and go through the benefits and limitations for ginseng growers. Furthermore, it is important to collect and submit samples properly for an accurate diagnosis.
The best time to sample for plant parasitic nematodes is before leasing the land. Ideally this should be done in the spring or fall when soil temperature is moderate and the field is relatively bare or the existing crop is dying down. In-season collection of soil or roots has some major limitations for ginseng. Click here for a comprehensive guide to sampling soil and roots for nematodes. Most ginseng soils will contain root-lesion nematodes, so there presence shouldn’t be too surprising. There are no specific thresholds for ginseng, but relatively low levels (>100 nematodes/kg of soil) appear to cause a significant amount of damage to seedlings. However, fumigation will control most nematodes, so levels higher than this can be tolerated if you are testing soil before fumigation. There are a lot of unknowns about root lesion nematodes in ginseng, but previous observations suggest they cause most of their damage when the roots are forming in the seedling year and cause limited damage thereafter. They are likely to die off rapidly during the seedling year, since it appears that ginseng is not a good host. Thus, soil or root samples collected after the early seedling stage will not be useful for diagnosis.
Root-knot nematodes are patchier in their distribution, and their presence in a sample is a cause for concern. Fumigation does not control root-knot nematodes well and ginseng is a definite host of root-knot nematodes throughout the life of the garden. In-season collection of soil or root samples for root-knot nematode diagnosis is not necessary for two reasons: 1) it is easy to identify the galls on ginseng roots if they are present, and 2) there is currently nothing you can do about root-knot nematodes at that stage anyway.
Regardless of the nematode that you are trying to control, you will need to keep the sample cool until it can be submitted to a lab. We usually bring coolers out to the field with ice packs and put each sample in the cooler as they are collected. Even having the sample sit on the ground in the sun for a few minutes could heat it up enough to kill some nematodes. It is best to collect and submit them on the same day if possible so there is little chance that the nematodes will die before submission. If they cannot be submitted on the same day, they should be kept in a refrigerator or cooler overnight and submitted the next day. It is best to submit them early in the week so the lab can deal with them before the weekend. If you submit on a Friday, they may sit around the lab all weekend.
Sampling for soil pathogens can be more of a challenge than sampling for nematodes. Growers are often interested in testing new land for soil pathogens to determine if there will be any issues in the new garden. Laboratories offer DNA soil scans for various soil pathogens that affect ginseng including Cylindrocarpon destructans, Phytophthora cactorum, Pythium species, Fusarium species, Rhizoctonia solani, and Verticillium dahliae. However, without an understanding of what the presence of these pathogens in the soil mean, it may be difficult to know what do about it. For example, Pythium and Fusarium are present in virtually all soils, so their presence in a soil test does not say anything about the potential for disease.
Our research experience shows that the test for Cylindrocarpon destructans in soil scans is not sensitive enough to identify the ginseng-specific strain/species (Cylindrocarpon destructans f.sp. panacis – which is now called Ilyonectria mors-panacis). There is a separate PCR test that can identify the ginseng-specific species, but it too does not detect Cylindrocarpon unless it is at very high levels. If either test is positive for Cylindrocarpon in soils, the field should probably be avoided because it will likely be an issue in the crop. However, if the test is not positive, it doesn’t necessarily mean you won’t have an issue. We are currently working on improving the test.
The presence of Phytophthora cactorum, Rhizoctonia solani or Verticillium dahliae in a soil test would be cause for concern, since these pathogens are not as widespread as Pythium and Fusarium. It is these pathogens that make the test useful for growers.
Soil samples for fungal identification can be collected similar to nematodes except there is no need to remove the top 3-5 cm of soil from the soil core. If they cannot be submitted right away, they can be stored in a freezer until submission and there is not as much a rush to get them to the lab right away. However, it is still important to avoid heating them up.
There are also tests available for diagnosing root or leaf diseases during the crop, which also include some DNA scans for pathogens but also microscopic identification. For the main diseases of ginseng these tests can be helpful. However, for more obscure diseases and for rusty root-type symptoms they will likely not be able to identify a cause. A positive test for Fusarium or Pythium in these tests may not necessarily mean that they are the cause of the disease, since these soil organisms quickly invade diseased tissues.
When collecting roots or leaves for diagnosis, it is important to collect tissues that are still at least partially alive. Fully dead tissues are usually invaded with many secondary fungi and bacteria, which make identifying the cause nearly impossible. Root and leaf samples should be collected and submitted or shipped the same day. If they are being delivered straight to the lab, avoid letting them dry out by putting them in a plastic bag and into a cooler. If they are being shipped or submitted the next day, samples should be put into a paper bag or wrapped in dry paper towels and then the whole thing put into a plastic bag. The paper bag/paper towels will prevent excess moisture from causing further rot and the plastic bag will prevent the sample from drying out too much.
Diagnostic tests are a useful tool for ginseng growers, but they will not give you all of the answers. Misinterpreting them may lead to a control measure being applied that will have no effect on the cause.