Diagnostic Laboratories offer varying tests that can detect DNA of plant pathogenic fungi in the soil or in plant tissues. These can be useful tools in the management of diseases of ginseng, but it is important to understand their limitations.
The tests can detect very small amounts of DNA of all of the major plant pathogenic fungi and water moulds that attack ginseng including Phytophthora, Pythium, Rhizoctonia, Fusarium, and Cylindrocarpon. When a sample is submitted for analysis, a very small amount is used for the test. The tests are mainly designed to detect presence or absence of the pathogen, but can also provide an indication of the quantity of DNA detected. There are several key things a grower should know about these tests before using them for their disease management decisions:
1. Several pathogens of ginseng are almost always present in every field in the ginseng growing area. Pythium, Rhizoctonia, and Fusarium are very common in the soil. Detection of the presence of these organisms in the soil does not necessarily mean there is anything out of the ordinary for a ginseng field or that they will cause disease. There are also many species of Pythium and Fusarium in the soil and they do not all cause disease on ginseng. When testing root or leaf tissues, Fusarium will almost always be present. It is important to know that Fusarium is one of the most common secondary rot organisms. It moves into tissues damage by some other factor and causes additional rot. Its presence on plant tissue does not indicate it is the cause of a disease, although it could be. Phytophthora cactorum and Cylindrocarpon destructans, the causes of Phytophthora root rot and disappearing root rot, respectively, are less common soil pathogens. They will often be found in soils previously cropped to perennial horticultural crops such as tree fruit. DNA based soil tests may be more useful for detection of these pathogens. For plant tissues, there presence will usually indicate that they are causing disease, but often the diseases they cause can be diagnosed based on symptoms alone.
2. These tests will often indicate the relative amount of DNA of each pathogen that was detected (e.g. high, medium, low). However, no studies have been conducted to relate these levels to the potential for disease in ginseng. In some cases low levels could mean there is a high disease risk and in other cases high levels may result in no disease at all. Testing soil prior to ginseng production each year for many years, and then comparing those tests to the disease pressures in those fields over time, may result in a much more useful soil test. Growers should be cautious in interpreting the level of each pathogen until they have something on which to compare that level.
3. These tests are very sensitive. If a grower collects 100 soil cores from a field, and only one of them contains a pathogen (ie. only a small patch of the field contains the pathogen), the test will likely show the presence of the pathogen. As a result, there is the potential of inflating the level of risk if the sample is improperly collected or prepared for submission. Multiple samples may be required for larger fields and this can be very expensive. If the soil sample is not thoroughly mixed, additional issues can result. For example, if the lab happens to collect the soil from one core that contains the pathogen, the test might detect high levels. On the other hand, if that core was missed, a potential disease issue may go undetected. It is important to thoroughly mix soil samples before submission. No studies have been conducted to determine how fields should be sampled, how many soil cores should be collected, how many samples should be collected from each field, or how the results should be interpreted.
4. These tests can be very useful for confirming a pathogen is causing a disease when combined with a comparison of symptoms. For example, if a grower is unsure whether a particular root rot is caused by Rhizoctonia or Cylindrocarpon based on the symptoms, a DNA-based test can confirm which pathogen is causing the disease. They can also be useful for comparing soil around diseased vs healthy roots. If a fungus is present in the soil around disease plants and not around the healthy plants, the fungus may be a cause of the disease. The exception to this is if the detected fungus is Fusarium or another common secondary rot organism, because these can build up in decaying plant matter and may not be the initial cause of the disease. This illustrates the importance of leaving interpretation of the results to a trained professional.
DNA-based tests for Cylindrocarpon have not proved very useful for detecting the potential of replant disease on replanted sites. The five main soil-borne pathogens of ginseng will likely be present in any soil following ginseng production. More research is required on surveying soils prior to ginseng production to determine disease risk on replanted sites. Until then, growers should understand the limitations of these tests. They can be very useful when combined with traditional diagnostic techniques and when the results are interpreted by qualified personnel. Growers should discuss the different options with the lab upon submission and determine which tests will have the greatest chance for a successful diagnosis.