Berlin Nelson Jr., Professor, Department of Plant Pathology, North Dakota State University
Until recently, there was little concern about the effect of soybean cyst nematode (SCN) (Figure 1) on dry bean production in the United States since most dry beans were produced in areas where SCN was not reported or not common. However, as SCN moved into North Dakota and northern Minnesota, where the majority of dry bean production is centred, research was conducted to determine if SCN was a threat to production. Even though dry beans have been known as a host since the 1930s, there was limited research on the effects of SCN on dry bean yield.
In research conducted at NDSU since 2005, SCN was shown to cause significant reductions in dry bean yield under conditions favourable for infection and nematode activity. In addition, SCN reproduced on various classes of beans and that, in general, most dry bean types were not as susceptible to SCN as soybeans. This research was conducted with HG type 0 (the old race 3), which was the most common type found during that time. However, with the introduction of HG 0 resistant soybeans into the area after 2003, new HG types began appearing, such as HG 2.5.7, which can reproduce on soybean varieties resistant to HG 0. We know that HG 2 types can be more virulent on dry bean that have some resistance to HG 0. So, the changes in HG types in the region will have an effect on how serious SCN becomes on dry bean production. There is still considerable research that is needed to answer many of the questions relating to SCN and its effects on dry beans.
An example of yield loss in pinto bean is shown in Figure 2. The plant in the middle is growing in soil with no SCN, while the plant on the left is growing in soil with 5,000 eggs/100 cc of soil and the one on the right with 10,000 eggs/100 cc of soil. Notice the plant on the left has no obvious above-ground symptoms other than it is not as robust and has fewer pods than the plant in the middle. The plant on the right is significantly stunted and has no pods. The yield loss in that experiment at 5,000 egg/100 cc of soil was about 50%. These are high egg levels, but we have fields that are infested at those levels and even higher. An important point about SCN is that you can have a yield loss without any obvious above-ground symptoms. The same is true in soybeans. It is one of the reasons that SCN is often not detected in the early stages of infestation until there is serious damage to the crop with more obvious above-ground symptoms.
There is resistance in dry bean to SCN and we have shown there are sources of high levels of resistance to both HG 0 and HG 2 types. We also know that among dry bean varieties within a bean class such as in pinto and navy, there can be large differences in susceptibility or resistance of varieties. Susceptibility in dry beans has been measured against a susceptible soybean and the use of a female index (FI). For example, we use a very susceptible soybean called Barnes, and we inoculate the soybean and a dry bean variety with the same number of SCN eggs, under the same conditions in the greenhouse. Then, 40 days later, we count all the SCN females produced on the soybean and the dry bean roots. We then divide the number produced on dry bean by the number produced on soybean times 100 to get a female index. If the number on the Barnes was 250 and on the dry bean it was 60 then the FI for that dry bean would be 24. At present, we consider an FI of 10 or less to be a high level of resistance. That may change as we conduct more research on the SCN x dry bean interaction.
There are, within bean classes, varieties that have high to moderate levels of resistance based on the female index. The goal of the bean breeding program at NDSU is to bring high levels of resistance into the breeding material with the thought that in the future, resistance may be an important characteristic of varieties that growers will need to manage SCN. At present, in North Dakota most bean fields are not yet infested with the nematode, but SCN is spreading throughout the bean production areas. In 2019, a survey of 116 bean fields found 16% with evidence of SCN. However, most had low egg levels. Several things to keep in mind about this nematode are that high populations in the soil can develop when conditions are favourable for reproduction and the nematode survives very well during our cold winters. Warmer soils and lighter soils favour reproduction by SCN, but we have seen good reproduction even in heavy clay soils, although it takes longer in those soils to build up higher populations. A key factor in damage to dry bean or soybean is the level of eggs in the field. The higher the egg level, the more likely the damage to the plant. There are no threshold egg levels we can use to determine damage to plants because there are numerous other factors involved in disease development, such as susceptibility of the variety and the weather conditions during the growing season.
The most important practice a grower can do is first determine if you have SCN in your fields. That means sampling the soil and sending the soil sample to a laboratory that can determine the egg count. Generally, this is reported as eggs per 100 cubic cm of soil. Once you know a field is infested, then you want to determine if the egg levels are increasing after you grow a susceptible crop such as dry beans. If you can, avoid growing the bean crop in that field. Rotation to non-host crops such as small grains, corn or other row crops, can help reduce egg levels. The longer the rotation, the better, but even a one to two-year rotation is helpful. Once you have SCN, it is almost impossible, even with long rotations, to eliminate it from a field. The best advice for preventing damage to dry beans by SCN is to grow a resistant or less susceptible variety if such information is available and maintain the egg levels as low as you can in the field. Highly resistant dry bean will likely tolerate levels of SCN over 1,000 eggs/100 cc of soil, but at some higher levels, there could be damage to the plants. There are also commercial products for seed treatments that may have future use in managing SCN on dry beans.
There are numerous websites with information on the basic biology of SCN and how to sample soils for determining egg levels if additional information is needed (www.thescncoalition.com). NDSU made a video about the threat of SCN to dry beans with information on various aspects of the disease: www.youtube.com/watch?v=abOt45qj_2E