Dr. Mario Tenuta, Professor of Soil Ecology, Department of Soil Science, University of Manitoba – Fall/Winter (December) Pulse Beat 2021
The soybean cyst nematode (SCN), Heterodera glycines, is the most damaging pest or disease of soybeans in Ontario and the U.S. The nematode reduces the vigour and increases the susceptibility of soybeans to stresses (e.g., moisture) and promotes soybean sudden death syndrome (SDS) caused by Fusarium virguliforme. SCN causes yellow stunting of soybeans where plants are visibly delayed in growth and canopy closure, and chlorotic indicating nitrogen deficiency (Figure 1). Yield can be reduced by as much as 70% in heavily infested fields. Yield depression results from a combination of decreased pod setting and filling, weed presence from delayed canopy closure and the eventual establishment of SDS.
SCN has steadily expanded its geographical range as the soybean crop has expanded its range in North America. Important to Manitoba, SCN has moved northward along the Red River Valley through South Dakota, Minnesota and North Dakota. It is currently present in counties of Minnesota and North Dakota bordering Manitoba. SCN is also a pest of edible beans, particularly kidneys. A dark-red kidney bean field in Minnesota in 2016 was diseased by SCN. Further, SDS was reported this summer (2021) in a county in North Dakota bordering Manitoba. Taken together, Manitoba is destined to have to deal with SCN as a major, if not the major, disease issue of soybeans within a decade.
With prior support from MPSG, the University of Manitoba found SCN in four fields in the province in 2019. Levels in the fields were very low (< 50 eggs/100 mL soil), indicating the pest had recently been established. In July 2021, we observed a patch of yellow stunting of soybean (Figure 2) and nematode cysts on roots (Figure 3) in the headland area of a field in the R.M. of Thompson. Our doctoral student, Nazanin Ghavami, confirmed the presence of cysts filled with eggs in the soil. Egg levels were 1,250–1,700 eggs/100 mL soil, a low to moderate level. The field is on the light side and there was a drought, which is consistent with SCN-affected fields first being noticed on light soils in dry years. Nazanin’s molecular analysis of roots with cysts from both the field and greenhouse in pots containing field soil (Figure 4) confirmed the cysts to be SCN (H. glycines).
In early September 2021, the field was revisited. The SCN-affected patch was very weedy, which was not a surprise. However, we were surprised to see the soybean plants were green and growing, whereas the rest of the field was maturing (Figure 5). There were good rains before the revisitation, which alleviated water stress and the plants were able to green up and try to fill what few pods were present.
The affected patch was sampled intensively in a grid pattern for the distribution of SCN levels in the soil. Sampling was done from the roadway edge, 24 m into the field and 70 m across. The sampled area extended beyond visibly diseased soy plants into good areas of the field. The student found that SCN levels were moderate to high in the centre of the patch and tapered to zero or near zero into visibly good soy growth areas (Table 1). This showed us the diseased areas were linked to higher levels of SCN in the soil. It also means that as the pest expands from this patch, the rest of the field will be affected.
With SCN now present in five rural municipalities in Manitoba and the first confirmed field with levels of SCN that are high enough to produce low-yielding diseased patches, what should we do? For starters, the chance of moving the pest between fields should be reduced. This means air or water pressure washing of implements between fields. Also, include SCN resistant varieties in your soybean rotation, regardless of whether you have SCN or not. If SCN is present, consider lengthening the rotation time between soybean years.
Scout fields in early July for yellow stunted and chlorotic soybean patches. Headlands, depressions, entrance ways and drainage courses leading to the edge of fields are often the first areas to encounter SCN. Light soils are more prone to see SCN damage. It is not that these fields may have more SCN in them, it is that moisture stress compounded by SCN damage is more apparent in light soils than in heavier soils. Heavy soils are just as susceptible to the establishment of SCN as light soils. Dig up roots of suspected SCN plants and look for the immature, white cysts of the females. This is a very effective scouting tool. In fact, the SCN field observed this past summer was spotted by a consulting agronomist that scouted roots and alerted the MARD pulse and soybean specialist, Dennis Lange.
If you find suspected cysts, try popping them between your fingernails. If the suspected cyst pops like a small zit, it isn’t a young nodule but likely SCN. Affected field areas often go years without being identified to be caused by SCN. This is because SCN-affected areas are easily confused with other issues such as iron chlorosis, field variability in moisture availability, waterlogging damage and herbicide damage. Scouting and soil analysis are the most effective tools for figuring out the presence of SCN. If you have a suspected SCN field or need help with scouting, contact MARD or MPSG extension staff. We all work closely together and can help. Our laboratory can assist with identification and soil analysis. Commercial lab services are available from Agvise Laboratories. I’d use a commercial service to track the success of management practices and soil levels if you know SCN is in a field.
Lastly, MPSG maintains an up-to-date web page on our latest information on the distribution of SCN in Manitoba and has great scouting and management tips (manitobapulse.ca/soybean-cyst-nematode/).