Although named for its affliction on soybean plants, soybean cyst nematode (SCN) can cause comparable damage to dry edible bean crops, and can adversely affect yields and economic benefit, as well as the future use of Ontario’s current dry bean-producing land. Poor yields associated with SCN infestation can be exacerbated by other environmental factors such as soil type, other diseases, and dry, hot weather.
SCN is a tiny roundworm that appears in the spring when soils reach adequate moisture and temperature conditions. They infect and feed on plant roots, which can impair the plant’s ability to uptake essential nutrients, as well as provide an entry point for other diseases. The female nematodes then form cysts on the roots which can contain as many as 400 eggs. These cysts are white or yellow and are visible on the roots but are smaller than the plant’s root nodules.
SCN is a root parasite and commonly does not produce noticeable impacts above-ground until the infestation is considered a serious threat to production. Often, the first symptom observed is lower yields, but plant discolouration and stunting may also take place with the presence of large SCN populations.
Although all types of dry edible beans can be affected by SCN, some have demonstrated more susceptibility, while others have shown more tolerance to SCN. For example, black beans have typically shown more resistance, but little research has been done to test the resiliency of different dry bean classes and varieties. Numbers of SCN cysts present on roots also fluctuates over time, making current evidence not as straightforward when comparing bean varieties.
Currently, SCN continues to work its way through Southwestern Ontario. Growers should be aware of the threat and be proactive by following strict biosecurity measures, testing their soil for SCN, and maintaining recommended crop rotation regimens to mitigate the risk for both SCN and other pests.
The purpose of this project is to establish the presence or absence of soybean cyst nematode (SCN) in some Perth and Oxford County dry bean fields, and to determine if and how SCN egg counts vary from the beginning to the end of the growing season.
Sample fields were determined by voluntary participation by dry bean growers. Those who volunteered did not report a known history of SCN in their fields. Therefore, the fields in this study cannot be representative of all of Perth or Oxford counties, or other regions of Ontario. One fresh pea field in Oxford County with a known history of SCN was also sampled to note any distinctions between fields positive or negative for SCN, and to help validate lab results.
Soil samples were taken from 26 dry bean fields and one pea field between May 26 and June 11, 2020, either before planting or directly after planting (before any roots had formed), to determine if SCN was present.
In each field, between 15 and 20 8-inch soil cores were taken within a 20-foot area, delineated by GPS coordinates, in two different locations in each field. Samples were taken from either knolls in the field or flat areas, never in lower lying areas that may retain more moisture.
Consistent biosecurity measures were used, including removing soil from all equipment, cleaning buckets with soap and water followed by rinsing in bleach, and soaking boots in Virkon disinfectant between fields. As a precaution, gloves were always worm while handling the soil samples to eliminate the risk of damaging SCN cysts or eggs with the natural contents of human skin.
The cores from each location were broken up and mixed together, stored in 1 lb sample bags and placed in a cooler with ice packs until they could be refrigerated at 4 degrees C. Samples were then spread out to dry indoors at 28 degrees C for a minimum of two days. Dry, re-bagged samples were taken to the University of Guelph Agriculture & Food Laboratory Services in Guelph, Ontario.
Results and Discussion
Results were received from the University of Guelph Laboratory on July 6, 2020. Neither eggs nor cysts were detected in any of the dry bean fields, although there were positive results for the fresh pea field with a history of SCN. In the pea field, the two sampling locations resulted in counts of 200 and 250 cysts per kg of soil and 0 eggs per 100g of soil. Both of these results are below the economic thresholds reported by OMAFRA for soybeans, but a threshold specific to dry beans is unknown. This project acts as a good starting point for SCN testing in Ontario dry beans, and SCN results, even negative ones, are valuable because they provide insight to their current and possible future distributions. Not all counties have been thoroughly tested and an effort should be made to test more in the future. It is worthwhile to track the travel of SCN through the province and it gives growers a better understanding of their risk.
The purpose of this project ties into the importance of having up-to-date information on the SCN status of fields over time because SCN levels can change rapidly. Knowing SCN trends, such as increasing or decreasing populations over the long-term, highlights why testing once is simply not enough. This is also important because SCN eggs can remain viable in the soil for many years and become active when a suitable host and favourable environmental conditions arise. The costs of precautions are minute compared to the considerable losses in yield, economic benefit, and the frustration that accompanies a pest infestation.
Although it is impossible to completely eradicate SCN once it is established, successful management with minimal yield losses is possible. However, prevention is key, and biosecurity should be common practice to prevent a multitude of pest invasions.
Some simple strategies include soil testing every couple of years, in the spring before planting and/or in the fall after harvest, maintaining best soil conservation practices to prevent SCN from spreading with soil erosion and wind, and limiting any unnecessary traffic from crossing the field. Further, establishing biosecurity procedures such as washing all equipment when moving between fields is advantageous, as well as following a crop rotation that includes non-host crops such as cereals, corn, and many cover crops.
Overall, the results are good news for these volunteer dry bean growers, although it is essential to acknowledge that there are SCN populations in the area, and that only a handful of the fields in Perth and Oxford counties were tested.