Known Distribution of the Soybean Cyst Nematode, Heterodera glycines, in the United States and Canada Through 2023

The soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is considered the most-damaging pathogen of soybean (Glycine max [L.] Merrill) in the United States and Ontario and has been so for several decades, reducing yields twice as much as any other pathogen (Bradley et al. 2021). Numerous factors contribute to the substantial yield loss caused by SCN, including rapid increases in nematode population densities in single growing seasons (Tylka et al. 2023), long-term survival of the nematode in the absence of its host (Inagaki and Tsutsumi 1971), narrow genetic diversity of resistance in available soybean varieties (Tylka and Mullaney 2023) resulting in loss of effectiveness of the resistance (McCarville et al. 2017), significant reductions in soybean yields with no accompanying aboveground symptoms in soils with moderate to high nematode population densities (Wang et al. 2003), and widespread distribution of the pathogen (Tylka and Marett 2021).

Interest in managing SCN has increased as the pathogen spread throughout North America after its initial discovery in 1954 (Winstead et al. 1955). Currently, there are soybean varieties available that farmers can grow that contain two different sources of resistance to SCN, namely breeding lines PI 88788 and Peking (Tylka and Mullaney 2023). A great majority of the SCN-resistant soybean varieties available throughout most, if not all, soybean-producing areas of the United States and Canada were developed with resistance genetics from PI 88788. Peking has different genetic SCN resistance than PI 88788 and is also available in some soybean varieties, but considerably fewer varieties contain Peking resistance than PI 88788 resistance (Tylka 2023). An additional SCN management tool available to farmers growing soybeans in SCN-infested fields is seed treatments, of which there are several that can provide protection against yield reduction.

It is important to periodically assess the known geographical distribution of SCN because the nematode has not been found in every soybean-producing area of the United States and Canada, and plants growing in fields infested with SCN may not display obvious foliar symptoms. Farmers cannot actively manage SCN unless the pathogen is known to be present in their fields. Knowledge that SCN has been found in a county or rural municipality may increase scouting for and discovery of SCN in fields. This publication contains tables listing counties in the United States and counties and rural municipalities in the provinces of Manitoba, Ontario, and Quebec, Canada, in which SCN was first discovered from 2020 through 2023 and also contains a map showing the overall known distribution of SCN in the United States and Canada and where SCN was first discovered as infested from 2020 through 2023.

Methodology

Beginning in January 2024, university nematologists, plant pathologists, agronomists, and governmental plant health officials in soybean-producing states in the United States and the provinces of Manitoba, Ontario, and Quebec, Canada, were queried about the known distribution of the nematode in the territories in which these individuals worked. Several surveys of the known distribution of SCN using this approach have previously been conducted, and the results were published in Plant Health Progress (Tylka and Marett 2014, 2017, 2021). Illinois and Iowa in the United States were not included in the current survey because SCN has been found in all counties in those states.

Distribution information obtained in the current survey was compared with the known distribution through 2019 (Tylka and Marett 2021). Counties and rural municipalities in which SCN was first identified between 2020 and 2023 are shown in Tables 1 and 2. Figure 1 shows the location of all counties and rural municipalities in which SCN was found between 2020 and 2023 shaded blue among those previously known to be infested with the nematode shown in red. An animated map illustrating the expansion of the known distribution of SCN in North America from 1954 through 2023 is provided in the Supplementary Materials.

TABLE 1 Soybean cyst nematode-infested counties discovered in the United States by state: 2020 through 2023

State	Counties
Kansas	Ellis, Meade, Washington
Kentucky	Lewis, Lincoln
Nebraska	Lincoln
New York	Broome, Erie, Genesee, Herkimer, Oneida, Schenectady, Tioga, Yates
Ohio	Athens, Gallia
Oklahoma	Delaware
Pennsylvania	Centre, Cumberland, Lackawanna, Lebanon, Lycoming, Mercer, Northampton, Perry
South Dakota	Potter
Virginia	Augusta, Orange, Rockingham, Shenandoah
Wisconsin	Barron

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TABLE 2 Soybean cyst nematode-infested counties and rural municipalities discovered in Canada by province: 2020 through 2023

Province	Counties and rural municipalities
Manitoba	Thompson
Ontario	Dufferin, Durham
Quebec	Antoine-Labelle, Deux-Montagnes, Joliette, La Nouvelle-Beauce, La Vallee-du-Richelieu, Laval, Les Chenaux, Les Jardins-de-Napierville, Matawinie, Temiscamingue

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Continually Expanding Geographic Distribution

The survey described herein obtained reports that SCN was found in numerous counties and rural municipalities in the United States and Canada that were not known as infested with the nematode through 2019 (Tylka and Marett 2021). Specifically, there were 31 counties in 10 states (Table 1) and 13 Canadian counties and rural municipalities in the provinces of Manitoba, Ontario, and Quebec (Table 2) in which SCN initially was identified between 2020 and 2023. New York and Pennsylvania each had eight newly reported SCN-infested counties in the United States between 2000 and 2023, which was more than any other state. West Virginia continues to be the only soybean-producing state in the United States in which SCN has not been found. Within Canada, SCN was found for the first time between 2020 and 2023 in 10 rural municipalities in Quebec, in 2 counties in Ontario, and in 1 county in Manitoba in the same time period.

Considerations and Practical Implications

Several points should be considered about this survey and its results.

• There were a few SCN-infested counties not identified as infested in the last report of the nematode's distribution through 2019 (Tylka and Marett 2021) that were identified in this current survey as being first identified as infested with SCN prior to 2020. The years of initial discovery of these SCN-infested counties were not known or specified. This situation is uncommon but not rare, having occurred in previous surveys as well.

• The absence of knowledge of the presence of SCN in a county or rural municipality is not evidence of absence of the pathogen. Fields may be infested with the nematode for many years before infestations are discovered. Additionally, agribusiness and independent plant health professionals were not consulted in this survey, and these individuals may know of SCN-infested fields in areas that were not identified in our survey.

• As with all previous reports of the known distribution of SCN in the United States and Canada (Tylka and Marett 2014, 2017, 2021), the reported presence of the nematode in a county or rural municipality is a measure of incidence or occurrence of the pathogen. The map does not represent and should not be interpreted as indicating prevalence or pervasiveness of SCN within a county or rural municipality.

• The discovery of new SCN-infested counties or rural municipalities is affected by the survey activities that occur. New discoveries of SCN are more likely to happen where active sampling surveys for the pathogen are occurring than where discoveries are the result of observations or samples collected for testing by individual farmers or those who advise them.

• Fields that are newly infested with SCN may not exhibit aboveground symptoms of damage for many years after the introduction of the nematode into the fields. Furthermore, distribution of SCN within a field is usually initially limited and can be aggregated for many years after the first infestation, moving only a few feet or meters each growing season (Gavassoni et al. 2007). Consequently, one cannot assume that SCN was introduced into the counties and rural municipalities identified during the years covered by this survey. Instead, the counties identified should only be considered newly discovered as infested with SCN from January 2020 through 2023.

• As with past surveys and maps of the known distribution of SCN in the United States and Canada, it should be noted that soybeans have not been grown for several decades in some of the counties and states identified as infested with SCN in Figure 1, and SCN may no longer be present in fields previously known to be infested if soybeans have not been produced for many years. Whether SCN still survives in fields in these areas or not, its presence is of little agricultural consequence until soybeans are grown again.

• The map in Figure 1 represents the cumulative known distribution of SCN in the United States and in Manitoba, Ontario, and Quebec, Canada, since the pathogen was initially discovered in North Carolina in 1954 (Winstead et al. 1955).

Summary

The results of this survey show that SCN continues to spread throughout the United States and Canada. As a result, it is reasonable to conclude that increased soybean yield losses due to the nematode will follow if not already occurring in these areas.

Continued and increased efforts to scout for SCN are recommended in all fields in which soybeans are produced in the United States and Canada. Also, more soybean varieties with Peking SCN resistance and with novel SCN resistance genetics (i.e., not PI 88788 or Peking) are needed as well as continued development of nematode-protectant seed treatments.

Acknowledgments

The authors thank the 30 individuals who provided information about the known distribution of SCN in the counties and rural municipalities in the states and provinces in which they work. Their knowledge and information are critical to compiling and publishing updates of the distribution of SCN in the United States and Canada.