Genetic Resistance in Soybean Is Effective Against Southern Root-Knot Nematode
Southern root-knot nematode (SRKN) is one of the top yield-limiting pathogens of soybean in the midsouthern US. In fact, it was estimated to be responsible for the second most soybean yield loss in the region in 2019. In previous years, it was always one of the top three yield reducers. It is most damaging to soybeans grown in Arkansas, Louisiana, and Tennessee.
SRKN tends to be associated with sandy soils on sites that have previously been devoted to cotton production in the Midsouth, where the combination of root damage and the reduced water-holding capacity of the soil can result in wilting of infected plants during the heat of the day.
Management of SRKN by crop rotation is complicated by the wide range of hosts for the nematode. This is especially true for Midsouth producers where the common rotational crops of corn, cotton, and wheat all serve as hosts for SRKN. Thus rotation of soybeans with these crops is not a management option for this nematode.
The use of resistant varieties is the most effective tool for management of SRKN; however, the number of current varieties that are resistant to colonization is low. Using varieties that are only moderately resistant will allow SRKN populations to be maintained or increased (Kirkpatrick and Thomas, University of Arkansas). Resistance to SRKN is more prevalent in MG 6 through MG 8 varieties than in MG 5 and earlier varieties. Wide-spread use of early-maturing varieties in the Midsouth points to the need for SRKN genetic resistance in these varieties.
Ratings for selected soybean varieties in a SRKN-infested environment can be found here. These ratings were made by Univ. of Arkansas scientists/specialists on varieties grown in a field that had a known high population density of SRKN. Click here for a video collection that covers SRKN issues in southern soybean fields.
Results from research conducted in southeast Missouri and reported in an article titled “A major quantitative trait locus resistant to southern root-knot nematodes sustains soybean yield under nematode pressure” provide evidence that genetic resistance to SRKN in newly-developed soybean varieties could be forthcoming. This article is authored by Vieira et al. and appears in the journal Crop Science [Vol. 61:1773-1782 (2021)]. Details about and results from that research are summarized below.
• Although the major quantitative trait locus (QTL) on chromosome 10 represents the primary source of resistance to SRKN in soybean varieties, limited studies have been conducted in the field to evaluate its efficacy in minimizing soybean yield reduction under high SRKN pressure. Thus, the objective of this research was to evaluate the yield performance of SRKN-resistant and -susceptible soybean lines in field conditions with variable levels of SRKN pressure
• Elite soybean lines–85 predicted as resistant and 105 predicted as susceptible to SRKN–were grown in yield trials on three soil types at two locations in southeast Missouri in 2017 and 2018. Soils (Tiptonville silt loam and Sharkey clay) at two of the sites had no SRKN pressure, and a third site (Malden fine sand) had high SRKN pressure.
• Soil samples were collected at soybean maturity in all plots of each line grown in each environment at the high SRKN pressure site to determine nematode concentration numbers. The sampling protocol did not detect a uniform pattern of SRKN distribution across the infested site in either year, which reinforces the importance of a thorough soil sampling protocol to capture this variation in nematode numbers on an infested site.
• At the two sites with no SRKN pressure, no significant differences were measured in yield between the resistant and susceptible lines in either year. At the site with SRKN pressure, the susceptible lines were reduced in yield by an average of 19% and 17% in 2018 and 2019, respectively. This indicates that lines carrying the major resistance QTL on chromosome 10 have the ability to protect yield performance in fields with moderate to high SRKN pressure.
• Results from this research did not show a yield drag of SRKN resistance in the absence of SRKN pressure.
• The authors concluded that incorporating this resistance into breeding programs is a simple and straightforward process since highly accurate molecular markers are available.
• The authors also proffered a caution. The impact of resistance-breaking SRKN populations in soybean, although very rare, would be dramatic because of the high concentration and wide distribution of SRKN, the present rather narrow base of genetic resistance, and the lack of alternative management strategies against this pest. Therefore, they suggest that increased efforts are needed to identify and stack novel sources of SRKN resistance in soybean lines in order to enhance and increase the durability of SRKN genetic resistance for the foreseeable future.
The validation of the effectiveness of this genetic resistance in protecting soybean yield against SRKN is extremely important for midsouthern soybean producers since there are no other effective management options against this pest. Hopefully, both private and public soybean breeders will incorporate this added piece of genetic protection into their breeding programs so that the region’s producers will have this tool to minimize the deleterious effects of this significant nematode pest on soybean yield.
Composed by Larry G. Heatherly, June 2021, larryh91746@gmail.com