Screening Commercial Resistance to Adapted Soybean Cyst Nematode
AbstractMSoybean cyst nematode (SCN) is the leading cause of yield loss for soybeans in North America, causing an estimated loss of 566.5 million bushels between 2015 and 2019. While the situation was much improved for a few decades after the introduction of resistant cultivars, things have gotten worse over the past 15 years, as the resistant varieties that once held back this disease have become less and less effective.
Most commercial varieties derive resistance gene from one source, cultivar PI 88788. These resistant varieties have been grown season-after-season throughout most of the soybean-producing regions, putting massive pressure on SCN populations to adapt. In some states, such as Illinois and Iowa, virtually all SCN isolates from the last few years successfully overtake PI 88788 and resistant varieties derived from it. While most of these varieties are labeled resistant to SCN, there is an ongoing need to monitor which available varieties still limit SCN reproduction and soybean yield loss, and how well each does so.
This study set out to screen 128 commercial soybean cultivars against SCN populations isolated from Ohio fields. Three seedlings of uniform length from each variety were planted into separate cylinders. A baseline was established using the standard SCN-susceptible variety, Lee-74. Plants were inoculated with 1 mL of a 3000 eggs/mL SCN solution. Plants were grown for 30 days in the greenhouse to allow the nematodes to complete one life cycle. After this, plants were pulled out, and the females (or cysts) were extracted from the roots by wet sieving. Females from each were counted under 40x magnification. The number of females on each plant was divided by the number on the known susceptible variety to obtain the female index (FI). Plants were categorized according to Niblack s (2005) standard: a plant with less than 10% as many females as the susceptible is highly resistant, 10%-25% is resistant, 25%-40% is moderately resistant, 40%-60% is low resistance and greater than 60% reproduction is no resistance.
Of 128 varieties screened against this SCN population, more than 40 were classified as no resistance, 18 as low resistance, and only 10 as highly resistant. This specific isolate is highly virulent to PI 88788, so farmers planting in fields with this pathotype can expect significant yield loss on most commercial soybean varieties. While this pathotype is common in places like Illinois and Missouri, it is fortunately not common in Ohio yet, so these varieties are being screened again with another Ohio SCN population that is less virulent on PI 88788. That data will be available beginning in early January 2023. Overall, this study shows the need for new soybean varieties which incorporate other known resistance genes for SCN. Some varieties with resistance genes from Peking have successfully come to market, but farmers must be able to rotate these with other resistance sources (which have yet to be bred into commercial cultivars) and with non-hosts to prevent SCN populations from adapting as quickly as the population used here did.