Millions of dollars have been spent to combat the most damaging soybean pathogen in North America. The defense discovered decades ago is beginning to falter and researchers are concerned about the challenges ahead. As a consequence, nearly 40 university, checkoff and private scientists and growers gathered in mid-December to share management information and discuss the development of the second Soybean Cyst Nematode Coalition.
“There’s a long history with the SCN Coalition because in the 90s when it originally started SCN was causing a lot of yield loss and growers didn’t have the information to manage it,” said Sam Markell, Ph.D., associate professor and extension plant pathologist for North Dakota State University. “We’re in almost the exact same situation again because resistance is starting to fail. We need to look at it a different way.”
The original SCN Coalition lasted only a couple years, but in that time thousands of growers across the North Central United States began testing for SCN and actively managing it. However, those same management tools are not working as well as they used to, and many growers don’t realize that the pathogen is changing.
This issue doesn’t affect one state or even one region. As of December 2016, SCN had been confirmed in more than 30 states, Puerto Rico and Southern Canada. SCN continues to spread within the states and is being confirmed in new counties and fields every summer. Unfortunately, the pace of spread isn’t expected to slow down any time soon.
Not helping the issue are several challenges unique to the SCN pathogen and how it has been handled over the last few decades. The most concerning of which is farmer’s apathy toward the situation. As part of a 2015 survey, researchers found that 45 percent of farmers didn’t think identifying SCN was important and of these farmers, 69 percent didn’t think SCN was a serious issue. This means not only is SCN the biggest yield robber for North American soybean farmers, but those same farmers aren’t aware of the damage being caused.
“Growers a generation ago quickly learned about SCN and started managing it with the best tools they had,” said Markell. “Once something is under control and being managed you don’t tend to actively think about it as much. The problem with SCN is that it’s been changing so the old tools aren’t as good meaning the growers aren’t managing it as well as they thought.”
Other challenges facing SCN researchers include biology, math exercise and the definition of resistant. The way SCN attacks a soybean plant is through the roots, so until the damage is extremely severe, the leaves and canopy may appear healthy. This means casual scouting would not determine the issue. Soil sampling, testing and egg counts are necessary to determine how severe the problem is.
To explain how severe the SCN problem can get if not caught early, Greg Tylka, Ph.D., Iowa State University, offered a math exercise. If half a cup of soil starts with 100 eggs, around half of those eggs will be female and produce 250 additional eggs each. Even with a 95 percent egg mortality rate, after three generations there would be 24,414 eggs in that same half-cup of soil. Depending on the environment most north central states will experience 3-6 SCN generations in one growing season, so that number could be exponentially higher for some farmers.
“Every 24 days a new generation of SCN is born,” said Tylka. “That means they can go from below the threshold to problematic very quickly.”
Definition of resistance
Many farmers who know they have an SCN issue are planting a ‘resistant’ variety, but what does resistant mean? In science the definition is less than 10 percent reproduction across a single generation, measured in a greenhouse test. Legally there is no definition for SCN resistance so a bag of seed with 75 percent (or higher) susceptibility could be labeled resistant.
Research presented at the coalition showed that in a greenhouse study examining the level of reproduction on 61 different soybean varieties, 58 of which were labeled resistant, all but one of them allowed reproduction above the 10 percent scientific threshold. With the same varieties in a field setting, 40 of the 61 varieties allow high rates of reproduction. In other words, the majority of those varieties tested were technically not resistant.
Adding to the resistance problem is the fact that the nematode is overcoming the two most common soybean breeding lines, PI 548402 (commonly known as Peking) and PI 88788. Or stated another way, SCN populations are becoming resistant to the resistance. Reproduction rates of SCN on both sources of resistance has risen above the scientific threshold in most areas. In the early 90s there was almost no reproduction on the varieties with Peking and PI 88788 resistance, but overuse is leading to resistance problems for many farmers.
“There is a lack of resistance diversity in a commercially available varieties leaving farmers with little to no choice in what type of genetic resistance they will use,” said Tylka. “The usefulness of traditional resistant varieties will continue to decline and unfortunately new varieties with novel sources of traditional resistance aren’t likely.”
All of these factors led to the need for the second SCN Coalition. The first part of which was to bring a diverse group of academia, industry and commodity groups with farmers to discuss how to combat this pathogen. The event began with the 2016 National SCN Conference and concluded with the SCN Coalition meeting. Two days of research presentations and discussions led to a greater understanding among all of the groups of the collaboration that will need to take place to wipe out SCN.
“We’re all going to have to work together on this.” Markell said, referring to the diverse group in attendance, and “It’s critical that growers have a strong voice in the development of the 2nd SCN Coalition.”
All 12 of the North Central Soybean Research Program states are involved in this project along with Kentucky, Oklahoma, Tennessee and Virginia, as well as a university in Ontario, Canada.
Source: IA Soybeans