By Dusty Sonnenberg, CCA, Ohio Field Leader
Soybean Cyst Nematode (SCN) for decades has been considered the most damaging pathogen throughout North America. It is estimated soybean farmers have lost $1.5 billion per year since 1996.
“Every 5 years a survey is conducted to determine what counties have SCN present,” said Dr. Greg Tylka, Morrill Professor in the Department of Plant Pathology and Microbiology at Iowa State University. “In the most recent survey, 55 new counties were identified in the Unites States, with most of the new findings in the state of New York. That was largely due to an intentional survey that was conducted, which further proves we often don’t know if SCN exists until we look for it.”
SCN can be causing yield losses in soybean fields and have no above-ground symptoms. In Ohio, 70 of the 88 counties have fields where SCN has been found. In a random survey of Ohio soybean fields conducted by the USDA in 1995 and 1996, 60% of the samples submitted found SCN. Doing a soil test is the only way to know the SCN population numbers.
There are three main management tactics to combat soybean cyst nematode. These include rotating to non-host crops; using nematode-protectant seed treatments, and planting SCN-resistant varieties.
“In general terms, SCN-resistant varieties suppress or reduce SCN reproduction, resulting in less yield loss and greater yields,” Tylka said. “When cyst resistance works, it is great. It is even better when you consider that there is no extra fee charged when you plant a SCN-resistant variety compared to non-SCN resistant variety.
“Scientifically, the resistance of a plant to SCN is gauged by the number of SCN females that form on the plant compared to the number that form on a standard susceptible variety in a 30-day greenhouse experiment. The 30 days is the time needed for SCN to complete one generation, resulting in adult females visible on the roots.”
A resistant variety is considered to have 10% or less of the SCN females on a susceptible variety. A moderately resistant variety is considered to have more than 10% up to 30% of the number of SCN females on a susceptible variety. A moderately susceptible variety will have more than 30% up to 60% of the number or SCN females on a susceptible variety, and a susceptible variety will have more than 60% of the number or SCN females on a susceptible variety.
“It is important to note that not every variety with a specific SCN germplasm provides the same level of SCN control,” Tylka said.
Planting SCN resistant varieties also provides nematode control in those fields. “When a resistant variety is planted, the SCN egg count in those fields increases very little, often to a level almost not measurably different than before the beans were planted,” Tylka said. “The fields planted with susceptible soybean varieties had SCN numbers increase over 5 times.”
There are seven registered germplasm lines that are sources of SCN resistance for breeding. These include: PI 548318 (Cloud), PI 89772, PI 209332, PI437654, PI 90763, PI 88788, and PI 548402 (Peking). Of these seven lines, almost all SCN resistance available to farmers is from PI 88788.
“In 2021 there were 872 soybean varieties that were SCN resistant. Of those, 836 had the PI 88788 gene. SCN populations have increased their ability to reproduce on and damage varieties with PI 88788 resistance, and this continues to increase,” Tylka said. “We have been stuck at less than 5% of the SCN resistance being from non PI 88788 genetics for 15 years.”
“SCN populations continue to increase in their ability to reproduce on and damage varieties with PI 88788 resistance, and this will continue. There are no indications that more soybean varieties with Peking or any of the other SCN resistance genes are on the way. The cost in time and money of developing new soybean varieties with non-PI 88788 sources of resistance is much greater than developing new soybean varieties using SCN-resistant materials already in the breeding program.”
Soybean farmers are encouraged to rotate between soybean varieties. Even though the same resistance source, such as PI88788, is the most common source used today, there are many varieties of soybeans that have that resistance source, but different genetic background in each variety. Those different genetic backgrounds help to mix things up. Using nematode-protectant seed treatments and rotating to non-host crops will help reduce the chance of SCN numbers increasing.