Fall Soybean Disease Considerations (Part 2)

By Dusty Sonnenberg, CCA, Ohio Field Leader, a project of the Ohio Soybean Council and soybean check-off

Knowing the Soybean Cyst Nematode (SCN) numbers in a field matters. Knowing which sources of resistance that the nematode is resistant to also matters.  

“SCN is rated by the number of eggs found in 100 cc of soil,” said Horacio Lopez-Nicora, Assistant Professor at The Ohio State University in Soybean Pathology and Nematology. “A soil sample sent into the lab will help us decide what management should be implemented. If a sample is pulled in the fall and the SCN numbers are high, we can help identify if that population can reproduce on PI 88788 and at what level they can reproduce. That gives you the winter to figure out what other source of resistance should be used.”

Once a farmer knows their numbers, they can create a plan of action to address their best management options to reduce the SCN population in their field. Having a team of advisors to work with is also beneficial. Once the level of SCN pressure is known in a field, and which resistance trait will be effective, a farmer can work with their seed advisor or agronomist to select a variety with a resistance package that will manage the pest and reduce the population.

It is important to remember that if a field sample shows that SCN was not detected, it is a good thing, but it does not mean it is in the clear forever. Even if the results show a field sample did not detect SCN there could still be a “hot spot” somewhere in the field, or SCN could be in pockets at low levels in an area where the sample was not taken from. Those negative test fields should still be monitored each time a soil test is pulled.

SCN can be introduced to a field that has a negative test.

“Nematodes are in the soil and move very slow on their own. Nematodes can be carried quickly by wind and water on soil particles,” Lopez-Nicora said. “If a piece of farm equipment moves from a field that has high levels of SCN and soil is carried from that field on the equipment to a field where SCN was not detected, the SCN can be introduced.”

It is advisable to clean the soil off equipment prior to moving from field to field if known high SCN numbers are in certain fields.

There are key areas to initially pull samples from if SCN is suspected.

“Fields with high alkaline soil types, or field entry ways where equipment moves in and out of, or those areas in a field that have symptoms of Sudden Death Syndrome (SDS) are all good areas to pull samples from,” Lopez-Nicora said. “Field areas that show symptoms of SDS can potentially be hot spots for SCN.”

Statewide, SCN is a widespread issue.

“In Ohio we have processed soil samples from more than 800 fields. More than 60% of the samples tested positive for SCN populations,” Lopez-Nicora said. “Of those that tested positive, we have tested them to see which can survive on PI 88788. Over 80% of those positive tests also showed that they can reproduce on PI 88788. This is problematic because most of our SCN population can reproduce on the most commonly used source or resistance. The good news is that the majority of the 60% of the fields that tested positive for SCN were at very low levels.

“Tests were also conducted on two different soybean varieties that were very closely related in terms of their relative maturity and pedigree, but the difference was that one had PI 88788 and one had the Peking resistance,” Lopez-Nicora said. “These were placed in an environment with very high SCN populations that can reproduce on PI 88788. We were looking at the rate of reduction in the reproduction of the nematode population when we used different sources of resistance. The PI 88788 is still doing something positive compared to a susceptible variety. We are also looking at how much more the Peking resistance may be reducing the reproduction. We also applied a fungicide trial as a part of this research to determine how much a difference is made if we can keep that soybean plant healthy otherwise.”