Soybean cyst nematode (SCN) continues to migrate across Ontario and is adapting to resistant varieties.
Albert Tenuta, field crop pathologist with OMAFRA, says SCN is showing up outside the traditional southwestern Ontario infection areas and into Huron, Bruce and Simcoe counties. Resistant varieties are beginning to show symptoms on the roots.
Why it matters: Soybean cyst nematodes can cause more than 30 per cent yield loss in affected soybean fields. Understanding the pest is critical and practising proper management can help to lower the spread of the nematodes and its resistant traits.
“(The resistance) is found more in traditional areas such as Chatham-Kent, Lambton, Middlesex and Elgin counties. (SCN is) adapting, evolving and able to bypass the resistance trait (in SCN-resistant varieties).”
The nematodes are beginning to bypass the resistant trait PI88788 found within 90 to 95 per cent of SCN-resistant varieties, which growers have been relying on for many years.
“We’ve relied everything on this PI88788 source of resistance. Early on there wasn’t a lot of SCN-resistant varieties available. Unfortunately, the same varieties may have been grown in the same field many years in a row which of course pushes selection pressure to those new adaptive populations.”
SCN has been around since 1988 in the most southwest regions of Ontario with up to 50 per cent yield loss due to lack of knowledge and limited varieties availability.
“SCN never takes a day off and unfortunately it’s not one that can be eradicated from the field once it’s there,” said Tenuta.
Soil types make a difference
Sandy- and loam-type soils with better drainage and aeration are preferred by the nematodes and are ideal for their reproduction.
Within these fields, areas already experiencing stress are where growers are more likely to find the pest.
“Anytime that you have hot, dry conditions, it promotes soybean cyst nematode. As well, in areas of the field that are stressed — knolls, headlands, compacted areas, nutrient deficiencies, pH issues.”
To best scout for SCN start by recognizing fields that usually experience stress. Eventually they may see these areas of the field begin to show large SCN yellow patches.
“When you dig up those roots to examine, SCNs are very, very small – about the size of a pinhead… Don’t ever pull the roots out of the ground, you end up leaving the fine roots and that’s’ where the SCNs are,” says Tenuta.
Soybean cyst nematodes are best distinguished from nodules on the roots as they are much smaller and will pop, much like a cyst, when put them between your fingernails.
The value of rotation
Crop rotation is a best practice when managing soybean cyst nematode. A two-year break between soybeans, such as non-host crops like corn and wheat is ideal, but a two-year crop rotation between corn and soybeans is still better than soybeans on soybeans.
As well, in areas where soybeans are yet to show resistance, varieties with SCN resistance can still be used, so long as farmers understand the risk and rotate their varieties.
Tenuta says soil samples help growers to have an idea of their SCN numbers and if management strategies are working to lower those values.
“The nematode numbers should stay, or start to decrease over time. If you see a rapid increase, something is going on in that field. If you are in the southwestern region, that is because the PI88788 isn’t as effective in other areas.”
OMAFRA is part of a soybean cyst nematode coalition, a partnership between government, research, grower commodity groups and industry partners throughout North America with the goal to better educate growers on SCN.
The organization’s website, thescncoalition.com, has information for growers to learn more about SCN and how to manage the growing pest.