When Manitoba farmers first started growing soybeans it seemed to be a crop without pests.
That changed as the acres grew. Some years aphids and white mould have been problems, and eventually soybean cyst nematode will turn up too, says Albert Tenuta, an extension plant pathologist with Ontario’s Ministry of Agriculture, Food and Rural Affairs.
“One thing to keep in mind is that wherever soybeans have been grown soybean cyst nematode has followed,” Tenuta told the Manitoba Special Crops Symposium in Winnipeg last month. “It is the No. 1 soybean (pest) in North America and the yield losses on average are probably twice those of what we’d see (in Ontario) with the next group, which would be the seedling root rot diseases.”
Tenuta said Ontario farmers probably lose $10 million to $30 million a year, or have in the past, to soybean cyst nematode, though losses have declined recently.
Farmers need to respect soybean cyst nematodes, but not fear them, Tenuta stressed.
“We can manage it, particularly at those low levels, quite easily with early detection, non-host crops in the rotation as well as the use of resistant soybean varieties,” he said.
Soybean cyst nematodes are small parasitic roundworms (Heterodera glycines) that attack soybean roots, robbing plants of nutrients, disrupting water uptake and making it easier for root diseases to infect plants.
“The most common symptom of soybean cyst nematode, in the field above ground, is no symptoms whatsoever,” Tenuta said.
In other words, when nematode populations first move in and are low, there isn’t enough subterranean damage to cause visible above-ground symptoms. That’s the time to be controlling them — before the population explodes.
“By the time we start seeing stunting, yellowing (chlorosis), etc., in most cases producer losses are about 25 to 30 per cent,” Tenuta said.
Another telltale sign is what appears to be poor weed control in part of a field. When plants are smaller and under stress they are less competitive. It might be the plants failing, not the weed control.
Soybean cyst nematodes are soil borne and are spread through the soil via wind, water, machinery or plant transplants.
Be proactive, Tenuta said. Scout soybean fields for problems and dig up plants in search of cysts on soybean roots. Start looking in areas where foreign soil could have been deposited, such as along fencelines or in low spots, Tenuta said. Also check stressed areas such as knolls or headlands.
No soybean cyst nematodes have been found in Manitoba yet, but they’re not far away. The pest is working its way north in Minnesota and last year was found in North Dakota’s Pembina Country, which borders Manitoba on the west side of the Red River.
There was major flooding in Manitoba last year, including the Red River Valley, with floodwaters flowing through Pembina County into this province.
“Flooding is an ideal way to move them,” Tenuta said.
Adult soybean cyst nematodes are so small that magnification is required to see them. The lemon-shaped egg masses, or cysts, they deposit on soybean roots, can be spotted with the naked eye. But they are also small — about the size of a period in a sentence, Tenuta said. They are much smaller that the nitrogen nodules found on soybean roots, he added.
The cysts pop when squeezed.
“I call this the zit test,” Tenuta said.
The cysts contain 50 to 400 eggs, which can become adult nematodes and attack future crops. The nematode’s life cycle is 25 to 30 days so a field can have three or four cycles in a year, resulting in a fast buildup of the pest.
Some varieties resistant
While there are no totally nematode-resistant varieties, some are better at tolerating them, Tenuta said. When used in combination with rotating to non-host crops, the combination can effectively manage the pest, he said.
There are new tools too, including nematocides applied as seed treatments.
Non-host plants include corn, cereals and alfalfa. Edible beans are susceptible to soybean cyst nematodes but are more tolerant than soybeans.
Soybean cyst nematodes probably first arrived in southwest Ontario more than 20 years ago, Tenuta said. But they went undiagnosed until populations were so high they caused major soybean crop loses. One farmer’s field had 90,000 cysts per 100 grams of soil. Yields in the infected areas of the field dropped to around 15 bushels an acre from 60 or 70, Tenuta said. Yields were restored over time by planting soybeans with a higher nematode tolerance and crop rotation.
To put it in perspective 500 cysts per 100 grams of soil is a low nematode population, Tenuta said.
“While it’s low it can be managed very effectively,” Tenuta said. “What you don’t want is for it to build up over time.”