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Soybean cyst nematode is coming

Soybean cyst nematodes (small yellow) compared to soybean nodule (larger brownish). Photo by Sam Markell, NDSU Extension Service.

In 1789, the saying, “In this world, nothing can be said to be certain, except death and taxes…” was attributed to Benjamin Franklin. Now, about 225 years later, Manitoba growers can add soybean cyst nematode to the saying, as other soybean growers around the world have sadly come to realize.

“Soybean cyst nematode has not been confirmed in Manitoba, but I can confirm that you will [see it] at some point. I will guarantee it,” says Sam Markell, extension plant pathologist at North Dakota State University (NDSU). “The good news is that you can manage it if you are proactive. Bad news is that if you are not, you will be behind the eight-ball.”

The soybean cyst nematode (SCN) was first identified in Japan in 1880, and subsequently identified in the United States in North Carolina in 1954. It came in via soil imported from Japan for use as rhizobium inoculum. That soil brought along the nematode, and for North American growers, the spread of soybean has been accompanied by SCN ever since. The nematode has spread throughout much of the U.S. to Minnesota in 1978 and North Dakota in 2003. It was also found in Ontario in 1987. Today, the nematode is moving northward up the Red River Valley and is knocking on Manitoba’s door, if not already through it.

“Back in the ’50s, most of the nematologists in the U.S. didn’t think the soybean cyst nematode could survive further north because it doesn’t like cold soil. They were wrong,” says Mario Tenuta, a soil scientist at the University of Manitoba.

In the U.S., SCN is the number one soybean pest. Not only does it rob yield all by itself, it can exacerbate other problems. Markell says yield reductions can be 15 to 30 per cent before above ground symptoms are noticed. Soybean producers in the U.S. lost more than 300 million bushels to the SCN from 2003 to 2005. More yield is lost to SCN than any other soybean pathogen. It now occurs in all major soybean production areas worldwide.

Long-lasting cysts
SCN is a plant-parasitic nematode. Essentially it is a roundworm; a very simple animal related to parasitic roundworms found in livestock, pets and humans.

Juvenile nematodes hatch from eggs and are invisible to the unaided eye at about 1/64 inch long. The juveniles penetrate soybean roots and feed inside the root. Male juveniles leave the root after a few days. The female breeds with multiple males, ensuring genetic diversity. The female swells into a lemon-shaped object and loses the ability to move. Most of the mature female with eggs breaks through the root surface but remains attached to the root. The white females yellow as they age and turn brown and hard after they die. This brown stage is called a cyst, and typically contains 200 to 400 eggs. The life cycle typically takes about 28 days. The eggs in the cyst can survive several years in the soil.

“The juveniles emerge from the eggs when a soybean root is near. They can actually sense a soybean root,” explains Tenuta.

Markell says the hardiness of the cyst is one of the reasons the nematode is difficult to control. Short soybean rotations mean the nematode has multiple opportunities over several years to reproduce. But the nematode can thrive on some weeds, so even longer rotations are at risk. This hardiness also contributes to the spread of the nematode. It spreads by anything that moves soil such as water, wind, animals, birds, machinery and even mud on boots. For Manitoba growers, the biggest risk is the movement of dirty equipment from areas with the nematode, and floodwaters carrying soil northward up the Red River.

What to look for
Damage from SCN happens primarily because of the disruption of root mechanisms. The feeding inside the root interferes with nutrient and water uptake and movement, and can interfere with nodulation and nitrogen fixation. The holes in the roots made by juvenile nematodes can also allow the introduction of other pathogens such as root rots.

Field symptoms can be seen as yellowed and stunted patches, especially in lower areas of the field. The symptoms can be confused with drowning due to excess moisture or iron chlorosis. The patches will grow over the years and eventually coalesce together to take over large areas of the field.

However, yield losses can occur without visual symptoms. Research trials in the U.S., especially north of Kentucky, have found that yields of resistant varieties are consistently higher than susceptible varieties, even when no visible symptoms are observed. In high-yielding fields of more than 40 bushels per acre and in years with good soil moisture, visible symptoms are rare but rather observed as poor yields, uneven growth and delayed canopy closure.

One of the most important management tools for growers and agronomists is to scout for the nematode. Tenuta says if you have grown soybeans more than three times on a field, you should be scouting for nematodes. That means walking fields starting 30 days after emergence, carefully digging up plants in susceptible areas, and looking for small white lemon-shaped cysts on the soybean roots. Risk areas are where soil may be transported onto a field, such as field entrances, fencelines, headlands, sloughs, waterways and depressions.

“You have to get out of the truck. You can’t do it with a [Ford] 150,” says Tenuta.

Markell notes if fields are turning yellow in June, the cause may not be SCN, but if yellowing starts to turn up in later July and August, he says the SCN is one of the causes to think about.

In the U.S., soil test labs can analyze for the presence of cysts. In Canada, Tenuta cautions that soil samples submitted to commercial labs may not properly identify the SCN, because there are other nematodes in the soil. Tenuta’s lab can find the nematode at lower levels in soil and precisely identify the type of nematode. He is collaborating on a research project to replace current commercial test methods with a species-specific quantitative PCR test for nematode.

Tenuta says that in Manitoba, soil sampling was done in 2012 and 2013 to survey for SCN. Surveys are continuing with support from the Manitoba Pulse Growers Association, Western Grains Research Foundation, and Manitoba Agriculture’s Agri-Food Research and Development Initiative. While nematodes were identified, none were the SCN.

Tenuta adds if SCN has been identified from soil samples by a private lab or if cysts are discovered on soybean roots, growers or agronomists should call him to discuss next steps to verify the nematode identity.

Markell says the reason scouting is so important is that if you know you have SCN, then you can take steps to manage the impact on yield by keeping the egg levels low.

Management practices
Preventing movement of soil between fields is an important first step. Purchase clean, used equipment and clean it thoroughly, especially if it comes from areas with SCN. Wash implements and tires between fields and don’t drive pickups between fields. Use clean footwear when scouting in fields.

“Equipment sanitation is a tough one to sell,” admits Tenuta. “When time is pressing and you have to get to the next field, it’s understandable that cleaning equipment and vehicles falls by the wayside. But consider the monetary and time costs of getting the nematode; having good biosecurity is worth it.”

Migrating waterfowl can also transport soil in the spring when flying to northern nesting grounds. Keeping birds off you fields is another management tool that can be difficult to implement.

Crop rotation to non-hosts like canola and cereals can help keep egg levels low. Tenuta cautions the SCN has other possible crop hosts in Manitoba. Some other pulse varieties, including dry beans such as pinto, navy, kidney and great northern, are hosts. Lengthening the rotation to non-hosts can help reduce egg levels. Markell says crop rotation can produce significant egg population reductions.

“With cysts, you get the biggest drop in eggs in the first year rotation [out of soybean]. The second year you get some, and the third year is pretty flat. After that you get into the land of diminishing returns. The reason is that not all eggs are in the cyst and these eggs hatch in the first year no matter what. After that, it depends on whether the cyst opens up and the eggs hatch,” explains Markell.

Another good management tool is controlling weeds that are also alternate hosts for the SCN. Common weeds such as chickweed, field pennycress (stinkweed), purslane, shepherd’s purse and wild mustard are alternate hosts.

The use of resistant varieties is important for keeping egg numbers and yield losses low. Markell says resistance pays in two ways: the first is reducing yield loss; and the second – just as important – is keeping egg numbers low. Research at NDSU has illustrated the importance of good crop rotation and the use of resistant varieties. When corn and resistant soybean were grown in rotation over five years, egg levels were kept low. But in the sixth year when a susceptible soybean variety was grown, egg production skyrocketed.

“In the right environment with susceptible soybeans, egg levels will spike. In North Dakota, we’ve done some of this work and they appear to be spiking a lot faster in our soils than in Iowa. I’m not sure why that is. Probably a lack of predators here compared to Iowa,” says Markell. (See Fig. 1.)

Tenuta cautions resistant varieties aren’t completely resistant, but SCNs will grow on the roots. The resistance mechanisms aren’t well understood, but the nematodes don’t feed as much or reproduce as much in resistant varieties. That means yield isn’t impacted as much, either. Information from the U.S. shows that in general, the use of resistant soybeans will reduce the reproduction of SCN by over 90 per cent.

In Manitoba, Tenuta suggests soybean growers consider using resistant varieties even if SCN hasn’t been confirmed on their fields – especially in the higher risk area of the Red River Valley.

“Maybe you don’t have to consider using resistant varieties every time you grow soybeans, but maybe one in two or three years,” says Tenuta. “The good thing is that there are very good soybean varieties with resistance. There isn’t necessarily a yield penalty.”

In the U.S., rotation of resistant varieties is recommended since different companies use different sources of resistance. Rotating resistant varieties helps reduce the chances of SCN overcoming the resistance. So far, resistance has held up fairly well.

“We have two sources of resistance. Sources, not genes. Sources made up of multiple genes. If the pathogen is going to overcome resistance, it usually does it one gene at a time, so you see erosion of the resistance over time. It is happening in Iowa, but we haven’t seen that in North Dakota,” says Markell.

A final tool that American growers have is nematicide seed treatments. They are starting to be heavily promoted in the U.S and are a promising tool that may help to control the nematode.

“The one thing I want to emphasize is that they are meant to be used as an add-on to resistance and rotation. They are not meant to be a substitute for one of the two. The companies will not, in some cases, put them on susceptible beans,” says Markell.

Tenuta says SCN isn’t going to be just a Manitoba problem. With the expansion of soybeans into Saskatchewan and Alberta, growers there will also need to be on the lookout. He says Prairie soybean growers all need to be vigilant in scouting for SCN, and implement rotation of crops and resistant varieties as the foundation of SCN management programs.

For further information on SCN, the Soybean Research & Information Initiative has produced the fifth edition of their Soybean Cyst Nematode Management Guide available as a free download from http://www.soybeanresearchinfo.com.

If you suspect you have SCN, contact Mario Tenuta at the University of Manitoba’s Department of Soil Science, 204-474-7827, mario.tenuta@umanitoba.ca.

March 9, 2015  By Bruce Barker