Soybean cyst nematode spreading rapidly

Gregory Tylka, a professor with Iowa State University’s plant pathology and microbiology department, says soybean cyst nematode doesn’t always cause above-ground symptoms, which means farmers and agronomists must look for it on roots or collect soil samples. | Iowa State University photo

The nematode was found in 24 new counties and rural municipalities in Manitoba, Ontario and Quebec from 2017-20

Researchers at Iowa State University have found soybean cyst nematode is rapidly spreading.

SCN is the most damaging pathogen to soybean crops in the United States and Canada. First found in the U.S. in 1954, latest estimates show that it results in about $1.5 billion in annual yield losses

They can also make plants more susceptible to secondary infections by other pathogens.

According to the report published in Plant Health Progress, university nematologists, plant pathologists and plant disease specialists as well as state and provincial departments of agriculture in the United States and Canada were contacted in 2020 and asked to identify counties and municipalities in which SCN had been found since the last survey map in 2017.

Between 2017 and 2020, the nematode was found in 55 new counties across 11 states and 24 new counties and rural municipalities in Manitoba, Ontario and Quebec.

New York state, which saw SCN in only one county before 2017, reported the pest in 29 new counties in 2020. Manitoba saw SCN for the first time in 2017. The pest is found in all counties in Illinois and Iowa.

“There are two different aspects pertaining to SCN spread in Ontario,” said Albert Tenuta, field crop pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs.

“The first is detection of SCN in new areas of the province with the most recent being Simcoe County in 2020.”

The second aspect is that the nematode has developed resistance to SCN-resistant bred soybean crops.

SCN can be difficult to detect.

“SCN frequently does not cause any above-ground symptoms, so a farmer or agronomist usually has to look for it on roots or collect a soil sample to be tested for SCN,” said Gregory Tylka, Morrill professor in the department of plant pathology and microbiology at Iowa State. “Looking at roots and soil sampling isn’t done nearly enough.”

He said farmers usually won’t know if they have SCN if they don’t examine roots or take a soil sample. Anything that moves soil can move the nematode including wind, water, and farm machinery.

“I do not believe, based on experience and conversations with others, that there are a lot of fields being sampled for SCN by farmers or agronomists,” said Tylka. “If soil sampling were to increase, I believe there would be many more farmers and agronomists paying attention to SCN and thinking about managing it.”

Tenuta agreed.

“Many will have an initial SCN soil test to confirm its presence,” said Tenuta. “But follow-up testing to determine if populations are increasing/decreasing is critical to see if a farmer’s integrated SCN management program is working.”

To check for SCN on soybean roots, Tylka said to gently remove a plant from the soil and shake or crumble the soil off. Cysts form on the secondary roots so care should be taken not to strip them away when removing the root ball. Alternatively, he recommends taking soil samples.

Each female cyst can contain 250 eggs, which disperse in the soil and remain viable for years. A new generation of nematodes is born every 24 days in summer. However, growing resistant soybean varieties in rotation with non-host crops such as corn and using nematode-protectant seed treatments can help prevent SCN populations from expanding.

“SCN only infects the roots of soybean plants but the roots support everything that goes on above ground in the plants including pod and seed development,” said Tylka. “The pest disrupts the functioning of roots in moving water and minerals from the soil to the top of the plant. It affects the nitrogen-fixation that soybean roots accomplish to capture nitrogen from the air and convert it to a form usable in the plants.”

Tenuta said that SCN damage can be viewed both by above ground and below ground symptoms.

“Below ground are obviously SCN cysts on the roots but also the wounding in the roots for many pathogenic root rots such as fusarium, rhizoctonia, oomycetes and especially sudden death syndrome caused by Fusarium virguliforme.

“Above ground, the most obvious symptom is often no symptom, which is a major issue if a farmer is not aware of SCN, especially in a new area.”

Tenuta said by the time visual symptoms appear such as stunting, leaf yellow/browning, plant death, etc., SCN populations have built up and yield losses in the 20 to 30 percent range will be common on susceptible varieties. He recommends digging the soil often and examining roots for cysts at least two to three times in a season.

Considerable research is ongoing on soybean and resistant varieties. Iowa State University annually compiles a list of SCN-resistant soybean varieties and the 2019 list contained information on 891 varieties, the majority of which had resistance from the Pl 88788 breeding line. However, the nematode has developed a resistance to soybean varieties with P1 88788 resistance, and those varieties are not as effective at managing SCN as they were 20 or 25 years ago.

Tylka referred to recent reports that recommended producers grow soybean varieties with a source of SCN resistance other than P1 88788, which could increase yields as a result of better nematode control.

“Basically, these new SCN populations can reproduce on the P1 88788, resulting in symptoms and significant yield losses. If the same SCN variety or resistance (soybean) source is used, the potential for the cyst field population to build and adapt to that crop variety increases rapidly. There are fields or U.S. states where over 80 percent have the adapted P1 88788 SCN population as the majority. In Ontario, we are around 30 percent of the fields having over 10 percent of the P1 88788 SCN population.”

Research in SCN control in both the U.S. and Canada continues.

“My research group studies the effects of new and existing SCN-resistant soybean varieties with different resistance genetics on soybean yields and also on SCN reproduction and numbers in field experiments,” said Tylka. “We also conduct field experiments studying the effects of nematode-protectant seed treatments on SCN reproduction and soybean yields. In the laboratory, we are studying the effects of seed treatment on the chemical ecology of SCN.”

Tenuta said that an integrated management program begins with genetics and SCN resistant varieties including the P1 88788 and some Peking varieties, which have been shown to be more effective in controlling SCN.

“Do not use the same variety consecutively or overuse it since this will allow for that portion of the SCN population (type) to reproduce on that variety since P1 88788 and Peking are not immune to SCN,” said Tenuta. “This is what is occurring throughout the U.S. Midwest and parts of fields in southwestern Ontario. Crop rotation with non-host crops such as cereals, corn, forages as well as soil testing on a regular basis and most recently the registration of nematicide seed treatments can all be used.”

The results of the published survey show a steady expansion of the SCN, and it is expected that the pathogen will continue to spread within soybean producing areas. However, according to the report, SCN has so far not yet been reported in the soybean-producing state of West Virginia or in Prince Edward Island.

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