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Do you know your SCN number?

By Dusty Sonnenberg, CCA, Field Leader

What’s your number? While this question sounds like the latest campaign to monitor your cholesterol or blood pressure, it is actually talking about a different health measurement. The health and yield of future soybean crops will be impacted by the level of soybean cyst nematode (SCN) present in fields.

SCN damages soybeans by feeding on roots. This takes nutrients from the plant, and creates wounds for fungi to enter. The past few years the SCN Coalition, funded by the soybean checkoff, has been running the “What’s your number?” campaign to renew attention to the yield robbing pest. Presentations about SCN were common on the agenda of many farm programs in the mid to late 90s. The relative ease of Roundup ready soybean production increased the common practice of no-tilling soybeans back to beans, which created a wonderful environment for SCN populations to grow. In the years following, thanks in part to an increased awareness of SCN associated yield losses and the development of resistant varieties, SCN saw a decline in many Ohio fields.

Recently, though, SCN has quietly been overcoming the resistant varieties. Most SCN resistant varieties have the same source of resistance: PI88788. For more than 20 years, more than 95% of all SCN-resistant soybean varieties have contained resistance from the PI 88788 breeding line.

The good news has been that the PI8878 resistance is free and worked well for several of those years. The bad news is that now nematodes are becoming resistant to this gene. By definition, a resistance soybean variety will allow less than 10% reproduction by the SCN compared to a susceptible variety. The resistant variety should stop 90% of the SCN in a field from reproducing. Now, after over 20 years of using the same gene, natural selection is occurring and the nematodes seem to be adapting. In some fields surveyed, 50% of the nematodes can reproduce on a resistant variety.

According to Horacio Lopez-Nicora, who is a graduate student in the Department of Plant Pathology at The Ohio State University, over 80% of Ohio fields sampled tested positive for SCN. Lopez-Nicora works with soybean nematologist Terry Niblack, and plant pathologist Anne Dorrance at OSU.

They have discovered another concerning number: over 60% of growers with those fields that tested positive for SCN were unaware that they had a problem.

“SCN can reduce yield without the plants showing symptoms,” Dorrance said. “SCN injury can also easily be confused with other crop production problems such as nutrient deficiencies, injury from herbicides, soil compaction, or other diseases. SCN is the leading cause of soybean yield loss in North America.”

When sampling for SCN, it is recommended to pull a soil sample roughly every acre for each 10-20 acres sampled.

“A farmer should pull a sample randomly, as they walk in a zigzag pattern across the field. Because SCN feed on roots, they are found in the root zone of the soybean plants. Samples should be taken at least 6 to 8 inches deep. Each composite sample submitted to the lab for analysis should represent 10 to 20 acres, and it will be submitted as a composite sample, which means all the soil pulled will be mixed together. Approximately one pint of soil will be submitted for analysis. Once the soil is collected, it should be placed in a plastic bag and kept in a cool place out of direct sunlight,” Lopez-Nicora said. “Storing the samples in a cooler or box with an ice pack is a easy way to preserve the samples until they are sent to the lab for analysis. Be sure to mark the sample that is being submitted with the farm and field name or identification, location in the field, and collection date as well as anything else that will help you make sense of the results.”

Results will be listed as number of eggs per 100 to 200 cc of soil. (approximately half to 1 cup of soil.) Trace amounts are considered 40 to 200 eggs per cup. Low levels are considered 200 to 2,000 eggs per cup. Counts of 2,000 to 5,000 eggs are considered moderate, and over 5,000 levels are considered high. When large SCN populations are found in a field they can be difficult to remove, though the situation can be managed to minimize the negative impact on yields. At low levels, it is recommended to plant SCN resistant varieties and rotate to non-host crops. Levels at or above 2,000 may see some yield losses even on SCN resistant varieties. For moderate levels, the field should be rotated to a non-host crop, and when soybeans do return in the rotation, a SCN resistant variety should be selected.

Susceptible varieties in fields with moderate levels have recorded 25% to 50% yield losses in Ohio. Fields with a high level should rotate away from soybeans to a non-host crop (corn, wheat or alfalfa) for 2 to 3 years and then re-sample before introducing soybeans to the rotation again. When soybeans are reintroduced, an SCN resistant variety should be selected.

Some farmers have expressed concern about using legume plants as cover crops in case they could be inadvertent hosts for SCN. There are almost 100 different legume species reported in the scientific literature to be hosts supporting reproduction of SCN. Because of this, it is important to control winter annuals such as purple deadnettle that can serve as a host, and also avoid cover crops such as several of the clovers, cowpea and common and hairy vetch.

More information can be found on the SCN Coalition website: thescncoalition.com




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