Do you know your 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 of future soybean crops will be impacted by the level of soybean cyst nematode (SCN) present in your 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 soybean cyst nematode. Presentations about SCN were common on the agenda of many farm programs in the mid to late 90’s. The relative ease of round-up 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, SCN has quietly been overcoming the resistant varieties.
Most all SCN resistant varieties have the same source of resistance: PI88788. For more than 20 years, more than 95 percent 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 percent reproduction by the SCN compared to a susceptible variety. The resistant variety should stop 90 percent of the SCN in a field from reproducing. Now, after over 20 years of using the same gene, natural selection is occurring. 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 Dr. Terry Niblack, Soybean Nematologist, and Dr. Anne Dorrance, Plant Pathologist at The Ohio State University. Another concerning number is that 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,” said Dr. Anne Dorrance. “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.”
The SCN Coalition’s slogan is “Take the test. Beat the pest!” SCN sampling can be done in the spring or in the fall. Populations are highest in the fall after soybeans are harvested. SCN populations can increase as much as 10 to 40 times in a single growing season. More importantly, SCN is not distributed evenly throughout a field. “There can be SCN hot spots,” Dorrance said. 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 zig zag 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 6-8 inches deep. Each composite sample submitted to the lab for analysis should represent 10-20 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,” said Lopez-Nicora. “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 100-200 cc of soil. (100-200 cc of soil equals approximately ½ to 1 cup of soil.) Trace amounts are considered 40-200 eggs per cup. Low levels are considered 200-2000 eggs per cup. 2000-5000 levels are considered moderate, and over 5000 levels are considered high. When SCN are found in a field they can be difficult to get rid of. SCN 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 2000 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 25-50 percent yield losses in Ohio. Fields with a high level should rotate away of soybeans to a non-host crop for two to three years and then re-sample before introducing soybeans to the rotation again. When soybeans are reintroduced, and SCN resistant variety should be selected. Rotating to a non-host crop such as corn, wheat or alfalfa.
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 (support reproduction) of SCN. If you have SCN in your fields, 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 & hairy vetch.
More information can be found on the SCN Coalition website: thescncoalition.com