Test for SCN, soybean’s silent killer
April 12, 2019
Written By Linda Geist
Data from MU researchers shows soybean cyst nematode (SCN) field populations are becoming more virulent on commercial soybean cultivars, says University of Missouri Extension plant pathologist Kaitlyn Bissonnette. Testing fields for SCN is typically conducted once harvest is completed; however, spring is not too late to send soil samples for analysis if it couldn’t be done last fall.
SCN quickly began spreading in Missouri in the 1970s and gained a strong foothold in most of the state’s soybean-growing counties by the 1990s. Easily transported by nature, cysts and eggs can be spread within a field or to new fields by soil, equipment, water or wind. Today, it is the No. 1 soybean disease in the United States and Canada.
Yields drop by as much as 14 bushels per acre in infected fields when SCN reproduction is high, according to the SCN Coalition, a public-private partnership of researchers, extension specialists and industry representatives. Populations can increase exponentially, with 100 females capable of producing 39,062 eggs after four generations in one growing season, assuming each female produces 250 eggs, only half become female and only 1 percent of eggs will survive.
SCN is difficult to detect without testing because damage occurs to the root system before it can be seen. Symptoms include stunted plants, yellowing and yield loss. Yield loss can occur even when there are no visual symptoms, Bissonnette says.
Nematodes are becoming increasingly resistant to PI 88788, the genetic source of SCN resistance used in about 95 percent of all SCN-resistant soybean varieties.
Bissonnette suggests two ways to test for SCN: Dig a month-old soybean plant, gently shake the soil from the roots and look for white females, or collect soil samples for testing.
Collect 15 to 20 core samples for every 20 acres. Cores should be 6 to 8 inches deep and an inch in diameter. Collect in a zigzag pattern and divide each field into management zones. Include high-risk areas such as the field entry, flooded areas, low spots and historically low-yielding areas.
For each collection zone, mix the core samples together. Moisture content is important. “It’s difficult to get an egg count out of concrete or sludge,” Bissonnette says. Ideally, cores will stay intact during collection, but will easily fall apart upon mixing. When in doubt, err on the side of dry.
Samples should be put in bags and labeled. If possible, mark down the GPS coordinates of the field where samples were collected, and then send the soil to a testing facility.
Know your baseline SCN egg count and test every three to five years, Bissonnette says. Comparing SCN egg counts tells you if your management plan is working long-term. She recommends working with crop advisers and extension agronomists in your area to develop a management plan.