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Soybean cyst nematode discovered

Just when you thought you were getting soybean aphid management figured out, a new pest has make its appearance in the area that may make aphids seem like a minor inconvenience.

The soybean cyst nematode (SCN) is one of the most destructive pests affecting soybeans in the United States. Traditionally, this pest has been considered a "southern" pest, but has been gradually moving north and was recently discovered in Red Lake and Norman counties in northwest Minnesota. Although it has not been officially confirmed in Polk, its presence is quite likely.

The soybean cyst nematode is a microscopic worm that feeds on and within the roots of soybeans, dry beans and a handful of weeds. High levels of this parasitic worm can have a devastating effect on yield, which was the one of the symptoms that led to its discovery this past summer. In fact, a common symptom is a farmer's complaint of disappointing soybean yields; SCN can reduce yield by 30 percent without any visible symptoms.

SCN is difficult to detect because symptoms often mimic other common crop problems. Plants appear stunted and yellow so the problem is easily confused with wet soil conditions, iron chlorosis (IDC), or herbicide damage -- the three most common soybean yield thieves.

The life cycle for SCN is typically 3 to 4 weeks depending on geographic location, soil temperature, and nutritional conditions. It likes warm soils and needs 75 degrees Fahrenheit for egg hatch, 82 degrees for root penetration, and 82-89 degrees for juvenile and adult development. Little or no development takes place below 59 degrees or above 95 degrees. In southern Minnesota, SCN can complete three to four generations during the growing season. In our neck of the woods, the nematode may complete only two, possibly three generations.

The best time to scout for SCN is late July into early August. Using a shovel to dig the root mass of a symptomatic plant, carefully remove the soil from the fine roots, and look for females with white egg masses and/or the yellow egg cysts attached to the root of an infested plant.

So how did they get here? SCN juvenile worms can move on their own only about 2 inches, so they need help to move any further. Longer distance movement from field to field is typically blamed on the movement of farm/construction equipment from infected to non-infected fields, runoff, wildlife, wind born dust, sugar beet tare, and possibly through seed lots containing infected soil peds. Once present, close rotations with host crops, such as monocropping soybean or soybean in sequence with dry beans, aggravate the problem and enable populations to increase at exponential rates.

If fields currently do not have infestations, producers may want to rethink their sanitation procedures to prevent infestation. Many company's offer resistant varieties, but few are short-season varieties appropriate for our area of Minnesota. Plus, some varieties with SCN resistance may not have agronomic traits that fit a particular producer's field.

There is no magic bullet to control SCN and its management leaves producers with disappointingly few options, beyond rotations with non-host crops. Rotations with non-host crops such as small grains, alfalfa, corn, sunflower or sugarbeet are currently our most effective tool. There are no chemical or bio-controls so producers should closely watch rotations of soybeans or dry beans in the cropping system, and should consider sampling, yield monitoring/mapping, and scouting for detection of this pest.

If you would like to learn more about SCN, join us for the Annual Crops Day at the Community Center in McIntosh, Minn., on Tuesday, Feb. 23. SCN will be one of the presentations of the day-long  Soybean Update/Private Pesticide Applicator Training. This event will held in conjunction with the East Polk County Crop Improvement Annual meeting. If you cannot attend this meeting, SCN information will be offered at other Soybean Update sites.  For more information, contact me at 800-450-2465, or if e-mail is your thing, contact me at Source: University of Minnesota Plant Pathology.