By Melissa Bartels and Tamra Jackson-Ziems
Planting will be here before we know it. But even in the winter, management decisions can be made to reduce losses caused by some important diseases. The most damaging pathogen of soybean is soybean cyst nematode (SCN) and winter is the time to make decisions to control it. If you have confirmed soybean cyst nematode on your farm, it’s important to actively manage them to reduce population densities and keep them low to minimize the damage they can cause (Figure 1).
Figure 2. Larger Rhizobium nodule (blue arrow) and several white SCN females (red arrows) on neighboring roots. Note the size difference and that SCN are much smaller than nodules. (Photo by Kyle Broderick)
Management of SCN can be easy and may not require additional input costs. In fact, the first management recommendation is to rotate the infested field(s) with a nonhost crop — most Nebraska producers are doing this already by rotating soybean with corn. For the first 1-2 years of corn production, SCN population densities are expected to decline 50-75% each year. Growing continuous corn for additional years still helps reduce SCN, but the rate of reduction is expected to decline and population densities will never reach zero SCN eggs (Figure 2). Other nonhost crops, such as small grains and alfalfa, are also good nonhost crop rotation options to help reduce SCN populations.
In addition to growing corn or other nonhost crops, it’s very important to carefully select soybean varieties that have SCN-resistance and rotate the sources of resistance year-to-year that are used. Unfortunately, we have historically had limited choices for SCN resistance. More than 95% of SCN-resistant varieties were derived from a single source of resistance — PI 88788. In Nebraska, almost 50% of SCN populations are able to reproduce on PI 88788 (Broderick, 2016). In some other states, up to 100% of their SCN populations can reproduce on PI 88788! When selecting your resistant varieties, try to identify ones with SCN resistance that comes from other sources, such as Peking, which is becoming increasingly common in commercially available varieties. The good news is we now have another source of resistance — PI 89772 —which is available this year in two new varieties from Syngenta brands: Golden Harvest and NK. These varieties are in maturity group 2.3. Sometimes you may find it difficult to find a resistant soybean variety in your desired maturity group with a different source of resistance (other than the common PI 88788). In that case, you should at least select soybean varieties with a high level of PI 88788 resistance and switch varieties. That information is available in ratings printed in seed company catalogs or contact your seed company representatives for more information.
In addition to selecting the best SCN-resistant variety for your farm and rotation, you also have several seed treatment nematicide products now available. These products may provide some additional protection and can increase yield 1-5 bushels per acre, based on the results from Iowa State University. Seed treatments are not a substitute for the use of effective SCN-resistant varieties whose yield can vary by 10 or more bushels per acre in fields infested with SCN.
Figure 3. Sampling for SCN can be conducted any time of year and during any crop since these nematodes remain shallow in the soil. Click here to review sampling recommendations from the SCN Coalition. (Photo by Kyle Broderick).
Remember, weed management can be important as there are several common winter annual weeds that are hosts for SCN and can allow them to infect and reproduce to higher population densities, even when there’s no crop in the field. Winter annuals henbit and purple deadnettle are strong hosts for SCN. Field pennycress, shepherd’s purse, small-flowered bittercress and common chickweed can also be hosts for SCN.
In 2020, the UNL Plant & Pest Diagnostic Clinic processed 459 samples from 37 counties across Nebraska as a part of an annual survey funded by the Nebraska Soybean Board — 32% of these samples were positive for SCN and 12% of samples had populations over 500 eggs per 100 cc’s of soil (~3 oz.). The highest population recorded was 85,760 eggs per 100 cc’s of soil from Antelope County. View more results from the
2020 SCN Survey and results of the 10th Annual ‘Tode Awards.
If you don’t know or haven’t tested a field(s) recently to check SCN numbers, you can collect a sample any time of the year, in any crop field, as long as you can get a soil probe into the ground (Figure 3). Submit a soil sample for SCN analysis at no charge thanks to support from the Nebraska Soybean Board.
Source : unl.edu