September 15, 2016
Alphabet Soup: BSR-SDS-SCN
Don't you just love trying to remember what abbreviations and acronyms represent? I have to admit that every so often I will stare at an acronym and try to remember exactly what I should associate with the hodgepodge of letters in front of me. Today I want to talk about three of these that are all related.
BSR stands for brown stem rot and SDS stands for sudden death syndrome, two fungal diseases of soybeans that are easy to confuse unless a little detective work is done. SCN stands for soybean cyst nematode, the most destructive and yield-reducing pathogen for soybean growers in Nebraska and across the United States.
In fact, if you added the yield losses caused by brown stem rot, charcoal rot, frogeye leaf spot, phytophthora, pythium, rhizoctonia, stem canker, sudden death syndrome, white mold and all other soybean diseases, they would not equal losses caused by this one pathogen, soybean cyst nematode. And unlike any of the other diseases listed, SCN causes these losses on perfectly healthy looking soybean plants. That is why it often goes undetected.
Now you may be asking yourself, "What do two fungal diseases and a nematode have in common?" The short answer is, if you have SCN in your field, you are more likely to have BSR or SDS. Both of these diseases enter the soybean plant through the roots early in the growing season. The SCN juvenile creates microscopic wounds when it enters the soybean roots to feed on moisture and nutrients produced by the soybean. These wounds provide points of entry for the diseases to enter the plant.
I want to make sure I avoid any confusion. Your fields can have either, or both, of these diseases and not have SCN. Or you can have SCN in your fields and neither of these diseases. However, if you have SCN in your field, or areas of your field, you are more likely to have either of these diseases where SCN is present, particularly where SCN is concentrated.
That's important to remember at this time of year because late summer is when BSR and SDS symptoms occur on the leaves. The plant was actually infected last spring, but the foliar symptoms don't show up until now. Toxins produced in the roots by the disease move up to the leaves, causing the whitish to yellow spots that may eventually coalesce and kill the tissue between leaf veins. Because the infection occurred last spring, there is nothing you can do now to lessen the impact of these diseases.
BUT, you can take two steps this fall to help manage these pathogens when soybeans are next planted in this field. First, soil test for SCN. You cannot tell if it's present or the level of infestation just by looking at a field. Second, if you have BSR- or SDS-like foliar symptoms in your field, identify which disease is present. BSR and SDS cause similar lesions and it's impossible to identify which disease is causing them just by looking at the leaves.
The pith, or spongy white tissue in the middle of the stem, will turn dark brown if the plant is infected with BSR. If the plant is infected with SDS, the pith will remain white, but there may be a tan discoloration to the stem tissue outside the pith, particularly at ground level or in the lower part of the stem.
There are a couple reasons why it's important to know which disease is in your field. First, some soybean varieties have good BSR tolerance and poor SDS tolerance, or vice versa. Knowing which disease is present can help with variety selection. Second, if you have significant problems caused by SDS, you might want to consider a seed treatment containing ILeVO. This product has performed well in reducing yield loss caused by SDS, but will not be as effective in reducing yield loss from BSR.
When you're selecting your soybean varieties, you also will want to select an SCN-resistant variety if soil tests indicate its presence. You would take a sample much as you would take a topsoil sample for fertilizer recommendations. Take 15-25 cores, six to eight inches deep, mix them thoroughly and submit it for analysis. If you have BSR or SDS in pockets in a field, I recommend taking one soil sample where either disease was present and another sample where it was absent and compare the levels of SCN.
Results from 29 trials conducted by the University of Nebraska in SCN-infested fields show an average six bushel-per-acre advantage by planting SCN-resistant varieties over susceptible varieties. When the same varieties were planted in non-infested fields, susceptible varieties had an average two bushel-per-acre advantage which is why you shouldn't automatically just plant resistant varieties. If SCN is not present, you may be sacrificing yield.
In Nebraska, the Nebraska Soybean Board has partnered with the University of Nebraska to provide a soil analysis for SCN at no cost to the grower. The cost is normally $20 per sample. This is an excellent way to get a return from your soybean checkoff dollars right on your own farm. Bags for submitting these samples are available at your local Nebraska Extension office.
So take your soil samples now, or make some notes where you observed these diseases and sample after harvest, to determine if SCN is present in your fields. If it is, this should also influence your variety selection the next time soybeans will be planted in this field.
For more information on BSR, SDS or SCN management in your soybean fields, contact your local Nebraska Extension office. We can help make it as easy as ABC to manage these problems in your soybean fields and improve your profitability.