Recently a local farmer asked me if all the rains and flooding earlier this spring drown the soybean cyst nematodes, or SCN, he knew he had in his fields. I think he knew the answer, but he was looking for something positive out of all the weather challenges he and many farmers faced this spring. Unfortunately, SCN drowning in flooded or waterlogged fields was not going to be one of them.
SCN are microscopic worms that do require oxygen which they absorb through their body wall. Waterlogged soils greatly reduce oxygen levels, but SCN has been shown to survive long periods without oxygen. An older study in Arkansas showed SCN survived 630 days under flooded conditions… probably longer, but the experiment ended after 630 days.
More recently, I checked some soybean fields in 2012, where I knew there was an SCN infestation and had been under water for 3 months in 2011, and SCN was still present and active in those fields. Not only will SCN survive flooded conditions, but floods also have the potential to move SCN from one field to another. So flooded fields where SCN wasn’t present in the past may now have SCN.
The only possible silver lining to the wet conditions this spring is it may reduce the potential for SCN building up in a field. SCN normally completes its life cycle in 25-30 days, depending on soil temperature, so delayed planting may result in one or two fewer generations this summer.
It takes a long-term, integrated approach to manage SCN. This includes rotating soybeans with non-host crops such as corn, planting SCN-resistant soybean varieties when they are planted, and periodic soil testing to determine if SCN levels are increasing or decreasing in known infested fields. This is because about half of the infested fields in the state have populations of SCN that can reproduce on varieties that use PI88788, the most common source of resistance in SCN resistant varieties.
If you have been rotating and using SCN-resistant varieties and your SCN levels continue to increase, you will want to use a variety of soybeans that has a different source of resistance, such as Peking, the next time you plant soybeans. You may also want to consider a nematode protectant seed treatment which bolsters the performance of SCN-resistant varieties.
Now I want to switch gears and talk about a problem that is showing up in some cornfields. I was on a conference call last week and the person who runs our Plant Diagnostic Clinic at the University of Nebraska indicated he was getting a lot of corn samples that had bacterial leaf streak. This disease is relatively new to Nebraska and the lesions on corn leaves can mimic that of gray leaf spot.
The reason this is important is bacterial leaf streak is caused by a bacteria and will not be controlled with fungicides. If it is misdiagnosed as gray leaf spot, needless, ineffective, and costly fungicide applications may be made. A couple of tips on distinguishing between BLS and GLS are:
g Bacterial leaf streak often shows up earlier in the growing season than gray leaf spot.
g Bacterial leaf streak often appears in the mid- to upper-canopy, gray leaf spot starts in mid-canopy.
g Although the lesions are similar in appearance, bacterial leaf streak lesions are usually not as straight along the sides or square on the ends as the lesion as gray leaf spot. Also, bacterial leaf streak is more likely to have a yellow halo around the edges of the lesion than gray leaf spot when held up to a bright light, but this is not consistent because different hybrids react differently.
Unfortunately, there are no good treatments for bacterial leaf streak, but observations indicate there are differences between hybrids in their susceptibility. More testing is being done to identify and rate these levels of resistance. If you need help identifying which disease might be in your cornfield, bring leaf samples to your local Nebraska Extension office.
John Wilson is a Burt County Extension Educator specializing in innovative cropping and water systems, with emphasis in pest management, crop disease, pollinators, and pastures and grazing. He can be reached at firstname.lastname@example.org.