Work through soybeans-after-soybeans dilemma

Soybean Pest Beat: How can you limit disease and insect issues if you must follow beans with beans in 2020? 
Sep 03, 2019
I ended up with soybeans after soybeans in several fields where I intended to plant corn. Are there signs of certain diseases I should look for at harvest which tend to be worse in beans after beans? To get back on rotation, would it be risky to plant beans in those fields again in 2020?
The Indiana certified crop adviser panel answering this question includes Betsy Bower, Ceres Solutions, Terre Haute; Traci Bultemeier, Corteva/Pioneer, Fort Wayne; and Dan Ritter, Dairyland Seed, Wabash.
Bower: There are several root and foliar diseases that can cause issues in beans after beans. They include phytopthora and pythium root rots, sudden death syndrome, and brown stem rot, plus root infections. White mold and frogeye leaf spot both overwinter in soybean residue. Soybean cyst nematode can be more problematic in beans after beans.
Look for areas of the field or even single plants that die prematurely. Get with your crops adviser and see if he or she can help you identify the root cause of death, especially if you plan to follow with soybeans next year. Notify your crops adviser on the early side of the infection, if you can. It can be very hard to identify the cause of early death at harvest.
While beans on beans isn’t highly recommended, it still may be an ideal rotation for a specific farm or field. Conditions that support beans on beans include good soil fertility. Be sure that you fertilize for beans after beans.
Soybeans are a heavy user of potassium, but also of phosphorus, sulfur and zinc. Ideal potassium soil-test levels improve the chance to reduce potential for certain diseases. Other conditions that support a bean-on-bean rotation would be little evidence of SCN, frogeye leaf spot or white mold. Early-season root infections by pythium and phytophthora can be reduced with seed treatments. Fields with good drainage are good candidates. I personally know of a few fields that have been in beans year in and year out with little difference in yield.
Bultemeier: The key word for success in beans back to beans for two or more years is variation! Vary your herbicide program, vary your soybean variety, vary your soybean maturity, and you may need to vary your tillage practices. If white mold is an issue, consider deeper tillage to bury the sclerotia. You’ll also want to soil-test again, which may mean soil-testing back-to-back years to be sure fertility levels aren’t being depleted.
Scouting, scouting and more scouting for diseases and insect presence is the norm instead of just planned passes of fungicide and insecticide. Soybean cyst nematode is also a major consideration — not only on light soils, but it needs to be tested for even on heavier soils. SCN can quickly remove top-end yields.
The benefit of crop rotation to soil quality and soil fertility must be weighed carefully against the potential long-term costs of going to three years of soybeans. It can be managed, and it must be managed.  
Ritter: The best time to assess disease issues would have been in the growing season. During harvest, we still can see where there were problems. However, it may be more difficult to determine what those were. Agronomically, monocultures tend to increase pest issues. So, to directly answer your question, yes, it’s riskier to run three years of soybeans. The most concerning pest for me is SCN.