• By Dominic Reisig, Lindsey Thiessen and Rachel Vann •
Soybean seed quality issues are widespread across the North Carolina Coastal Plain and parts of the Tidewater region this year in indeterminate varieties planted prior to mid-May. We do not have a complete understanding of the drivers behind the observed issues this year; however, these are likely a result of an environment by genetic by management interaction in many fields.
We have been involved in calls with our county Cooperative Extension agents in many fields across the region over the past month and will discuss some commonalities and differences in damaged fields below.
Use of indeterminate varieties: The vast majority of damaged fields have been from a <MG5.5, with the majority of the varieties having an indeterminate growth habit. These varieties were approaching maturity when excessive rainfall was encountered in early September.
It is worth noting that at this point it is still too early to determine if these issues will persist into later-maturing (determinate) varieties or even earlier maturing varieties planted at later dates; however, we have research trials with soybean maturities from 2-7 this year where we have observed seed quality issues in our earliest maturing, indeterminate varieties (<MG5) that have yet to be observed in our later maturing varieties planted at those same planting dates. In general, in our trials, seed quality has improved in later planting dates with all maturity groups, as was observed in 2019.
Early planting: At this point, most of the problem fields were planted in mid-May or earlier.
Excessive moisture in early September: Many of the areas with the worst observed damage had 4-8 inches of rain from the end of August through mid-September. The rainfall events were persistent, often dropping 1-2 inches of rain over several subsequent events during that time period.
Early season stressors: We have been concerned about our soybean crop since the spring when many of our fields received excessive moisture, leading to replanting on many acres. Our concern, especially in the Coastal Plain, is that this excessive moisture limited root development early in the season.
With limited root development, our crop had reduced ability to withstand stress events, like the heat and drought we experienced during July in parts of the state. These compromised root systems not only limit the plants ability to withstand stress, but in doing so, compromise the plants own defense mechanisms.
When issues like seed decay pathogens are prevalent at this point in the season, the plant does not have a robust ability to fight off those issues using their normal defense mechanisms.
Lack of scouting past early reproductive development: This was not the year to give up on soybean scouting in July. We have had heavy foliar diseases show up in parts of the state, primarily Cersopora leaf blight, and we have had a heavy stink bug year in soybeans.
Many of the visited fields were not scouted past the early reproductive growth stages. Fungicide and insecticide applications in later reproductive development may not have prevented all observed issues, but could reduce some of the synergies we are now seeing between these stressors.
Homogenized damage: In many of these fields, but not all, damage is remarkably consistent across the field. This pattern is typically indicative of environmental drivers behind observed issues rather than solely biotic drivers (i.e., diseases and insects).
Green stem is often present.
Variety: While at this point we have observed these issues primarily in indeterminate varieties, beyond that we cannot isolate this issue to any one variety or company. We have observed seed damage in varieties from many different seed companies. It is obvious from walking across our OVT (on-farm variety trials) plots that there are differences in varietal response to these seed decay issues; however, there is not current information available on differences that could aid growers in variety selection to avoid this situation next year.
Confirmed pathogen(s): There are many different pathogens contributing to the seed quality declines we are observing this year including phomopsis seed decay, purple seed stain, Fusarium and anthracnose. Some of these diseases are present alone, while in many circumstances several diseases are observed in the same field; however, there is no consistency between field to field on what has been confirmed. We have seen more anthracnose in 2020 than other recent years.
Stink bug pressure: Like many late-season soybean issues, stink bugs are taking the heat for much of what we are observing now in 2020. However, many of the fields we have visited have little indication that stink bugs are the primary driver of observed seed decay issues.
Are stink bugs present in many of the fields at this point in the season? Yes, of course, because these soybean fields are often the only green crop remaining in the environment. But the classical stink bug injury (pierced pod and seeds), are not present at high enough levels to indicate that stink bugs are the primary driver in these issues.
Stink bug injury can open up opportunities for movement into the pod by other seed decay issues; therefore, stink bugs (and other insects) should be scouted through R7 (beginning maturity) in soybeans to prevent yield loss and seed quality declines in soybeans.
Soybean sprouting in the pods: Soybean sprouting in the pods is present in some but not all of the affected fields. Premature seed sprouting is generally rare but can be an issue when the moisture of the seed drops below 50%, and then quickly goes back above 50%.
If growers have experienced severe seed quality declines, there are limited options at this point in the season. Please contact your County Cooperative Extension agent to discuss these options. Many of these seed decay diseases can survive in soybean residues, so growers should rotate out of severely impacted fields next year.
We are starting to get questions from growers about avoiding using earlier maturing varieties next year because of these issues. We have always recommended that growers diversify their risk by planting a range of maturities across different planting dates if this is possible from a management perspective in their operations.
Most fields where we have seen seed decay in the earlier maturing varieties this year have also had high yield potential. Our 2019 data indicated that seed decay issues could be reduced if planting dates were pushed into May without sacrificing yield; we look forward to providing more data on this in 2020 and upcoming years.
The authors are Extension specialists with North Carolina State University.