Understanding the Yield Impact of Septoria Brown Spot on Soybean

by Terry Spurlock, Extension and Research Plant Pathologist - June 29, 2022

Key Points

• Septoria brown spot frequently occurs in soybean fields in Arkansas.
• This disease typically does not cause yield losses.
• On-farm trial results in 2020 and 2021 have shown Septoria brown spot occurring in the top third of the canopy during the reproductive stages to cause significant yield losses.

When does Septoria brown spot impact soybean yield?

Septoria brown spot is a foliar disease caused by the fungus Septoria glycines. Most people scouting soybean in Arkansas have noticed this disease beginning in the lower canopy. It is likely detected before other foliar diseases develop. Septoria brown spot is often the first one we see. In the lower canopy, it impacts leaflets that do not contribute to pod formation and fill during the later reproductive stages of soybean development and therefore does not impact yield. However, like many other foliar diseases, if brown spot occurs in the upper third of the canopy, it can damage green tissue that would otherwise be making energy for pod fill before R6 (Figure 1). In this case, our data shows that significant yield losses can occur.  

Figure 1.  Severe Septoria brown spot occurring in the nontreated control in an on-farm trial in Arkansas County, 2021.

Effective Recommendations for Severe Septoria brown spot

Generally, the recommendation for in-season management of foliar diseases that occur on soybean would be to scout. If the disease is spreading due to favorable weather conditions, apply a foliar fungicide to protect tissue not yet impacted. Products containing multiple modes of action have been effective in yield protection in our trials where Septoria brown spot was severe (Tables 1 & 2).

Table 1.  Results of an on-farm foliar fungicide trial in Lonoke County in 2020. 

Means with the same letter are not significantly different using Tukey’s honest significant difference test at P = 0.05.

Table 2.  Results of an on-farm foliar fungicide trial in Arkansas County in 2021.

Means with the same letter are not significantly different using Tukey’s honest significant difference test at P = 0.05.

Weather Patterns and Septoria brown spot

We would also expect much of the damage that we see from foliar diseases in our state to occur on later planted or later maturing soybean when the temperatures have cooled slightly, and rainfall is becoming more frequent. However, in on-farm trials in 2020 and 2021, fields where yield losses occurred due to Septoria have been R3 in late June. These trials were all 25-50 acres with fungicide treatments replicated three times and a nontreated control block included in each replication. Weather data collected from on-farm trial locations between R3 and R6 indicated that when compared to fields where Septoria did not occur or was not yield-limiting, the average rainfall was less, but the humidity was around 2 percentage points higher. Temperatures among these locations were similar (Table 3). Based on my observations and conversations with others, I don’t think it’s coincidental that we are currently seeing Septoria brown spot advancing up the canopy in many fields while experiencing high heat and humidity around the state.

Table 3.  Weather data compiled from 8 locations in 2020 and 2021 where Septoria brown spot occurred and was severe or not. 

Practices to Consider

If Septoria brown spot does occur in fields this season and is severe, take action to limit its impact next season and into the future. Since we do not have a comprehensive list of varieties that are more or less susceptible to Septoria, choosing less susceptible varieties may be difficult. Because the Septoria glycines fungus overwinters on soybean debris, tillage and rotation to another crop, at least for one season, are good practices to consider. Some evidence suggests Septoria brown spot is more severe in fields grown in continuous soybean every year.

As always, should you need assistance, do not hesitate to reach out to me or Travis Faske.

Acknowledgements 

We thank the Arkansas Soybean Promotion Board for funding the research discussed in the blog article as well as the cooperating farmers, consultants, and county agents that helped coordinate the on-farm fungicide trials mentioned here.