Time to think about Brown Spot control

Septoria brown spot (SBS) is the second biggest disease threat to soybean yield after soybean cyst nematode.

Since SBS can potentially reduce soybean yield more than most farmers and agronomists realize, consider additional attention and control measures to manage this disease.

SBS, also known as brown spot, is a fungal leaf spot caused by Septoria glycines, a common disease of soybeans not only in Illinois, but throughout the entire Corn Belt. Nearly every soybean field in Illinois experiences some level of brown spot infection annually. However, for years none of us considered this disease a real threat to yield.

Warm, moist weather conditions, combined with high humidity and poor drainage, favor the development of brown lesions on lower leaves. These lesions can reinfect and move upward through the canopy. Moderate to severe infection can lead to premature senescence and leaf defoliation, resulting in loss of stored nutrients, smaller seed size with lower seed weight, and yield reduction.

In Illinois, soybean diseases that usually are associated with the greatest yield losses include soybean cyst nematode (SCN) and sudden death syndrome (SDS). While yield loss estimates continue to indicate SCN is the #1 yield robber in Illinois, estimates provided by the North Central Soybean Research Program in 2015 show SBS was the #2 yield robber. During 2015 in Illinois, SCN was estimated to cause yield loss of 18.8 million soybean bushels, while SBS caused yield loss of 12.5 million bushels. By comparison, SDS was estimated to cause 6.2 million bushels of yield loss.

These staggering yield loss estimates suggest SBS requires more attention and management for high yield soybeans than previously thought. In most years, I haven’t considered SBS to be a significant yield-limiting factor because it usually stays lower in the canopy without much upward progression throughout the growing season. Therefore, we generally never see it. But think about it for a minute. A lot of the yield on a soybean plant comes from the middle 50 percent of the plant and the foliage that resides there. The unobserved presence of SBS may have a bigger impact on yield than we thought.

A recent review of SBS research in scientific literature suggests yield losses can depend on several factors including genotype vulnerability, environmental conditions (rainfall and drainage), timing of infection, and use and timing of a fungicide application. Uncontrolled natural infections of SBS in field trials during the past four decades have resulted in yield losses of up to 10.2 percent.

Recent research and farm practices have revealed that applying a fungicide (strobilurin, triazole or a combination of strobilurin + triazole) near R3 (beginning pod) has been effective at reducing SBS infection and preventing yield loss when infections occur later in the season

Research has also indicated the infection of SBS during early vegetative stages, as early as V2 to V8 and before R1, can lead to greater disease pressure and yield loss than initiation of infection during mid-reproductive growth stages (R3 – R5). Infection of SBS is common in Illinois soybean fields during early vegetative stages. As mentioned previously, disease progression depends on varietal tolerance, rainfall during early vegetative studies, and adequate soil drainage. This information raises many valid questions regarding the need to better manage SBS to increase soybean yield. There are many questions to be answered.

1. Should SBS be controlled early in the growing season if disease is present and conditions favor continued development?
2. Would an application of a fungicide, when tank-mixed with a postemergence herbicide, result in higher crop yield and a positive ROI compared to a single fungicide application near R3 growth stage?
3. Can Illinois soybean growers increase yield and profit by utilizing sequential foliar fungicide applications, i.e., fungicide near V2-V3 followed by a fungicide application near R3?
4. Is a sequential foliar fungicide application, using different modes of action for disease control, a responsible approach for disease resistance management?

Figure 1.  Septoria Brown Spot (SBS) infection on lower soybean leaves.  Picture taken June 15, 2016 in Champaign County, IL (Chad Kalaher).

Chad Kalaher is an agronomist in East-Central Illinois at Beck’s Hybrids and a Farm Management Advisor in the Midwest. He is a 2017 Illinois Soybean Association CCA Soy Envoy.