According to the “Southern United States Soybean Disease Loss Estimates” for the 2014, 2015, and 2016 growing seasons, Septoria brown spot has become a problematic soybean disease in Tennessee and Mississippi. Annual yield losses have been estimated to be over 1 million bushels in each state, which translates to a significant economic impact.
Septoria brown spot can cause premature defoliation that may contribute to yield losses when susceptible varieties are grown and conditions favor disease development. Septoria glycines, the fungus that causes brown spot, overwinters in crop residue and in infected seed. Early-season infection may occur on cotyledons and leaves (lower soybean canopy), especially since optimum conditions (79 to 83○F and wet weather) for disease development are likely to occur at this time (Faske and Kirkpatrick, UAEX). According to Dr. Tom Allen (MCS blog, Soybean Disease Update: July 22, 2017), diseases that stay in the lower soybean canopy likely do not contribute to yield loss.
In 2015, research was conducted to determine “Effect of Seed Treatment on Early Season Brown Spot Caused by Septoria glycines of Soybean” (Vol. 17, p. 223-228 of Plant Health Progress). The addition of fluopyram to a base seed treatment resulted in a 2.5-fold reduction of brown spot, where average brown spot was 27% for the base seed treatment without fluopyram, and only 11% with the base seed treatment + fluopyram. The results from these studies provide evidence that seed treatments that include fluopyram (ILeVO) are effective at controlling early-season brown spot. [The ILeVO label indicates this when applied at a rate of 0.15 mg a.i. per seed, or 1.18 fl. oz./140,000 seed (equivalent to 50 lb. of soybean seed of a variety that has 2,800 seed/lb.)]. However, results from controlled-environment chamber studies the authors also conducted suggest that the fluopyram seed treatment is not likely to provide long-term protection from the disease.
Foliar fungicides in more than one class have very good efficacy against brown spot (Kelly, UTIA). The automatic fungicide applications that are usually made at the R3/R4 soybean stages will likely keep the fungus in the lower canopy where it will have negligible effect on yield.
While no soybean varieties are specifically promoted as being resistant to brown spot, the incidence and severity of the disease can differ among varieties. Thus, there appears to be potential for genetic resistance to this disease in soybeans.
In a previous blog on this website, I cited a PMN article about the first report of sudden death syndrome (SDS) in the lower Midsouth. As indicated in the Seed Treatments White Paper on this website, ILeVO is an effective seed treatment against SDS. The above results indicate that this seed treatment is also effective against early-season brown spot. Thus, in production environments where either or both of these fungal pathogens have the potential to cause soybean yield loss, producers should consider the addition of this seed treatment fungicide to their base package. This addition will increase the seed treatment cost by about $8/acre.
Brown spot has not caused the soybean yield losses that other more prominent diseases such as frogeye leaf spot (FLS) and cercospora leaf blight (CLB) have caused. However, producers need to at least be aware that there is potential for this disease to cause yield loss as indicated by the above cited survey results. The above research results provide confirmation that there is a management option for this disease in environments where it has potential to threaten yield. And this option also manages SDS, which may become a prominent yield reducer in the lower Midsouth.
One final note. Dr. Kelly’s above-linked foliar fungicide efficacy table (Kelly, UTIA) shows that several of the foliar diseases of soybean can be controlled by fungicides from more than one class, or by combining fungicides that have different modes of action. To prevent or delay resistance development among fungal pathogens, producers should rotate efficacious fungicides from different classes, or apply fungicide mixtures that have more than one mode of action. Or better yet, apply fungicides only when there is a problematic pathogen present or potentially present, or apply a product that will specifically address the problem that is identified. Producers cannot afford to lose presently available effective foliar fungicides because of resistance development in problematic pathogens.
Composed by Larry G. Heatherly, July 2017, firstname.lastname@example.org