Soybean disease update: June 20, 2026

Honestly, the statement to this point in the season is the disease situation has been rather quiet.  Normally, foliar diseases start to pick up at this point in the soybean crop with growth stages of some of the earliest planted fields well into reproductive stages.

Septoria brown spot

Septoria brown spot is one of the most common lower canopy diseases in the Mississippi soybean production system.

For the second year in a row, I continue to receive mostly questions related to managing Septoria brown spot.  Honestly, Septoria brown spot is one of the most common soybean diseases in MS.  The disease is most common in the lower-to-middle canopy depending on the time of the year and growth stage of a given field.  Brown spot most often in fields of continuous soybean and can be observed from early vegetative stages (V2/V3) through late reproductive growth stages (R6/R7).  I suspect that brown spot is present in every soybean field in MS by the end of the season.  However, not every field has an “issue” with brown spot.  In my 19 years in MS, I can count the fields that I would have considered suggesting receive a fungicide application to manage the brown spot on one hand.  During this season in particular (2026), of the fields that I have observed where Septoria brown spot was considered a significant disease there were additional, underlying agronomic issues.  In fact, over the past two seasons, the fields I’ve seen with the worst Septoria brown spot had some form of hard pan, were reduced tillage, and in some cases there were major fertility issues present.  In those field situations, I hardly considered a fungicide application because the disease was knocking off a significant number of leaves on already stressed plants.  However, in both those cases (one field in 2025 and one in 2026) the fields received a fungicide application and in neither instance did the fungicide produce much in the way of “disease control”.  In fact, the specific field in 2025 was twin-row soybean on 38” rows that never lapped the middles because of the inherent agronomic issues and not as a result of the disease.  The Septoria was not the major issue in that field…..the lack of good agronomics caused the soybean plants to be held back.  Be mindful, Septoria brown spot is a lower canopy disease that likes row middles that have lapped where increased levels of humidity can help the disease move through the canopy.  Wide open fields that appear to have a lot of Septoria brown spot likely have some additional stress that’s helping the disease along.

Fungicide applications may ultimately prevent the loss of foliage through premature defoliation and may reduce the movement of the disease throughout the canopy.  However, in my experience, you can make a fungicide application to attempt to manage brown spot, come back in two weeks and observe more disease in the canopy.  Not every field needs a fungicide application to manage Septoria brown spot.

I honestly consider Septoria brown spot one of our more inconsequential diseases and not a major yield reducer.  I think in 2026 we are more focused on brown spot because it’s one of the only foliar diseases present in the canopy.  Be mindful that a foliar fungicide application cannot make up for poor agronomics and will not alleviate stressful situations if those are in fact contributing to increased levels of disease.

Frogeye leaf spot

Frogeye leaf spot more commonly occurs in the upper third of the plant canopy generally after R3/R4.

I have had several conversations over the past week related to frogeye leaf spot.  Based on observations over the past several years there are likely some varieties that tend to be more susceptible than what we have been accustomed to in the recent past.  However, with that said, I don’t think anything is as susceptible as the old DeltaKing 4744 that then went to Armor and became Armor DK 4744.  I have not seen anything as susceptible in any of the portfolios since that soybean was discontinued around 2016/2017.

Focus on scouting for frogeye in the upper third of the canopy beginning at R3.  Lesions tend to form on the newest leaf material and will be maroon with a gray center.  Observing and taking photos of the lesions on the underside of the leaf makes it easiest for diagnosis since the fungus produces spores within the lesion on the underside of the leaf.  Halos do not generally form around lesions so disregard leaf spots that may have a yellow halo around them as these tend to be caused by something else.

Fungicides are effective at managing frogeye leaf spot and provide a good return on investment especially when applications are made to susceptible soybean varieties.  One lesion does not indicate that a variety is susceptible and in fact, even resistant soybean varieties can ultimately develop one to a few lesions.  However, proper diagnosis of the disease as well as correct fungicide selection are important.  Do not expect to apply a stand-alone QoI-based product and manage frogeye leaf spot.  Widespread resistance to that particular class of chemistries means that a DMI or triazole will most likely be needed.  Applications can be made as late as R5.5 (and likely later) and still produce a desired effect.

R3 fungicide applications

I continue to field calls related to this former practice.  With that said, over the recent past I have started to hear that “MSU is anti-fungicide”.  That’s simply not the case.  Put more eloquently, we are in the practice of helping soybean farmers make money.  At present, our data suggest that an “automatic” fungicide application is not economical and does not produce a positive return on investment.  For more specific information related to this see:

https://www.mississippi-crops.com/2026/06/12/the-r3-r4-fungicide-application-reconsidering-the-practice-from-large-plot-strip-trial-data/