Foliar Soybean Disease Update: August 19, 2017

Severe aerial web blight (photo courtesy of retailer).

Aerial web blight

When the environment remains conducive for an extended period of time, aerial web blight can be one of the most devastating diseases. Earlier this week I received photos of severely affected soybean plants.  The field in question contained plants at the R4 growth stage.  Photos showed plants with no pods, no blooms, and very few lower canopy leaves (see attached photo).  The field was planted to a maturity group (MG) V, determinate soybean variety.  The suggestion to the retailer was to not spray the field since the cost of the fungicide may be lost since the plant has finished flowering and may not flower again.  Scout for the presence of aerial blight early in the morning when dew is still present on the foliage.  The webbing that is produced by the fungus is much more readily observed when dew is present.  Later in the day distinguishing aerial blight may be different as the symptoms generally appear as burned leaf tissue.  Fungicide products that contain a strobilurin (or quinone outside inhibitor, QoI) are preferred for aerial blight management.  If a fungicide application is warranted, make the application via ground in high water volume (15-20 gallons per acre) and as high a pressure as possible (> 60 psi would be preferred).  Aerial application is not the preferred method to manage aerial blight as canopy penetration of the fungicide is the most important aspect.

Severe Cercospora blight symptoms as expressed with bronzing, blighting, and symptom expression on petioles.

Cercospora blight

One of the most common foliar diseases of soybean annually. Cercospora blight occurs in every field of soybean grown to one degree or another.  Leaves that develop a bronzing coloration or a thickened feel are infected with the fungus that causes Cercospora blight.  Managing Cercospora blight has proved to be difficult since fungicides appear to be ineffective on the fungus and resistance within the commercially available germplasm is lacking.  The symptoms associated with Cercospora blight can also affect pods, petioles, the main stem, with the most notable symptom being observed on leaf tissue.  Research conducted at the LSU AgCenter suggests that two phases of foliar symptoms present post-infection: bronzing, which can appear as slight yellowing to maroon or purple coloration on leaf tissue (depending on variety and location) and blighting, which most notably appears as leaf scorching at the leaf tips.  Fungicide application once symptoms are observed is not a good management alternative.  In addition, fungicide timing trials as well as efficacy programs suggest that strobilurin (QoIs) are no longer effective on the fungus, more than likely due to widespread fungicide resistance.

A large research project, funded by the United Soybean Board in coordination with the Mid-south Soybean Promotion Board has been looking at management alternatives for Cercospora blight as well as determining whether or not germplasm offers any resistance to the disease. The project is currently in its third funding cycle with locations in AR, LA, MS, MO, and TN.

Soybean rust as observed on the upper leaf surface. Pustules are formed by the fungus on the underside of the leaf.

Soybean rust

Over the past two weeks multiple counties have been observed to contain either soybean rust infected kudzu (Clarke, Monroe, Winston) or soybean rust-infected soybean (Claiborne, Copiah, Lee, Lowndes, Newton, Noxubee). In all instances where soybean rust infected soybean plants were observed at growth stages beyond the need for a fungicide application (≈R5.5).  Commonly soybean rust is observed during this time of the year and is most likely the result of the recent rainfall received throughout much of the state.  Since soybean rust was first identified in the region, in 2004, Mississippi has likely not observed widespread yield losses as a result of this particular disease.  The most notable exception was in 2009 when several fields around Brooksville, MS were observed to have severe soybean rust and likely observed a 20-25% yield reduction as a result of severe soybean rust.

Young soybean fields in early reproductive growth stages should be scouted regularly as in situations where increased inoculum may be present soybean plants in early reproductive growth stages can be infected by the fungus. But, be mindful, that if necessary (and warranted due to the presence of soybean rust), a carefully timed fungicide application could be beneficial, but that will depend on growth stage, severity of infection, expected yield potential, and the prevailing environment post-application.  Be mindful that distinguishing between soybean rust and several other lower canopy diseases can be difficult.  Quite regularly, soybean rust will first be observed in the mid-canopy and is not going to start in the top of the plant.  Septoria brown spot as well as bacterial diseases (blight and pustule) can all produce symptoms similar to soybean rust.  Pustules on the underside of soybean rust-infected leaves should be observed with the aid of a hand lens (20x is all you will need to complete that task).

Target spot lesions can appear similar to other diseases depending on position in the plant canopy as well as variety susceptibility to the fungus.

Target spot

Much like Cercospora blight mentioned above, target spot is one of the most common soybean diseases in MS every year. Target spot is always present in the lower canopy, but if the proper environment persists whereby plants remain wet for extended periods of time with ample rainfall and overcast conditions the disease can move up the canopy.  In addition, varietal susceptibility plays a large part in whether or not the disease can move into the upper canopy.  The increased severity of the disease that occurred during 2016 greatly increased the coverage of this particular disease.  Even though target spot has been severe during 2017, the severity associated with the disease has not been as widespread as occurred during 2016 and defoliation in some field situations is not near as severe as observed last season.  In the lower canopy, target spot produces lesions with concentric rings, hence the name target spot.  However, on susceptible varieties lesions can oftentimes appear quite similar to frogeye leaf spot and can also occur on pods, petioles and the main stem.  Some fungicide products are effective at keeping the disease in the lower canopy; however, product application timing and the environment post-application likely dictate how high the disease moves.  Stand-alone strobilurin fungicides are not effective at target spot management due to the presence of fungicide resistance.

Numerous foliar fungicide efficacy trials have produced some good results thus far and should be excellent tools for messaging in the fall and winter meetings.