Soybean Disease Update: September 7, 2016

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The bronzing phase associated with Cercospora blight is a commonly observed disease in MS soybean fields.

Foliar diseases continue to be observed throughout the MS soybean production area. In addition to some diseases that have increased in incidence and severity, additional observations have been made in some field situations to determine the impact of the inclement weather received over the past month.  Many areas in MS received several weeks of extremely wet, overcast weather that increased conducive conditions for the development of foliar diseases.  Most of the comments below deal with the incidence of particular diseases and overall lack of a management practice at this time.

Cercospora blight

Over the past few seasons the appearance of Cercospora blight has seemed more extreme. Soybean leaves that develop a leathery appearance, thickened, puckered, and in some cases with a bronzing coloration can all be considered to be exhibiting symptoms of Cercospora blight.  Two phases of the disease exist: the bronzing phase that turns leaves several different colors depending on the variety and a blighting phase that can make leaves appear “burned”.  Bronzing of pods, stems and petioles can also be observed depending on how severe the disease has been during that season.  Fungicide applications once the symptoms have been observed are not effective at reducing symptoms associated with the disease.  Purple seed stain, even though considered to be the result of the same fungus, is not correlated to the presence of Cercospora blight on leaves.

Frogeye leaf spot. Maroon lesions with gray centers. Fungal reproduction on the underside of the leaf.

Frogeye leaf spot

Over the past two weeks frogeye leaf spot has generally increased in severity at locations where susceptible varieties were planted. The cooler temperatures, coupled with ample moisture have increased observable levels of frogeye.  In general, R6 is considered to be a safe growth stage and fungicide applications would likely not provide an economic benefit when applied at that specific growth stage.  However, fungicide efficacy work done with frogeye susceptible varieties suggests that applications made as late as R5.5 can be economical.  Frogeye leaf spot, if present in a field, will occur on the youngest leaves in the upper most canopy.  If the leaf spots are only present in the lower to middle canopy make sure to verify the particular disease present by looking for fungal reproduction on the underside of the leaf.  Several other leaf spot diseases can produce lesions similar to frogeye (see below).  The main disease that could most easily be confused for frogeye leaf spot would be target spot.

Be mindful that data sets to support the response of susceptible varieties planted following wheat are limited regarding fungicide applications. In most of the situations over the past several years fungicides applied at R5 to later-planted soybean fields have not provided as much economic response.  Every field situation differs and the decision to make a fungicide application should be based on the specific response (resistant or susceptible) of the variety to the frogeye leaf spot fungus.

Soybean rust as observed from the upper leaf surface on an infected leaf.

Soybean rust

Low levels of soybean rust continue to be observed throughout parts of MS. The hot dry conditions experienced during the majority of the season have reduced the risks associated with this particular disease.  In general, much of the soybean acreage is at or beyond a growth stage where a yield loss could result from soybean rust infection.  The general environment throughout much of the season produced poor conditions for the development and spread of soybean rust within MS.  In addition, as it would take several weeks for enough inoculum to cause a yield loss, and with the current temperatures, soybean plants will likely not be affected by the disease.

Over the coming weeks and months, additional counties will likely be added where soybean rust is observed in the coming weeks as we continue to look at fungicide trial plots and variety trial locations (both OVT and on-farm trials).

As an update, for the 2017 season, soybean disease monitoring and the continued use of sentinel plots will be proposed to the Mississippi Soybean Promotion Board. However, the larger, regional disease monitoring situation that has been previously been funded since 2005 was not funded for 2017 by the United Soybean Board.  A general reduction of funds may limit the effectiveness of the disease monitoring effort as well as reduce the total number of sentinel plots planned for the state.

Target spot

Over the past several weeks, numerous fields with severe defoliation have been observed. In some cases, the symptoms observed in the fields may have easily been confused with numerous disease issues (e.g., stem canker, pod and stem blight, anthracnose) or with general cultural practices (e.g., irrigation issues).  The confusing symptoms have been difficult to diagnose at the field level through photos due to the presence of interveinal chlorosis in some field situations and small black to purple lesions on petioles, pods as well as main stems.  Symptoms associated with target spot are generally observed in the lower to middle canopy.  The most commonly associated symptoms that result from target spot includes lesions that tend to be a ¼ to a ½ inch around with a yellow halo.  Secondary infection as a result of the most mature lesions in the plant canopy tend to be smaller, maroon in coloration, do not tend to have the concentric rings and oftentimes are lacking the yellow halo.  On annual basis, target spot is observed at what could be considered low levels, a few lesions throughout the canopy.  In years when excessive moisture occurs during the mid-to-late reproductive growth stages, R4 to R6, severe levels of target spot can be observed.  The general symptom associated with fields exhibiting severe levels of target spot tends to include complete defoliation of the lower to middle canopy.

Target spot lesion on a main stem at the node.

Severe target spot secondary infection on leaves in the mid-canopy.

Target spot lesions on the main stem as well as petiole.

Observations made in foliar fungicide efficacy trials over the past several weeks suggest that a fungicide would NOT have prevented target spot from defoliating plants. As an example, Starkville fungicide trials were applied at R4 on some later planted Maturity Group IV soybean plants.  Minimal target spot was observed at the time of observations.  Four weeks after application some plots were observed to have greater than 50% defoliation of the lower to middle canopy (see photo).  Additional ratings in Starkville as well as Stoneville foliar fungicide efficacy trials will be made over the next several weeks.

Severe defoliation as a result of target spot in a fungicide trial conducted in Starkville, MS.