Soybean Disease Update: September 7, 2019

For the most part, 2019 has been rather quiet when it comes to soybean diseases. However, some things are starting to be observed across MS.

Soybean rust symptoms on the upper leaf surface should be confirmed by the presence of pustules on the leaf underside.

Soybean rust

We don’t talk about soybean rust much as an issue, because it rarely has impacted yield. But with some late soybean acres in MS, monitoring the disease this season might be important.  To date, soybean rust has been observed in six counties (Holmes, Leflore, Warren, Washington, Wilkinson, and Yazoo), but only on soybean in three counties: Leflore, Washington, and Yazoo.  In all three of the situations where soybean rust was observed on soybean, the fields in question were either at beginning R6 or close to R6.5.  Soybean rust does not like hot and dry weather.  But, as we move into September it is possible that the temperatures will decrease a little and with the excessive dews we are experiencing this could act to increase disease in fields that may be infected.  Soybean rust begins in the lower to middle canopy.  Lesions on the top of the leaf can appear similar to several other diseases in our production system with generally brown lesions surrounded by a yellow halo that look similar to bacterial blight and Septoria brown spot.  The pustules on the underside of the leaf are the most diagnostic symptom and appear raised and can range in color from tan (younger pustules) to almost dark brown (older pustules).

Monitor the Mississippi Crop Situation Blog, Twitter, and follow the new mapping website on https://soybean.ipmpipe.org/soybeanrust/. Please be mindful that there is no longer an official monitoring program for soybean rust and that planted sentinel plots are no longer in place in MS.  All of the scouting conducted, save for that in Alabama, is being done at random and most states no longer have a designated person to monitor for the disease.

Target spot of soybean. Note the concentric rings within the lesion.

Target spot

I continue to receive lots of questions regarding target spot. Honestly, target spot is one of the most common diseases in the MS soybean production system on an annual basis.  One cannot find a field of soybeans and not find target spot somewhere in the canopy.  I don’t think that the amount of target spot observed during 2019 has been any more or less common than what we normally observe.  At present there are a lot of commercial varieties that exhibit a good deal of susceptibility to target spot.  In addition, having looked at the MSU variety trials for the better part of the last eight years, I would suggest that almost no resistance exists to this particular disease.  Fungicides are therefore the main management alternative, but they are only effective to a certain degree.  In the fungicide efficacy work conducted in Stoneville, fungicides have helped maintain some leaves in the middle canopy by reducing severe defoliation as a result of target spot.  However, target spot is still observed following a fungicide application and since the disease is a common inhabitant in the middle canopy maintaining leaves on the plant above the symptomatic leaves is all that can be expected when a fungicide is applied.  In trial work conducted during 2017, fungicides helped maintain between 2 and 8% of the lower canopy (the non-treated was observed to have approximately 60% defoliation).  A fungicide application, regardless of the specific active ingredient, helped reduce defoliation, but defoliation as a result of target spot continued to occur regardless of the specific active ingredient.  Lately I have observed several stories online that suggest substantial yield losses associated with target spot (10-20 bushels/Acre).  I do not know anyone in a university plant pathology program that has conducted unbiased fungicide efficacy trial work that suggests yield losses associated with target spot are generally greater than 5 bushels.  In fact, during 2017, the greatest yield difference observed in efficacy plots in Stoneville was an average of a 4.9 bu/A between plots that received fungicides that contained a triazole and an SDHI and the non-treated.

The hardest part about a disease such as target spot tends to be that other diseases occur at the same time and in the same field and the same part of the canopy (e.g., Septoria brown spot, pod and stem blight). Therefore, making a statement regarding the losses associated with a single disease are rather difficult, especially that result from a disease that generally occurs deep within the canopy and not in the upper canopy like frogeye leaf spot.  Moreover, during the year that target spot was so bad, 2016 if memory serves me correctly, numerous fields were observed to also contain pod and stem blight and substantial defoliation as a result of both target spot and pod and stem blight that occurred prior to R6.  Separating the differences in a heavily infected field between target spot and pod and stem blight and the combined losses associated with the two working together would be extremely difficult.

In short, and the most important take home message: Finding target spot in a field does not suggest that a fungicide is necessary.

Southern blight of soybean. A) appearance of plants that have died as a result of southern blight, B) belowground mycelia growth, and C) normall appearance of mycelia and sclerotia on soybean plant crown.

Southern blight

Similar to 2018, a substantial amount of southern blight has been observed in fields across Mississippi. Small areas of fields or small clumps of plants where plants appear to have prematurely defoliated are a common observation this season.  In years past, looking for the fungal growth, in the form of white mycelia at the base of the plant, with sclerotia has been a telltale sign that southern blight has occurred.  However, in particular this season, finding the fungal growth at the base of the plant has been rather difficult.  I suspect a good deal of this difficulty has been due to the drastically changing environment.  One other way to potentially confirm the presence of southern blight would be to uproot plants and look for the fungal growth below the soil line.  In some cases, dead plants may produce sclerotia.  But, in most of the field situations I have observed this season (and last), in some fields where defoliation in the lower canopy has occurred in varieties with a dense canopy the protection offered by leaf litter on the ground and a dense canopy can sometimes help urge the fungus to produce mycelia and sclerotia at the base of the plant or on the soil surface.  I know not everyone likes to crawl around looking for fungi, but the best way to tell what occurred on dead plants is to get down on hands and knees and look at the stems and roots of plants.