Soybean Disease Update: July 5, 2015

Frogeye leaf spot on a susceptible soybean variety.

Frogeye leaf spot

Frogeye leaf spot (FLS) continues to be one of the hotter topics this season.  I have not received near the number of telephone calls this year as I did last year and I suspect a good deal of this has to do with more FLS-resistant varieties being planted.  However, many calls have requested information regarding the response of certain fungicides on FLS.  Managing the potential yield loss as a result of FLS is possible; however, strobilurin (QoI) stand-alone fungicides cannot be relied on to reduce FLS as has been the norm in the past.  On FLS-susceptible varieties when making an automatic fungicide application, rely on using either a pre-mix or tank mix that potentially contains a strobilurin + triazole or other curative-type component such as a carboximide.  Trial data for protocols that were conducted during the 2014 season can be found here:

https://www.mississippi-crops.com/2015/07/05/2014-frogeye-leaf-spot-and-cercospora-blight-foliar-fungicide-trials-trial-data/

Mystery disease update

Over the past two weeks the number of fields with observable symptoms associated with this particular root disease has increased.  In some cases, as the soybean plant matures the symptoms associated with the mystery disease (chlorotic leaves) are more noticeable as plants around them remain green.  Observationally, all soybean varieties appear to be susceptible to the root disease.  Researchers in Arkansas, Louisiana and Mississippi are working together to determine the specific causal organism.  Unfortunately, in the pathology world, before a management technique can be devised the organism responsible for the disease needs to be determined.  Once the organism has been positively identified research trials can move to the field to determine how potential management practices may reduce the severity of the disease.  I am hopeful that by the winter meetings a new disease name will be decided on and the specific organism will have been positively identified.  Keep in mind that in the past this particular disease has likely been referred to as: general root rot, Fusarium root rot, and black root rot.  In addition, depending on what state you are in the disease may have been given a different name.

Septoria brown spot. Note small brown lesions with expanding yellow margin.

Septoria brown spot

A tremendous amount of brown spot has been present in this soybean crop since the beginning of the season.  Much of this can be attributed to the environment we have encountered; cooler and wetter weather can increase observable disease, but the environment is typically pathogen dependent.  Septoria brown spot generally appears on the first true leaves that emerge from the soybean plant, especially if the field in question is in continuous soybean production.  Leaves with characteristic Septoria brown spot symptoms have small, 1/16^th to 1/8^th inch brown to maroon lesions oftentimes with a yellow halo.  Lesions can coalesce and form larger lesions in situations where the disease becomes severe.  Heavily infected leaves will turn yellow and generally senesce prematurely.  In a few situations over the past several weeks consultants have been concerned that irrigation has caused the disease to get worse and increase the leaf shed associated with brown spot.  Irrigation on particularly hot days could add additional stress to the soybean plant and result in the plant shedding brown spot infected leaves.  However, the irrigation did not result in the disease since the brown spot was already present in the lower canopy.  Rarely will brown spot occur in the upper part of the plant canopy.

Severe aerial web blight can defoliate the lower canopy and remove flowers as well as young pods from infected plants prior to R5.7.

Aerial web blight

Aerial blight can be regularly observed on an annual basis, typically at extremely low levels in some fields.  Currently, aerial blight is most severe in small fields with limited air flow south of I-20.  Aerial blight can be an extremely damaging disease if not managed properly.  Symptoms should be observed early in the morning when dew is still present on leaves since the fungal “webbing” can be more readily observed.  As the sun dries leaf tissue the lesions present on leaves will be more difficult to properly diagnose and can easily be confused with other foliar diseases such as FLS and Septoria brown spot.  Leaves with excessive water-soaking and when matted together in the middle to lower canopy make diagnosing aerial blight easy.

Managing aerial blight starts by carefully scouting fields.  All soybean varieties are susceptible to aerial blight and remain susceptible to yield loss until approximately R5.7 when soybean pods become woody.  Aerial blight can remove flowers and young pods from the plant as well as severely defoliate the lower canopy.  Fungicides are economically beneficial in reducing the yield loss associated with the disease.  Fungicides that contain a strobilurin (QoI) fungicide as either a stand-alone component or in a pre-mix are the products of choice for managing aerial blight.  However, fungicide applications should be made by ground applicators in a minimum of 15 gallons of water/acre (20 gallons would be preferable).  Aerial applications will not provide enough fungicide to the lower canopy.

For more information regarding aerial blight see:

http://www.mississippi-crops.com/2011/07/22/aerial-web-blight-of-soybean/

Target spot of soybean can produce symptoms with subtle concentric rings as well as yellow margins around the lesions.

Target spot

In some field situations target spot has been observed along with Septoria brown spot.  Target spot is generally a low to mid-canopy disease.  The symptoms associated with target spot include lesions that produce concentric rings oftentimes with a yellow halo.  Lesions range in size from 1/8^th of an inch to almost a ½ inch when at their most mature.  As soybean plants mature through the R5 growth stages, target spot can become more observable in some soybean fields.

Cercospora blight

Over the past week I have observed increased incidence of Cercospora leaf blight (late-season Cercospora), mostly in the western part of the state.  Generally speaking, Cercospora blight is observable in soybean fields that have matured past the early R5 growth stages.  Symptoms generally present themselves as bronzing of leaves, petioles and the main stem.  Leaves will oftentimes appear lightly bronzed, rolled at the end, flipped over so the bottom of the leaf is showing, and in most cases the affected plants will appear to be drought stressed due to the infection and how the leaf tissue presents itself.  Severe Cercospora blight can turn leaves almost completely purple as well as making the leaves develop a burned appearance with the development of ashen gray edges and leaf tips.  Once the disease is observed, making a fungicide application is likely not beneficial in terms of yield loss reduction or overall disease reduction.  Moreover, based on information from Dr. Trey Price at the LSU AgCenter, the likelihood of the fungus that causes Cercospora blight having developed resistance to the strobilurin fungicides is quite high.

Soybean vein necrosis virus. Mild symptoms typically develop along a vein and can easily be confused with other diseases as well as such things as herbicide injury.

Soybean vein necrosis virus

Symptoms associated with Soybean vein necrosis virus include a light mottling or chlorosis along veins of infected leaves.  However, the symptoms associated with the disease can be from mild to moderate on individual leaves and in some field situations the disease can be difficult to tell apart from other diseases as well as issues such as herbicide injury.  Insects vector the virus to soybean plants.  However, once the virus is observed on soybean plants the damage is done and an insecticide application will not limit the disease in the field.  Since the virus was first observed in the U.S., sometime around 2009, several extensive research projects have attempted to determine the potential yield loss associated with the disease.  At present, the potential yield loss associated with the virus has not been determined.