Soybean Disease Update: August 1, 2014

Numerous soybean diseases in addition to frogeye leaf spot continue to be observed throughout the MS soybean production system.  However, as of this morning (8/1/2014) soybean rust has NOT been observed in MS.

Cercospora blight

Over the past week I’ve observed light Cercospora blight (late-season Cercospora) in a few scattered fields throughout the Delta.  Fields with yellowing leaves or leaves that take on a purple discoloration are typical of the symptoms encountered.  Leaves with a leathery appearance or a thickened leathery feel are also quite common for fields with general symptoms of Cercospora blight.  Fungicide applications once symptoms manifest themselves in the field are likely not beneficial since the symptoms observed are likely more indicative of the toxin produced by the fungus that has been reported to be photosensitive.  Once the “disease” is observed applying a fungicide will likely not arrest further development.  Preventive applications that are generally made at R3/R4 likely provide a greater benefit in reducing yield losses attributed to Cercospora blight.  In general, Cercospora blight affects every field of soybean to one degree or another.  Moreover, information from Louisiana suggests that the fungus that causes Cercospora blight, Cercospora kikuchii, is resistant to the strobilurin class of fungicides as well as thiophanate-methyl, which has been used as a tank mix combination throughout the LA soybean production system over the years to manage Cercospora blight.

Aerial web blight as observed early in the morning when exhibiting the white fungal mycelium.

Aerial web blight

I’m still receiving a lot of telephone calls regarding aerial web blight in the Delta.  In the years I’ve been in MS I’ve only ever observed aerial blight one time in a small corner of a field.  Based on the images present in most of the diagnostic guides there are a large number of diseases as well as other maladies that can closely resemble aerial web blight.  When scouting soybean fields in the morning look for heavily water-soaked leaves that will contain white fungal growth between leaves, generally in the low to mid-canopy.  The hardest way to observe/diagnose the disease is waiting until the afternoon when dew is no longer present since the fungal growth will typically dry up and become difficult to observe.  In some rare cases aerial blight can favor frogeye leaf spot.  However, when aerial web blight favors frogeye leaf spot the frogeye lesions have likely coalesced and don’t resemble the typical frogeye lesions.  Aerial web blight will generally infect small fields with tree lines on three sides.  Over the years aerial web blight has been more readily observed in east and south MS.  The environment in south MS has been conducive for additional aerial blight development over the past month.  Some fields of late-planted, double-crop soybean following wheat have been observed to contain heavy aerial web blight even in situations where the canopy has not yet closed.  Fungicide products that contain a strobilurin active ingredient are excellent products to reduce the yield loss potential associated with aerial blight.  In general, fields where aerial blight is observed that are beyond R5.7 will likely not be injured by the fungus since the pods on the plant have become woodier and won’t be removed by a severe outbreak of aerial web blight.  Keep in mind, aerial web blight can remove leaves, flowers, and young pods from developing soybean plants.  Fungicide applications with products that contain a strobilurin fungicide will prevent the disease from progression.  However, when making a fungicide application to young soybean plants remember that in some cases if the environment continues to remain conducive for disease development that a second fungicide application may be necessary late in the growing season.

Brick red fungal structures (perithecia) associated with red crown rot.

Red crown rot

Red crown rot has been observed in east MS over the past several weeks.  Even though red crown rot is present in MS, the disease has only been observed in a few counties in east MS prior this season.  Early this week, red crown rot was observed in a soybean field in southern Sunflower County.  As of today (8/1/2014) no additional red crown rot has been observed in the Delta.  However, based on the environment that we encountered last season red crown rot appears to prefer a cooler, wetter environment.  Red crown rot will generally produce foliar symptoms that resemble SDS whereby the interveinal chlorosis will produce green veins and chlorotic to necrotic tissue between the veins.  Look at the base of the stem along the soil line for the characteristic brick red fungal structures (perithecia).  Areas affected by red crown rot can range in size from small (4 to 6 feet in diameter) to the size of a pick-up truck depending on the infestation of the fungus in the soil.  The disease is caused by a soilborne fungus that has been transported between fields in east MS on earth moving equipment as well as farm equipment.  Rotation is the best method to reduce the yield loss attributed to red crown rot so in subsequent seasons plant corn or cotton as they are not affected by the fungus.  However, peanut can be infected by the fungus so do not plant peanut in a field where red crown rot has been documented.

Frogeye leaf spot

Frogeye leaf spot continues to be a major concern throughout MS.  Fields of susceptible varieties in some areas of the state are exhibiting severe levels of infection, even in situations where a fungicide has been applied.  Keep in mind that in some cases a fungicide will only slow the progression of the disease.  However, based on observations from the production area this year as well as leaf samples collected during the 2013 season, if frogeye leaf spot is observed on a susceptible variety consider the fungus to be resistant to the strobilurin fungicides.  At present, 49 counties have been confirmed to contain strobilurin-resistant frogeye leaf spot.  Therefore, choose a fungicide product for an R3/R4 preventive fungicide application that contains two modes of action rather than applying a stand-alone strobilurin fungicide in situations where frogeye leaf spot susceptible soybean varieties are planted.  In addition, in cases where a fungicide has been applied and appears to get worse following the fungicide application remember that the fungicide prevents additional yield loss as a result of the disease and will not completely stop the disease.  For more information regarding the severity of frogeye leaf spot following a fungicide application refer to: https://www.mississippi-crops.com/2014/07/12/managing-frogeye-leaf-spot-with-fungicides-2013-ms-trial-data/ .

Septoria brown spot. Note yellow halo around lesions.

Septoria brown spot

Brown spot is generally a soybean disease observed in the mid to lower canopy.  However, in some rare instances brown spot can move into the upper canopy.  In the two instances I’ve observed over the years the two fields were substantially stressed from either too much rainfall (in 2014) or not enough irrigation (in 2010).  One field I scouted last week had lost a significant part of the upper canopy to brown spot as well as the lower canopy.  The field had advanced into the mid-R5 growth stages and would greatly benefit from a fungicide application in an attempt to keep enough leaves in the upper canopy to finish the season.  In the majority of cases brown spot will remain in the lower canopy.  A fungicide application in those situations will likely not reach the low to mid-canopy where the disease is present.  The most important thing to consider when deciding on whether or not a fungicide application may be beneficial is the general location in the plant canopy where the disease is present.  Fungicide applications made by aerial application may not reach the middle of the canopy.  In addition, a second fungicide application if one was already made at R3/R4 to attempt to reduce leaf shed in the lower canopy as a result of brown spot will likely not be economically beneficial.  Symptoms associated with Septoria brown spot will include reddish-brown lesions surrounded with a bright yellow halo.  Don’t confuse brown spot lesions with those associated with target spot.  In cases where target spot is observed the most mature lesions will generally have the appearance of concentric rings in addition to a yellow halo around the target-shaped lesion.

Sudden death syndrome. The necrotic tissue in between the green leaf veins is one of the specific characteristics associated with the soilborne disease.

Sudden death syndrome (SDS)

Reports of SDS have once again been widespread this season.  SDS is a disease that can be more severe in situations where the field contains high concentrations of the soybean cyst nematode.  However, even though several other diseases appear similar to SDS, the symptoms associated with SDS will only be observed in light soil classes.  Fields exhibiting symptoms similar to SDS where heavier soils, normally containing a high clay content, are not caused by the species of Fusarium that causes SDS.  Severe triazole phytotoxicity, stem canker, red crown rot, Phytophthora root rot, root-knot nematode (generally present in sandy soils), the root rot disease (see below) can all produce a symptom similar to SDS.  One other fairly easy diagnostic characteristic of plants infected with SDS.  Grabbing the stem and extracting the entire plant with the root from the ground will generally not result in a tap root broken off below the soil line.  See the information below regarding the “general root rot” to tell the differences between SDS and the black root rot disease that has been so widespread this season.

General root rot

For the better part of the last seven years, consultants as well as MSU extension/research personnel have encountered a disease that has been referred to as the “mystery disease” as well as “black root rot”.  The causal organism of the disease to date has remained elusive even though numerous individuals have attempted to culture the organism from infected root material.  The general symptoms associated with this particular disease include veins on leaves in the upper most part of the plant canopy remaining green in color while the rest of the leaf turns either yellow, or an orangish brown, or develops a “leopard-type” appearance.  Plants pulled out of the soil (as opposed to removed with a shovel) generally results in a large part of the tap root being broken off below the soil surface and left in the soil.  In addition, a black fungal growth will be evident on the main part of the root system.  Roots can easily be broken off the plant as well since the majority of the root material has rotted and the severely infected roots appear to be “dry rotted”.  Cutting into the pith of the plant tissue will result in a dark brown stain on the pith tissue suggesting a major vascular disorder.

General soybean root rot disease as observed throughout MS. Note the yellow leaf discoloration in addition to the green veins.

At present, pathologists from the University of Arkansas have numerous plant samples originating from a field in Leflore County, MS and are currently attempting to recover the fungus responsible for the root rot.  In addition, I am actively involved in attempting to determine the general distribution of the disease in some of the fields exhibiting severe symptoms of the general root rot.  As soon as additional information becomes available stay tuned to future posts made on the Mississippi Crop Situation Blog.