General soybean disease update: July 21, 2012

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 21, 2012 15:38

General soybean disease update: July 21, 2012

The lull in soybean disease observations we experienced over the past month is now over.  Over the past two weeks I’ve observed numerous fields with light foliar disease to situations where moderate to severe foliar disease with severe defoliation in the lower canopy were occurring.  However, there are still some locations in the state that are reporting no disease pressure.

In addition to foliar disease symptoms there are some soilborne/root-based disease issues occurring in some scattered fields.  Keep in mind, symptoms in the upper canopy that may appear similar to Sudden death syndrome (SDS) are not always SDS.  Annually, symptoms of SDS or disease that can produce an SDS-like symptom are observed in our production area.  Generally speaking, SDS is more of an issue (I hesitate to say always here) on lighter soils.  Plants infected by the fungus that produces SDS will typically be scattered throughout a field in either small clumps of plants or the occasional random plant.  In extreme cases, and when high populations of the soybean cyst nematode are present, SDS can be observed in large areas within a particular field.  In those particular situations a soil sample should be submitted to the Nematode and Plant Disease Diagnostic Laboratory for analysis.

Current national soybean rust distribution as of July 21, 2012.

Soybean rust – Soybean fields in George County remain the only location where soybean rust has been observed in MS (as of 7/21/12).  However, the environment over the past 10-17 days, depending on geography, has become much more conducive for soybean rust development.  Cooler temperatures with high relative humidity and scattered showers are optimum for soybean rust development.  Soybean sentinel plots throughout the state as well as commercial soybean fields in advanced growth stages and kudzu patches with a known history of soybean rust continue to be scouted to determine the overall distribution of this potentially devastating disease.  In addition, as soybean rust is detected in more locations in AL and LA a greater inoculum potential will be present since the fungus that causes soybean rust can be spread over long distances on air currents.  Soybean fields that are now at or beyond R5.5 are generally considered to be safe from the disease especially since the disease requires several generations (weeks) to produce enough inoculum to heavily infect a single soybean field.  However, as we have a young crop of soybean planted in some areas of the state we will monitor for the disease and present information on the blog as well as at www.sbrusa.net and through the FREE tri-state (AR, LA, MS) telephone hotline (1-866-641-1847) sponsored by the Mississippi Soybean Promotion Board and BASF.

Frogeye leaf spot, note the most mature lesions with the gray center and some immature lesions scattered around the leaf.

Frogeye leaf spot – Over the past 4 to 5 years frogeye leaf spot has not typically been observed with much frequency in MS.  However, this year in particular I’ve observed numerous fields with either scattered plants expressing symptoms or in one particular case an entire field planted to a susceptible variety.  Frogeye can be distinguished from other foliar diseases by the characteristic appearance of the lesion center.  Typically, as the lesions mature the center will turn an “ash” gray color and will more often than not fall out of the lesion.

Samples from 3 fields have been collected this year and sent off to Dr. Carl Bradley at the University of Illinois for fungicide challenge studies.  The fungus that causes frogeye leaf spot was recently determined to be resistant to the strobilurin class of fungicides.  Diseased leaf samples from some fields in IL, KY, LA, MO, and TN have been determined to be resistant to the majority of the strobilurin active ingredients commercially available (e.g. azoxystrobin, pyraclostrobin, trifloxystrobin).  In cases where resistance may have developed or to prevent the likelihood of future resistance developing, in situations where the disease is observed and a fungicide is deemed a beneficial management alternative, triazole products (e.g. Alto, Domark, Topguard) should be applied.  As of today (7/21/2012) we still have isolates of the fungus that are susceptible to the strobilurin class of fungicides based on an analysis conducted on one of the samples that was submitted on July 18, 2012.  However, as additional fields are located that contain frogeye please give me a call so that samples can be sent to Dr. Bradley to determine if strobilurin-resistant Cercospora sojina present in MS.

Aerial web blight of soybean with the typical spider web appearance of the fungal growth on the leaf surface as well as the heavy characteristic water-soaking.

Aerial web blight – At least one field in the Delta and several fields in the Hills have been observed to have aerial blight; however, not to the extent that was observed during 2010 and 2011.  The continued afternoon showers and warm temperatures present perfect conditions for the further spread and development of aerial blight.  Aerial blight favors continued hot and moist conditions and normally will not stop progressing within soybean fields unless the daytime temperature drops.  Much like the name of the disease, the fungus that causes the disease (Rhizoctonia solani) produces a white, fluffy mycelium that will appear like spider webs growing on portions of the plant.  In addition, infected leaves in the lower canopy will appear heavily water-soaked and in some cases matted together.  Soybean plants are most susceptible to a yield reduction prior to the pod becoming woodier, which generally occurs at approximately R5.5.  Thusly, depending on the particular growth stage when the disease is encountered a fungicide can benefit yield.  However, fungicides will be most beneficial when applied by ground.  Aerial application can’t be expected to apply the product to the most important parts of the plant since the fungus is soilborne and will move up the plant.  If at all possible, apply fungicides (that contain a strobilurin) with high pressure (> 60 psi) and high water volume (>15 gal/A) to increase the likelihood of good plant coverage.  In situations where fungicides have been used extensively, particularly in a rice/soybean rotation situation, and if a potential fungicide failure is presumed to be an issue please call as I would like to collect a sample.  R. solani has developed resistance to the strobilurin fungicides and has been observed in both rice and soybean fields in central LA.  Presently (as of 7/21/2012), strobilurin-resistant R. solani has not been detected in MS.

Target leaf spot of soybean, note the more mature lesions with concentric rings.

Target leaf spot – Over the past few seasons the occurrence of target spot has continued to increase.  However, one of the more difficult things to diagnose is the presence of the disease when the most mature lesions are not observed.  The mature target spot lesions have brown lesions with concentric rings surrounded by a yellow halo.  But, the younger lesions of target spot are difficult to diagnose without the presence of the most mature lesions (see attached photo).  Target spot can result in defoliation occurring in the lower and middle canopy, especially when high humidity is present.  Over the past 10 days I’ve observed fields in the Delta with severe defoliation as a result of target spot that followed several inches of rain.  Most fields contain some target spot and based on my observations over the past 5 years this is a disease that is increasing in occurrence.  To my knowledge, data regarding the efficacy of a fungicide on target spot are not available in our production system.

Severe Cercospora blight with symptoms present on leaves and petioles. The image was taken in Lousiaian during the 2009 season.

Cercospora blight (late-season Cercospora) – Based on the number of fields I’ve observed over the past two weeks and with the level of Cercospora blight present I suspect this year will be just as bad as last season for this particular disease.  Fungicides aren’t providing much protection against Cercospora blight.  In fact, Louisiana recently reported resistance to azoxystrobin and thiophanate-methyl within the greater population of Cercospora kikuchii from LA.  The fungus that causes Cercospora blight can produce symptoms on pods, peduncles, petioles, stems, leaves, and also produces purple seed stain.  However, research into the fungus has not produced a correlation between the level of disease in a field and the presence of stained seed.

Symptoms of Cercospora blight can produce purpling of leaves, or leaves can feel thicker and more leathery than normal.  Leaves can also have a characteristic purple appearance and in some literature the disease is referred to as “purple leaf”.  Abnormally shaped lesions on pods that can be dark purple to black can occur on some varieties.  In severe cases of the disease defoliation will occur and petioles will be turned a dark purple color.  In LA, in fields with severe Cercospora blight, the fungus will produce heavy sporulation on the infected leaves that appears to have a gray, ash-like appearance (see photo).

Foliar symptoms that can be associated with southern blight can easily be confused with SDS or other diseases (or disorders) that produce a pattern of interveinal chlorosis (necrosis).

Southern blight – In the 5 years that I’ve been in MS I could have counted the number of plants I observed with southern blight on one hand prior to 2012.  For whatever reason, I’ve observed numerous fields with southern blight symptoms, both early in the season and this past week in fields that were in the R5 growth stages in the southern part of the state as well as the Delta.  As stated above, symptoms that appear similar to SDS can oftentimes be a result of southern blight (or any one of a number of other diseases that includes stem canker, Phytophthora root rot (which we don’t have much of in MS), SDS, nematodes (either reniform or root-knot), scald, stress, red crown rot (that has only been observed in Chickasaw and Noxubee counties), and in extremely rare cases charcoal rot).  Essentially anything that stresses a soybean plant’s root system can produce a foliar symptom similar to SDS.  Observing the base of the plant at the soil line or plants that have been removed from the soil is the most important diagnostic technique.  Plants suffering from southern blight will oftentimes have a heavy, thick white mycelial growth at the base of the plant and/or on the roots and in situations where the disease has been present for a longer period of time sclerotia can be observed on the plant’s stem or on leaves that have fallen to the ground.  The fungus that causes southern blight, Sclerotium rolfsii, can infect the plant and also produce a vascular staining that can be confused with SDS.

Characteristic signs of southern blight at the base of the plant where a heavy white mycelial matte are typically present and sclerotia can be produced.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 21, 2012 15:38
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