Soybean Update and Additional Management Considerations

Trent Irby, Extension Soybean Specialist
By Trent Irby, Extension Soybean Specialist and Tom Allen, Extension Plant Pathologist August 3, 2012 16:08

Soybean Update and Additional Management Considerations

To date, the majority of Mississippi’s soybean crop is either blooming, setting pods, or turning color in places. Some isolated fields in parts of Bolivar, Issaquena, and Washington counties have been harvested over the past 10 days. Since July 4, rainfall amounts have ranged from 2 to 10 inches depending on location within the state. The rain certainly provided many benefits to our soybean crop. However, irrigation is back in full force and some dryland fields need a rain soon to help during pod fill.

Numerous calls have come in regarding what has been diagnosed at the field level as aerial web blight. However, in most cases, the symptoms in the field are being confused with other abiotic variables. Stressed soybean plants, in situations where either too much water (irrigation followed by heavy rainfall) or drought stressed soybean followed by irrigation can be observed to have a water-soaked set of trifoliate leaves in the upper plant canopy. In rare cases, leaves can be stuck together as if the symptoms on the leaf were produced by aerial web blight. In addition, in situations where the dectes stem borer has been present, symptoms that mimic stress or aerial web blight can be observed across a field.

Aerial web blight is caused by the fungus Rhizoctonia solani. R. solani inoculum can overwinter in the soil or on stubble left in the field between seasons. Fields in continuous soybean or in rotation with rice (another host but the disease is sheath blight) can lead to a build-up of the inoculum in the soil. The disease can completely defoliate the lower and middle canopy of severely infected fields. However, unless conducive environmental conditions occur for an extended period of time (specifically high humidity and overcast skies) and an excessive level of infected plant material is present, rarely will leaves become affected by the disease in the upper plant canopy.

Presently (August 3, 2012), aerial web blight has only been observed in isolated cases in the Hills in smaller (less than 50 acres) soybean fields surrounded on at least 3 sides with trees to limit the air flow. The warm temperatures coupled with the showers that we have received lately present an opportunity for development and spread of aerial web blight. With warm temperatures, high humidity, and chances of rain in the weather forecast, it is possible that this disease will flare up in other places. To date, fields with aerial web blight have not been observed in the Delta.

Symptoms of aerial web blight are unique. Symptoms, in the form of water soaked lesions, can be observed on pods, stems, or leaves of infected plants. The photo shows the typical water soaked lesions on the leaf and the mycelium (similar to a spider web) that can sometimes be observed when a group of leaves is held together. As the symptoms advance, the water-soaked leaves will become necrotic and die.  However, at different times of the day (especially after the dew has burned off) lesions on leaves will appear similar to the lesions produced when frogeye is present. In addition to the observable symptoms, in some cases the smell of rotting plant material can be detected in fields with heavy aerial blight.

One of the most frequently asked questions we have received is “at what growth stage will aerial web blight no longer threaten the soybean crop?” To answer this, proper identification of the growth stage is necessary. Generally speaking, soybean susceptibility to aerial web blight is greatest before pods have hardened and become “woody”. This typically occurs at or a little beyond R5.5. In addition, defoliation of the lower canopy after R6 will have less influence on soybean yield. With that said, the decision of whether or not a fungicide application could prove beneficial depends on the soybean growth stage at the time of disease development.

The majority of Mississippi’s early planted crop is beyond R6. This means that it is possible that this portion of the crop can finish the season without too many concerns of damage from aerial web blight or many other foliar diseases. The later planted crop, particularly the double-crop “wheat beans”, range from R1 to R5. This portion of the crop should be monitored closely for disease development, especially if scattered showers and high humidity persist.

Management Options

If a fungicide application is deemed to be necessary in instances where aerial web blight is positively identified and confirmed, it is recommended that application volume be 15 to 20 GPA at a high pressure (> 60 psi) to allow the fungicide to penetrate into the lower canopy. Aerial application is not the best option for fungicide application in situations where aerial web blight is observed to be a problem.  However, if aerial application is the only option, then a minimum of 7.5 GPA of water will provide the best control of the fungus.  At present, fungicides that contain a strobilurin are suggested for management of aerial web blight. During 2010 and 2011 fungicide trials were conducted in the eastern part of MS (Noxubee County during 2010 and Clay County during 2011) in fields with severe aerial blight.  Information regarding the 2010 trials has already been posted on the blog and is available at:

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

The second set of fungicide trials, conducted during 2011 are attached in this blog posting.  In both cases, essentially a fungicide product that contained a strobilurin provided an economic return.  Remember, aerial web blight can be a devastating disease.  In addition, based on the recently reported situation in Louisiana if you apply a fungicide that contains a strobilurin compound and believe you have a failure to the fungicide please call so that we can get a sample of the fungus from the field for strobilurin-resistance analysis.

2011 aerial web blight soybean fungicide data

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Trent Irby, Extension Soybean Specialist
By Trent Irby, Extension Soybean Specialist and Tom Allen, Extension Plant Pathologist August 3, 2012 16:08
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