Aerial Web Blight of Soybean

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 22, 2011 17:15

Aerial Web Blight of Soybean

Aerial web blight of soybean. Symptoms with observable fungal growth are typically present when dew is still on leaves in the early morning.

Soybean scouting continues throughout MS for foliar diseases.  Over the past week to ten days, aerial web blight has been identified in several fields in eastern and northeastern MS.  However, aerial blight has not been identified in the Delta and seems to only be present in the Hills at this time.  Last year was a bad year for web blight and it appears this year could be the same.  Typically, aerial web blight prefers hot dry weather with intermittent rain showers.  A field with trees around the edges is more likely to become infected first since this limits air movement and increases the chance of shaded areas where the fungus can more easily develop.  All soybean varieties should be considered to be susceptible to this particular disease.

Symptoms of the disease are most recognizable when the plant canopy is parted back.  Web blight is not one of those diseases that can be observed through the windshield of the truck.  Infection is most likely in parts of a field that receive shade during the day time and in areas of the state that have received more frequent rainfall.  Normally the disease is more of an issue in a rice-soybean rotation but last year in particular it was more commonly observed in narrow-row soybean fields planted throughout the eastern part of the state. 

Symptoms of the disease can appear on pods, stems, and leaves of infected plants.  When scouted early in the morning, when dew is still present, leaves can become matted together and appear with water-soaked areas across the entire leaf (see above photo).  Some individuals have called in the past and mentioned they saw “spider webs on the plants.”  The white material is the fungus growing between infected leaves.  When dew burns off, during the late morning or early afternoon, lesions on infected plant material can appear as reddish-brown spots (see photo below).     

Symptoms of aerial web blight when dew is no longer present on leaves.

Aerial web blight can be a problem until the pods harden and become woodier.  Typically this occurs around R6 so the most important stages to protect the plant are from flowering (R1) and pod development through R6 then aerial web blight is not near the yield reducing threat. 

In 2010, two fungicide trials were conducted in a soybean field in Noxubee County with what was considered to be continuous web blight across the entire field.  Trials were conducted in conjunction with Southern Ag and rated by Billy Moore and myself.  Fungicides were applied at approximately R5 in both trials and a second application was made at R5.5 (approximately 2 weeks later) in the second trial.  Plots were rated at 14 and 28 days after application by Billy Moore and myself by making 10 ratings from each plot recorded by a third person.  Disease observations are based on the average of those ratings.  Yield was determined at the end of the season by harvesting the center two rows of each plot.  Two different trials were conducted in the same field to rate single applications as well as two applications.  Keep in mind, differences between plots with regards to products and ratings were observable as is evident in the data.  As a general note, in the past, pyraclostrobin (Headline) was not a product that was typically suggested for the management of aerial web blight.  In addition, based on the data from 2010 a fungicide application, regardless of product was economical as late as R5 when aerial web blight was the disease present.  Products identified as “experimental” are not available on the market at this time but are included for comparison sake.

In addition to the products contained within the tables there are additional fungicide products labeled for use on soybean to manage aerial web blight.  However, keep in mind, last season in particular there was talk of some products being applied that were not labeled for aerial web blight.  Cercospora blight is not the same disease and those products that just contain a triazole should not be considered to be effective at managing aerial web blight.  Carefully read each fungicide label when it comes to the specific suggestions on the label for specific diseases as well as the mixing of a non-ionic surfactant or COC for fungicide applications. 

Application suggestions are as follows:

1) Strobilurin fungicides (or a product that contains a strobilurin) have proven to be the best products since they prevent the movement of the fungus to plant material that has not been infected.  One would perceive that curative activity would not occur following a fungicide application with a strobilurin chemistry.

2) Ground application will provide the best plant coverage.  Aerial application in the Hills would be difficult since fields are generally smaller and in the majority of the field situations where the disease has been identified there are trees present on all sides.

3) Application of the fungicide in 20 gallons of water per acre would be the most beneficial since this would increase the likelihood of spray coverage.  I realize that increased application volume can complicate matters so a minimum of 15 gpa would suffice when making a ground-based fungicide application in response to aerial web blight.

4) If flowers are being knocked off the plants in a soybean field throughout a field due to aerial web blight a fungicide application is warranted.  More simply, if the field is at R1 and you’re losing flowers already, make a fungicide application.

5) Increased pressure in the ground rig will help get more product where the fungicide is needed.  Aerial web blight will start in the lower canopy since the fungus is soilborne and move up the plant.  Pressure somewhere on the order of 60-70 psi is more preferred to help get the product deep into the canopy.  Slow your equipment down to provide maximum fungicide coverage.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 22, 2011 17:15
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