Soybean Disease Update July 20, 2018

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 20, 2018 21:52

Soybean Disease Update July 20, 2018

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In my short career at MSU I would say this is one of the cleanest soybean crops I have seen. A limited number of fields with disease have been observed.  To date, I have only received a single frogeye leaf spot call and have observed limited leaf spotting in my efficacy trial plots in Stoneville.  However, I think there are some important things to discuss at this point in the season.

Aerial web blight photo montage. Note how water-soaked leaves look. Later in the day leaves look “chewed” apart as in the photo on the far left.

Aerial web blight

Several calls over the past few weeks regarding the presence of aerial web blight. Of all the diseases we see in MS, this is one of the more difficult to diagnose in the field.  Scouting fields in the morning makes it easier to determine the presence of this particular disease.  In most situations, aerial web blight is a greater concern in narrow-row planted soybean fields and in situations where fields have trees on all four sides of the field.  Leaves infected by the aerial web blight fungus generally appear extremely water-soaked and early in the morning can be matted together.  The best time to scout for the presence of web blight is early in the morning when dew is still present.  Later in the day, when the dew is no longer present, is much more difficult to observe the presence of the disease as the symptoms will appear similar to several other diseases.

Management options for aerial web blight when multiple areas are infected with the fungus should be followed. Fungicide products that contain a strobilurin (quinone outside inhibitor, QoI) are the best options for the disease.  However, azoxystrobin and trifloxystrobin tend to be the best QoI options.  Make applications in a minimum of 15 gallons of water/A and as high a pressure via ground as possible.  Aerial applications are not suggested since canopy penetration with the product will be most beneficial and reduce the likelihood of a second application.

Phytophthora root rot photo montage. Foliar symptoms produce interveinal chlorosis, small patch of dead plants, shepherd’s crook appearance of some plants, and discolored vascular tissue.

Phytophthora root and stem rot

If I had to sum up this disease in one word, I would use to the word rare to describe the presence of this disease. In my career at MSU I have observed this disease personally five times and received one other set of photos that I diagnosed as Phytophthora root rot.  A specific set of environmental conditions are necessary for this disease to thrive.  In general, the main ingredient is too much water.  In the Delta, irrigation followed by a heavy rain has tended to be the main ingredient for Phytopthora root rot.  Moreover, this particular disease prefers a hot and wet environment.  In almost all of the situations where I was called regarding this particular disease the individual thought they had charcoal rot.  Charcoal rot prefers a hot and dry environment and in general is more of an issue on lighter soil classes.

Areas with Phythophthora root rot tend to occur in reproductively mature soybean fields. Plants will quickly yellow, lose their leaves and die almost overnight.  A slight shepherd’s crook can be observed in most plants and plants removed from the soil tend to have a canker that extends up the side of the plant.  In addition, splitting the plant longitudinally to observe the vascular tissue generally reveals a darkened vascular system as a result of the fungus that infects the plant.

Septoria brown spot as it appears in the mid-canopy.

Septoria brown spot

Probably the single most common soybean disease on an annual basis. In field situations with continuous soybean this disease is present from vegetative through reproductive growth stages.  During mid-reproductive growth stages this particular disease can be observed in the low-to-mid canopy.  Leaves tend to have brown lesions with stark yellow halos.  In situations where the lesions coalesce the leaves can develop a blighted appearance.  However, rarely does brown spot move into the upper canopy.  Most varieties sustain a normal level of brown spot in the middle canopy.  Moreover, as soybean plants mature and get closer to R6, the level of brown spot will increase throughout the canopy since the plant has shifted all nutrients to pod production.  Rarely is a fungicide application necessary and in most instances the product application would not be beneficial due to the general location of the disease in the canopy.

Target spot as observed in the middle canopy. Lesions are more commonly observed with concentric rings and a stark yellow halo.

Target spot

So far, limited target spot has been observed in MS soybean fields. But, the number of calls I have received have increased as the crop has matured.  Target spot is one of the most common soybean diseases in MS.  I doubt you can find a field of soybean that does not have some target spot in the lower-to-middle canopy.  Environmental conditions in the lower canopy as well as crop growth stage appear to dictate whether or not this particular disease is present and the level of disease within the canopy itself.  Lesions are commonly observed to develop a target-shape appearance.  However, if the disease moves up in the canopy in some cases lesions will not develop the common centric rings.

The environmental conditions that occurred during 2016 and 2017 were extremely conducive to the development of target spot. Rainfall at specific growth stages that results in the soybean plant staying wet in the middle canopy is likely the specific ingredient needed for target spot to explode.  Fungicides may seem like a good option to manage target spot.  However, a fungicide application will not make the disease disappear and in most cases do not prevent the disease from moving up within the canopy.  Fungicide efficacy trials conducted during 2017 confirmed that some R3 fungicide applications reduced defoliation that resulted from target spot, but defoliation was still observed regardless of product applied.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 20, 2018 21:52
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