Soybean Disease Update: July 1, 2017

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 1, 2017 11:13

Soybean Disease Update: July 1, 2017

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Frogeye leaf spot

Frogeye leaf spot

To date I have received no calls regarding frogeye leaf spot. With the milder temperatures and rainfall received to date keep an eye on fields for the presence of frogeye.  Be mindful that strobilurin (or quinone outside inhibitor (QoI) fungicides) are no longer effective at managing this fungus.  Annually I receive calls from individuals that have applied a stand-alone QoI and are disappointed that frogeye continues to increase.  Based on the number of efficacy trials conducted in Stoneville over the past four seasons to manage frogeye I can safely say that a stand-alone QoI will perform just as well as a tank of water in a susceptible soybean variety.

Soybean rust

The first soybean rust of the season was observed on kudzu in Wilkinson County, MS on Tuesday, June 27. Five to six infected leaves with extremely low numbers of pustules (less than 6 per leaf) were observed.  Historically, this is a month later than 2016.  However, I think the observation this season is on average based on the first observed soybean rust in most years.

Moving forward scouting of soybean and kudzu will continue throughout the state as normal. Sentinel plots are still present in the southern part of the state in key locations.  To date, soybean rust has not been reported on soybean in any of the states reporting active infection (AL, FL, GA, LS, and MS).  To stay up-to-date with soybean rust observations, refer to the maps available at:

http://sbrusa.net/cgi-bin/sbr/public.cgi

Interveinal chlorosis can be associated with several plant diseases. The chlorosis associated with taproot decline as observed on the left compared to that associated with sudden death syndrome on the right.

Taproot decline

Lots of calls over the past several weeks regarding root rot or plants that appear to have sudden death syndrome (see below). Taproot decline has become a commonly observed disease across MS.  Several other states in our region have reported observing more severe taproot decline this year as compared to year’s past.  Mild interveinal chlorosis with limited necrosis (see below and photo) are common with taproot decline.  Early in the season, prior to flowering or shortly before R3, leaves exhibiting interveinal chlorosis are present on infected plants in the middle canopy.  However, once plants reach R5 and in most cases R6, observing the plants is much easier as they oftentimes become visible in the upper canopy with expression of chlorosis in the uppermost leaf tissue.  Plants removed from the soil profile can be observed to have blackened roots and in most cases the taproot breaks off below the soil line.  Research is currently underway to determine the best management practices to address the disease in our production system.

Sudden death syndrome generally occurs in small groups of plants in MS soybean.

Sudden death syndrome

In general, sudden death syndrome, or SDS, is a rare occurrence. However, this season in particular, I have received several calls about active SDS.  I suspect a cooler, wetter spring has a lot to do with the appearance of SDS.  Be mindful that SDS generally occurs in lighter soil classes (sandy loams, silt loams) in MS.  Typically, SDS is observed in small areas, the size of a truck, and is not a disease that occurs across an entire field.  However, in field situations with large areas of SDS, several acres in size, can be associated with the soybean cyst nematode (SCN).  Soil sampling is one of the best ways to determine of SCN is present since observing the cysts on roots can be complicated.  Soil samples should be collected for an area of the field where plants look “healthy” as well as an area of the field where plants are exhibiting symptoms of SDS.  Submit samples to a diagnostic laboratory for analysis and to aid in decisions for the 2018 season.

SDS produces interveinal chlorosis as well as necrosis between leaf veins. However, the specific foliar symptoms can be confused with other diseases as well as a severe root-knot nematode infestation.  Always observe roots to verify whether nematodes are present.  Cutting into the vascular tissue generally reveals mild vascular staining.  In general, roots on SDS-infected plants stay intact when removed from the soil profile, unlike those affected by taproot decline.

Soybean target spot is one of the most common foliar diseases in the MS soybean production system. The general appearance of target spot in the lower canopy produces mildly concentric lesions with bright yellow halos.

Target spot

Over the past week I have continued to receive questions about target spot observed in soybean. Target spot is one of the most common diseases in the MS soybean production system.  Annually, target spot is observed in the lower-to-middle canopy once the canopy closes.  In extremely rare situations, target spot can move up the canopy and result in defoliation.  However, those rare situations are almost always associated with an environment that remains conducive for the disease for extended periods of time.  In the 10 years I have been in MS I can think of two years where I observed severe target spot: 2009 in some isolated geographies and 2016.  Both of those years had one major thing in common……..EXCESSIVE rainfall.  I do not think we have had the correct environment for target spot to be near the problem it was in 2016.  But, with that in mind, environment is more than just rainfall.

The panic and frenzy surrounding target spot is difficult to understand. Target spot is almost always present in any soybean field.  Prior to 2016, target spot in soybean was not even on the radar.  In fact, I do not remember being asked many questions about target spot and was not asked to make a presentation on that particular disease.  The occurrence of severe target spot during 2016 should not be considered to be an annual occurrence.  I have already observed target spot in some fields, but have also received numerous telephone calls regarding the presence of target spot and they almost always start with “here we go again”.  Do not panic if target spot is present…..it always is.  The occurrence of target spot in 2016 was target spot on steroids as a result of an extremely conducive environment at specific growth stages.  As of right now, I do not see target spot being the same issue during 2017.  If that changes, we can reassess at a later date.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist July 1, 2017 11:13
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