Soybean Disease Update: June 18, 2017

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 18, 2017 19:16 Updated

Soybean Disease Update: June 18, 2017

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Up until the past 10 days, the questions regarding diseases in soybean had been rather quiet. However, following the extended periods of cooler than normal weather conditions as well as excessive rainfall in some parts of the state disease calls have increased.

Lesions on leaves that can oftentimes be easily confused with plant diseases oftentimes resemble target spot.

Herbicide leaf spot

I really cannot come up with another name for this particular situation. Following last season, I think a lot of people are on edge regarding the potential for target spot.  First and foremost, it is much too early in the season to observe target spot.  On Wednesday I visited one of the most mature soybean fields I have seen this season.  The soybean field was completely canopied and I would say soybean plants were between R5.2 and R5.5 just based on glancing at the pods.  I was not able to observe anything other than some root-associated diseases (see below) and Septoria brown spot (again, see below).

Lesions that develop in the upper-most canopy, or within an area of the canopy as would be the case a week or more following drift or some other herbicide-related incident are generally not associated with disease. Lesions that form on leaves and tend to be more yellow, without a defined yellow margin and cover either a) only a few leaves across a large area (or in a part of the field that borders a field of soybean or another crop that recently received a herbicide application) or b) numerous leaves throughout the entire field and likely on some additional plants within the field or on the field edge are the result of herbicide injury.

Frogeye leaf spot on the underside of the leaf produces distinctive fungal structures that appear as grey to black tufts of fungus.

Frogeye leaf spot

I looked at fungicide efficacy plots in Stoneville on Thursday morning. A frogeye leaf spot-susceptible variety was planted in the field.  I observed six lesions across a 10-acre field.  I have received a few calls regarding the observation of frogeye in commercial fields and was able to observe one frogeye lesion in a commercial soybean field on Wednesday afternoon.  I think the above statement suggests we have drifted in and out of a conducive environment for the past two weeks.  But, keep your eyes open for frogeye since we have limited information regarding the sensitivity of some of the newer commercial varieties planted in MS this season.

Frogeye lesions tend to have maroon margins, brown/tan/gray centers, and are not surrounded by a yellow halo. Observing the underside of the leaf can oftentimes aid in distinguishing frogeye from other diseases or injury from a herbicide.  Gray to black fungal structures can oftentimes be observed with the naked eye (see photos).  But a 20x hand lens can greatly aid diagnostically.

Septoria brown spot is a quite common disease in continuous soybean fields.

Septoria brown spot

This year in particular brown spot has appeared to be more aggressive than normal. Much of that can be attributed to cooler than normal temperatures.  However, brown spot is one of those diseases that occurs in almost all soybean fields and appears more problematic in continues soybean situations.  Brown spot can be observed from vegetative growth stages all the way through physiological maturity.  Symptoms associated with the disease tend to arise as small brown-to-maroon lesions on leaves that oftentimes develop yellow areas around the lesions or the entire leaf turns yellow.  In vegetative soybean plants, the youngest leaves are generally infected early in the season in field situations where continuous soybean is planted.  The disease is not an issue in vegetative growth stages, but can result in defoliation when conducive conditions occur for extended periods of time during late reproductive growth stages.

Fungicide applications can be warranted if substantial defoliation begins in the mid- to upper canopy during mid- to late reproductive growth stages. Fungicide applications for brown spot in vegetative soybean plants are likely not economical.

Mild interveinal chlorosis as well as black root tissue are commonly observed with taproot decline.

Taproot decline

As compared to last season, a greater incidence and severity of taproot decline have been observed in MS as well as adjacent states (AR, LA). So far this season I have observed taproot decline in six additional counties.  But, with that said, I do not think the disease is spreading to new areas.  I think the conditions during the spring were conducive for the development of the disease earlier this season.

Be mindful that taproot decline will oftentimes be first observed in the lower canopy and can easily be misidentified as brown spot (see above). Leaves in the lower canopy can present themselves as completely yellow.  The best method for scouting is to walk down one row and look one or two rows to either side.  When observed in a field, the plants infected will not result in the spread of the organism to non-infected plants in the same row.  Plants generally develop interveinal chlorosis (areas of yellow leaf tissue with green veins) and the taproot of infected plants oftentimes rots, but can be observed to contain black fungal structures on the outside of the root.  The infection from the fungus requires a long period of time to develop symptoms on infected plants.  While visiting with a farmer on Thursday, in a field that had a sizeable area with plants exhibiting taproot decline, I think he was most concerned the disease would spread to the rest of the field.  Even though I have observed more of the disease this season as compared to last season I have not observed what I would consider to be a severe case of taproot decline.

Presently, research trials are underway at numerous locations in the region to determine the benefit of seed treatment as well as in-furrow fungicide applications on reducing the severity of the disease in inoculated plot trials. However, with that in mind, I suspect we will need multiple years of trials at multiple locations before we are comfortable making statements regarding the management of the disease.  Additional research is being conducted to determine more about the fungus that causes the disease.  Species of Xylaria, the genus of the taproot decline fungus, are not commonly associated with annual plants and tend to be more of a problem in trees as a wood rotter.  Finding Xylaria in soybean fields is a seriously strange situation and one that will require a good bit of time to sort out before we know a tremendous amount about the disease.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 18, 2017 19:16 Updated
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