Foliar Soybean Disease Update: September 3, 2014

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist September 3, 2014 22:04

Foliar Soybean Disease Update: September 3, 2014

Aerial blight

Throughout MS, low levels of active aerial blight can still be observed in numerous soybean fields.  Generally speaking, aerial web blight is more of a concern when soybean plants have not reached approximately R5.7.  Once soybean pods have matured to R5.7, the woody tissue is protected from the fungus that causes aerial blight.  Over the past several years I have continued to receive calls regarding the presence of aerial blight in the Delta.  Numerous diseases can produce symptoms similar to aerial blight, and in fact I observed several fields in south MS this season that appeared to have a heavy infection of frogeye leaf spot.  But on closer inspection the lesions on leaves were the result of aerial web blight infection that had dried once the dew had lifted.  The best way to diagnose aerial blight is to spend time in soybean fields in the early morning.  Parting the main canopy back and scouting for leaves that are stuck together, have heavy water-soaked lesions, or the fungal “webbing” between leaves with infection is the best way to confirm the presence of the disease.  Scouting for web blight in the afternoon hours is extremely difficult since numerous issues can be confused with the symptoms of web blight.  Aerial blight is one of those diseases where the pictures in the numerous diagnostic guides available for purchase don’t do the disease justice.

Severe Cercospora blight (late-season Cercospora).

Severe Cercospora blight (late-season Cercospora).

Cercospora blight

Cercospora blight is likely one of the most economically damaging diseases of soybean in MS.  On an annual basis, as the soybean crop approaches physiological maturity Cercospora blight can be observed in almost every soybean field.  The general symptoms associated with Cercospora blight will initially appear as a slight bronzing or yellowing of leaves in the upper plant canopy.  As the disease progresses, and soybean plants continue to mature, symptoms can be observed on pods, petioles, and stems.  A purpling or blackening of pods and petioles is associated with heavy Cercospora blight infection.  In the most advanced stages of the disease leaves will curl up, appear to be dried out, and purple lesions will develop a gray, “frosted” appearance.  Fungicide applications likely provide some protection from Cercospora blight; however, based on data from Louisiana, the fungus that causes Cercospora blight (Cercospora kikuchii) is more than likely resistant to the strobilurin class of fungicides.  Moreover, making a fungicide application specifically for Cercospora blight once the disease is observed is not a beneficial timing to prevent potential yield loss associated with the disease.  Keep in mind that the majority of the soybean varieties planted in MS are considered to be susceptible to Cercospora blight.

DSC_0909Frogeye leaf spot

Frogeye continues to remain a hot topic.  As of Friday (August 29, 2014), frogeye had been observed in 71 of 82 counties in MS.  The level of infection ranged from light (a single lesion on some leaves) to heavy (the majority of a leaf covered with lesions).  The majority of the frogeye observed has been in early planted soybean fields.  However, over the past two weeks I have encountered frogeye on soybean plants that were still in vegetative growth stages in field situations where soybean was planted following a wheat crop.  Fungicides are an economically beneficial way to manage (reduce) the yield loss attributed to frogeye leaf spot infection.  However, this season in particular, fungicides have appeared to not hold on the disease when applied in the presence of frogeye.  In general, fungicides work more effectively when applied in the absence of disease.  Even in situations this season when fungicides were applied at R3, frogeye leaf spot was likely already present and the fungicide application did not appear to be effective at reducing the level of disease.  Suffice to say, plenty of fungicide trial data will be available in the fall as I continue to rate fungicide trial plots in Stoneville and Starkville.  In addition, for specific information regarding the response of soybean varieties to frogeye leaf spot and how some fungicides responded last year refer to:

http://www.mississippi-crops.com/2014/07/20/foliar-soybean-disease-update-july-20-2014/

http://www.mississippi-crops.com/2014/07/12/managing-frogeye-leaf-spot-with-fungicides-2013-ms-trial-data/

When deciding on a fungicide application keep in mind that knowing the variety planted is extremely important.  In addition, keep in mind that some fungicides will not perform as they have in the past due to the presence of fungicide resistance.  Strobilurin-resistant frogeye leaf spot remains a new topic in our production system.  A graduate student (Mr. Jeff Standish) continues to receive infected leaf samples and conduct assays to determine if the fungus present is resistant to the strobilurin class of fungicides.  At present, strobilurin-resistant frogeye leaf spot has been positively confirmed in 62 counties since last season.  For the most current map regarding the presence of strobilurin-resistant frogeye leaf spot refer to:

http://www.mississippi-crops.com/disease-monitoring/

Soybean rust

To date (September 1, 2014), soybean rust had not been observed in MS.  Clearly, the much colder than normal winter we experienced during 2013/2014 reduced the level of inoculum necessary to initiate an early infection.  Based on the environment we have observed much of the 2014 season I can safely claim I think we dodged a serious bullet.  Stay tuned to the Mississippi Crop Situation Blog as well as the information contained on www.sbrusa.net for up-to-date information regarding the whereabouts of soybean rust.

Pod diseases

Several pod diseases have been observed over the past several weeks.  Anthracnose and Cercospora blight are the two most commonly encountered in our production system this season.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist September 3, 2014 22:04
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