Soybean Frogeye Leaf Spot: Management Decisions for the Disease Following an R3/R4 Strobilurin Application

Frogeye leaf spot of soybean.

Over the past two weeks the number of telephone calls regarding increased frogeye leaf spot in susceptible MG IV soybean varieties following a strobilurin application at advanced reproductive growth stages (> R5) have increased dramatically.  At this point, I am not going to say that we have widespread fungicide resistance in Mississippi.  I have observed many of the fields in question over the past week and can make a few general statements:

-susceptible MG IV varieties following a strobilurin application (4 or 6 oz @ R3/R4 of a stand-alone strobilurin; 4 oz @ R3 fb 4 oz @ R5 of a stand-alone strobilurin; or full label rate of a pre-mix application) all continued to have observable symptoms of frogeye leaf spot

-in most cases new growth (added nodes) following the fungicide application had more infection than leaf tissue present in the lower canopy at the time of the first fungicide application

-irrigated fields, regardless of irrigation method, appeared to have more frogeye than dryland fields, likely due to increased humidity

Severe frogeye leaf spot that has reached the leaf blighting phase (Photo courtesy of M. Newman – UT).

If you are dealing with soybean fields with an increasing amount of frogeye leaf spot following a fungicide application please call.  A graduate student in Starkville, MS is working on fungicide resistance within the greater soybean leaf spot complex and we would like to collect samples from as many fields as possible.  The project was sponsored by the Mississippi Soybean Promotion Board.  To date numerous leaf samples have been collected from throughout the state.

Fungicide effectiveness

With the recent observation of strobilurin-resistant frogeye leaf spot in MS and adjacent states I think there are some that would consider the above mentioned situations as “fungicide resistance”.  However, I’m not in a position to make that statement since in almost all of the cases a fungicide has been applied to a specific susceptible soybean variety.  But, keep in mind that fungicides work in several different ways.  First and foremost, a fungicide application won’t make lesions disappear on a leaf.  However, depending on the product, or product combination applied, the fungus that causes frogeye leaf spot (Cercospora sojina) should respond in a specific manner.  Essentially fungicides either prevent sporulation or they prevent the actual growth and development of the mycelium in the leaf tissue.  More specifically, a product that is considered to be “fungicidal” in its activity actually kills the fungus when it makes contact while a product that is considered to be “fungistatic” prevents the spread of the fungus.  The mode-of- action of the strobilurin fungicides would be considered to be fungistatic in their activity against frogeye.  Making an application with a stand-alone strobilurin would likely only prevent spread of the fungus to non-infected plant material that was present at the time of the fungicide application.  Thusly, new growth added onto the plant could become infected by the fungus and present symptoms of the disease.

Varieties and their specific response to frogeye leaf spot

The first, and one of the most important things to determine, is the particular variety planted in a field.  Varieties considered to be resistant or even moderately resistant (e.g., Asgrow 4632) should not have much if any frogeye leaf spot present (when rated and a single lesion presents itself in a plot with a resistant variety the score could be a 1=present) as opposed to a susceptible variety (e.g., Armor DK 4744).  But, with that said, I observed a field of what was termed a “moderately resistant” variety on Friday (7/26/13) with moderate levels of frogeye following a full label rate application of a pre-mix fungicide.  However, I think the pivot irrigation was contributing to disease spread and providing ample wet foliage for continued infection since a field of the same variety down the road with furrow irrigation had substantially less frogeye.  But, ratings for that particular variety from several sources appear to be confounding with regards to the tolerance to frogeye leaf spot.

Susceptible soybean varieties that received an R3/R4 fungicide application that contained a stand-alone strobilurin fungicide may in fact benefit from a second fungicide application.  However, if that is the case product choice will produce night and day differences with regards to the observable frogeye present on foliage.  But, the need for a second fungicide application should be based on 1) variety planted, 2) whether or not a fungicide has already been applied, 3) yield potential, and 4) growth stage.  Soybean plants at the R6 growth stage don’t lose much if any yield as a result of defoliation that might occur as a result of frogeye leaf spot.

For more information regarding the specific response of a variety on the MSU short list a table is provided of the specific disease reaction as assessed by the seed company as well as information provided from the Stoneville frogeye screening program conducted by Gabe Sciumbato using inoculated plots and information compiled from Melvin Newman’s screening program that uses natural inoculum.  Keep in mind that varieties may in fact respond differently across several environments so the scores contained in the table are based on the environment that occurred at that location (2013 soybean short list frogeye responses).

Fungicides labeled for frogeye leaf spot management

An increasing number of calls have been fielded regarding the need for a second foliar fungicide application following an R3/R4 application made with a stand-alone strobilurin compound.  Numerous compounds are labeled for frogeye leaf spot management (2013 soybean frogeye fungicides).  The attached table should provide some guidance regarding potential products labeled for frogeye leaf spot.  However, if you have already made an application with a product that contained a strobilurin (either stand-alone or as a pre-mix) rotating mode-of-action will help decrease the likelihood of resistance development.  Triazole fungicides, as stand-alone products (e.g., Domark, Tilt, Topguard) are labeled for frogeye leaf spot management.

By early this week I will include data from past fungicide trials where frogeye leaf spot was targeted with several modes-of-action to provide guidance if a second fungicide application is deemed necessary.

Reduced fungicide rates

If you have to make a fungicide application at R3/R4 to soybean apply the full labeled rate (e.g., 6 fl oz/a of Headline rather than 4 fl oz/a).  Over the past few weeks I’ve had an increasing number of telephone calls regarding cut rates of “newer” fungicide products so the application would consist of 4 oz of a strobilurin.  Pre-mix fungicides contain two modes-of-action.  By cutting the application rate of the product you are likely applying an extremely reduced rate of the curative compound than would be necessary to effectively manage the frogeye leaf spot fungus.

Labeled fungicides generally receive the approved rate structure based on years of research trials.  I realize that by reducing the applied rate the price of the product goes down.  However, even though that is likely a good advantage in the short term that type of application strategy increases the likelihood of fungicide resistance developing in C. sojina as well as potentially other, non-target organisms.  In the attached table of fungicides labeled for frogeye management the FRAC Codes (Fungicide Resistance Action Committee) are included.  The FRAC Codes present information regarding the specific chemical class of a fungicide and are also included on the fungicide label attached to the container.  Members of the FRAC Code 11 include the stand-alone strobilurin fungicides.  The development of resistance to members within FRAC Code 11 has been considered to be extremely high.  In addition, the fungus that causes frogeye leaf spot (C. sojina) and Cercospora blight (C. kikuchii) have both been identified as having developed resistance to strobilurin fungicides.  Therefore, use fungicides as a tool to prevent disease and attempt to reduce the likelihood of resistance developing by applying full rates and alternating modes-of –action when possible (strobilurin versus triazole).