Management Practices to Reduce the Development of Fungicide Resistance in Soybean: Part I, Improving Disease Management Practices

Over the past several weeks I’ve had several conversations within the agricultural community regarding the management of frogeye leaf spot for the upcoming 2014 season.  I’ll try to cover some of my suggestions and a few of the misconceptions regarding fungicide resistance in this three part blog series.  The information to support the third part, regarding fungicide resistance in the frogeye leaf spot fungal population in MS, should be available in a few weeks.

Fungicides have become the most notable disease management alternative, especially in situations where frogeye leaf spot susceptible soybean varieties are planted continuously.  Over the past decade timed fungicide applications at the R3/R4 growth stages have become a common practice in the MS soybean production system.  In addition, prior to 2010 when the first strobilurin resistant isolates of the frogeye leaf spot fungus were detected, strobilurin fungicides were more regularly suggested as a management alternative for frogeye leaf spot when the disease was present.  However, with the increasing number of counties throughout the United States that have reported strobilurin-resistant frogeye leaf spot isolates the management of frogeye leaf spot will likely become more challenging in the future (C sojina map – 2010-13).

Disease management alternatives outside of a fungicide application are rarely discussed.  In addition, rarely do plant pathologists talk about some of the important issues related to foliar fungicide application practices.

More often than not decisions regarding disease management relies on a fungicide application.  Last year in particular I struggled with making sound management suggestions when the particular soybean variety was unknown.  This might seem like a crazy comment since susceptible varieties had quite a bit of frogeye leaf spot present on the leaves and the more tolerant varieties clearly didn’t have observable lesions present.

Reducing the likelihood of fungicide resistance developing on a specific farm or as a widespread occurrence will require a holistic management approach that doesn’t just include fungicide application choices or major application decisions.

Variety choice

Even as a plant pathologist I understand that choosing a variety is likely the most important decision made all year.  Several soybean varieties exist with outstanding tolerance to frogeye leaf spot.  For more specific information regarding this topic, the foliar disease ratings (Cercospora blight and frogeye leaf spot) that were conducted in the 2013 MSU Soybean Variety Trials can be located at

Maturity Group IV frogeye leaf spot ratings: https://www.mississippi-crops.com/2014/01/23/2013-soybean-variety-trial-disease-ratings-maturity-group-iv-frogeye-leaf-spot-and-cercospora-blight/

Maturity Group V frogeye leaf spot ratings: https://www.mississippi-crops.com/2014/01/26/2013-soybean-variety-trial-disease-ratings-maturity-group-v-frogeye-leaf-spot-and-cercospora-blight/

Northeast MS, Verona frogeye leaf spot ratings: https://www.mississippi-crops.com/2013/12/24/east-mississippi-soybean-variety-trial-report/

In my opinion, yield performance of a particular variety drives on-farm variety decisions.  Most frogeye leaf spot tolerant varieties do not perform as well as the more frogeye leaf spot susceptible varieties when it comes to yield.  However, in situations where frogeye leaf spot has been a historical problem, choosing a variety with tolerance to the disease is an added benefit.  Not only will a tolerant variety not likely require a fungicide application to prevent yield loss as a result of frogeye leaf spot, in the long run the yield benefit of a susceptible variety may be negated by reducing the likelihood of fungicide resistance from developing.

Fungicide application decisions

Besides product choice, there are several other important factors regarding fungicide applications that could potentially contribute to the development of fungicide resistance over time.  The specifics regarding rate determination will be outlined in Part II.  All of the fungicide decisions outlined in these two posts are important and are potential issues that in some instances could result in an increased risk of fungicide resistant developing in a field situation.

Application volume

By air

Coverage is one of the most important pitfalls to protect against the development of fungicide resistance.  Aerial applications are generally made in less than 5 gallons of water/acre throughout MS.  As a realist, I understand why less than 5 gallons are typically applied with fungicides since time is money and we’ve got a lot of acres to cover at one time.  However, 5 gallon applications should be a minimum rather than applying fungicides in 2 to 3 gallons of water/acre.  Increased water volume will increase plant leaf coverage and aid the fungicide in reducing foliar diseases from increasing.  Decreased water volume can expose a fungus, such as Cercospora sojina that causes frogeye leaf spot, to a reduced amount of active ingredient and increase the potential for fungicide resistance to occur regardless of the specific fungicide product chosen for application.

By ground

With some disease situations, more is better when it comes to application volume by ground.  For example, aerial web blight is one of the situations that come to mind.  We suggest that a minimum of 15 gallons of water/A should be applied to guarantee good coverage.  However, much like the comments above regarding leaf surface coverage and amount of area to be sprayed it can be difficult to increase water volume due to the acreage that may require treatment.

Reducing the drift potential

In our production system we’re more concerned with drift from herbicides than we are when making fungicide applications.  However, I have oftentimes observed agricultural pilots making applications of what could only be fungicides on a windy day (based on the time of year and general crop phenology….more often than not this is during the R3/R4 soybean timing as well as post-VT corn).  Height of application as well as direction of wind and wind intensity at the time of application can prevent the leaf coverage that is necessary for a fungicide to prevent yield loss as a result of a foliar disease.  Even though most fungicides are considered to be systemic in their activity; however, the overall systemic activity of a fungicide once on the leaf surface is limited.  Most fungicides are locally systemic at best whereby the fungicide may move from the top of the leaf through the leaf to the bottom (translaminar movement) or from the point of the droplet towards the tips of the leaf or towards the petiole (acropetal movement).  Reducing drift potential and increasing leaf surface coverage can reduce the likelihood of fungicide resistance from developing.

In the second part we’ll discuss fungicide selection and rate decisions associated with fungicides.