Management Practices to Reduce the Development of Fungicide Resistance in Soybean: Part II, Choosing Fungicide Products and Rates

Frogeye leaf spot. Note smaller lesions indicative of secondary infection.

Prior to the first observation of strobilurin-resistant frogeye leaf spot (Cercospora sojina) in TN in 2010, as well as MS in 2012, strobilurin (QoI) fungicides were generally suggested to manage frogeye leaf spot once the disease was observed.  However, following the initial observation of strobilurin-resistant C. sojina in MS the specific suggested fungicide products have changed in the event of a frogeye leaf spot infection occurring due to the development of fungicide resistance issues.  More specifically, fungicide application strategies should be implemented to reduce the likelihood of fungicide failure as well as additional resistance development within the frogeye leaf spot fungal population.

Fungicide Product Choice

Last year a blog update was posted regarding the fungicides most commonly used in our row crop production systems.  To read the previous post go to:

https://www.mississippi-crops.com/2013/03/19/the-fungicides/ (Updated information included)

Specific differences can be attributed to the products we apply for disease management.  Generally speaking, fungicides perform best when applied prior to disease symptoms being observed.  However, this can be a difficult practice to implement.  More than likely this is why the R3/R4 fungicide application has been such a successful management practice.  When comparing the specific fungicide products and grouping them into chemical classes, several classes of products are used in our soybean production system.  The two most widely used classes of fungicides (strobilurin and triazole) are briefly outlined below as well as the resulting pre-mix fungicide products.

Strobilurin fungicides (FRAC Code 11)

The products that comprise the strobilurin class of fungicides are likely the most widely used class of fungicides in the MS soybean production system.  The strobilurin, or quinone outside inhibitor (QoI), fungicides are the main component in Headline and Quadris as well as several other products (e.g., Aproach, Evito, Gem).  The members of the class are considered to be preventive when it comes to disease management practices.  A preventive fungicide is most beneficial when applied prior to disease initiation.  Strobilurin fungicides don’t likely have any “curative” ability once disease is observed.

Active ingredient

I still receive questions regarding whether or not a specific product contains a strobilurin fungicide.  The easiest way to determine if a product is a strobilurin is to look at the name of the active ingredient on the product label.  If the name of the active ingredient ends in “-strobin” then the product is (or contains) a strobilurin.  Presently, several fungicide products (6 fungicide products) are stand-alone strobilurins as well as dual pre-mix products with strobilurin active ingredients (12 fungicide products).  Be mindful that annually, fungicide products are released that contain a strobilurin active ingredient.

Application rate

The specific application rates vary based on the percentage of the active ingredient contained within the fungicide product.  In general, single application rates range from 2 fl oz/A (e.g., Aftershock/Evito) on the low application rate end to 15.5 fl oz/A (e.g., Quadris) on the high application rate end.  Typically, application rate, based on the label jargon, is based on whether a susceptible variety is planted or environmental conditions are deemed conducive for a severe disease epidemic (see Quadris label).  In some cases fungicide labels call for the high application rate when foliar disease is present in the field at time of application.  See the above blog link for more specifics regarding the rates for each of the fungicide products labeled for soybean.

Risk of resistance development

When it comes to the potential development of resistance occurring within a fungal population, the strobilurin fungicide class of fungicides are considered to have a “high risk” since they affect a single-site in the fungus.  Repeated applications of stand-alone strobilurin fungicides can greatly increase the likelihood of resistance developing within a fungal population. Several scenarios can reduce the likelihood of fungi developing resistance to the strobilurin fungicides: applying the product(s) at the full label rate, making applications in a minimum of 5 gallons of water/A by air, rotating fungicide chemistries if multiple fungicide applications are needed in a single season, and not relying on the stand-alone strobilurin products to prevent yield loss in situations where frogeye leaf spot has been observed in the field.

Triazole fungicides (FRAC Code 3)

The products that comprise the triazole class of fungicides, much like the strobilurin products, are most effective when applied prior to disease symptom expression.  In general, the members of the triazole class can be considered to be “curative” since they can be effective when applied in the presence of a foliar disease.  However, no fungicide is truly “curative” and once the disease is observed, especially a disease like frogeye leaf spot, if conducive environmental conditions persist, symptoms of the disease may continue to worsen even following a fungicide application.  Fungicides act by preventing yield loss since affected leaf tissue around the lesions won’t be repaired following the application.

Active ingredient

For the most part, the members of the triazole class of fungicides have an active ingredient that ends in “-azole”.  However, there is one exception to this general rule, flutriafol (Topguard).  As outlined above, the specific rates associated with each of the products differ based on the percent active ingredient contained in each product.

Application rates

The specific application rates vary based on the percent active ingredient contained in the fungicide product.  In general, single application rates range from 2.5 fl oz/A (e.g., Proline 480 SC) on the low application rate end to 14 fl oz/A (e.g., Topguard) on the high application rate end.  Typically, application rate, based on the label jargon, is based on whether or not conducive environmental conditions persist for disease development to occur (see Topguard label).  Much like strobilurin fungicides, triazole fungicide labels call for the high application rate when environmental conditions persist that could translate into severe disease pressure.  See the above blog link for more specifics regarding the rates for each of the fungicide products labeled for soybean.

Risk of resistance development

Much like the strobilurin fungicides, the triazoles are also deemed to have a “high risk” for the potential development of resistance with the fungi managed.  Triazole fungicides also only affect a single site.  Continued exposure of a particular fungus to repeated applications of a triazole fungicide can increase the likelihood of resistance developing within the fungal population.  The same suggestions outlined above for the strobilurin fungicides can also reduce the likelihood of repeated exposure to the triazole fungicides.

SDHI (succinate dehydrogenase inhibitor) fungicides (FRAC Code 7)

Limited fungicide products contain members of the SDHI class of fungicides, especially for application in soybean production systems.  At present only Priaxor contains an SDHI fungicide as one component of the pre-mix.  Resistance has been reported from members in this particular fungicide class.

Dual mode-of-action (pre-mix fungicides)

The most widely used pre-mix fungicides all contain dual modes-of-action or multiple active ingredients.  More often than not, save for one product being actively marketed in our production system (Muscle ADV = chlorothalonil + tebuconazole), the pre-mix products contain a strobilurin as one of the actives.  The dual mode of action can be important if a fungicide product is applied in the presence of a foliar disease such as frogeye leaf spot.  Moreover, in the future, most chemical companies suggest that stand-alone strobilurin products will likely become a thing of the past.

Application Rate

Application rate of a fungicide is set for a reason.  Over the past decade farmers have become accustomed to making what amount to “reduced rate fungicide applications” using one specific class of fungicides, the strobilurin fungicides.  Research from MSU during the early 2000s suggested that a 4 fl oz/A application of a stand-alone strobilurin fungicide (e.g., Headline or Quadris) produces just as much yield benefit as 6 fl oz/A of the same fungicide when the application is made at the R3/R4 growth stage in the absence of foliar disease.  The line of thinking behind this type of fungicide application needs to be a thing of the past.  Moving forward we’re going to continue to encounter fungicide resistance within any of the foliar diseases in our main row crop systems especially in scenarios where stand-alone strobilurin fungicides are applied or in situations where reduced-rate applications of pre-mix products are conducted.  Fungicide applications can be expensive, but reducing the rate of fungicide to fit within a price point can lead to the development of fungicide resistance occurring much more quickly.

The newer, pre-mix fungicide products have continued to be released to the agricultural community.  Last year in particular I was made aware of several instances where applications were made using a product such as Quadris Top at a 6 fl oz/A application rate rather than the 8 fl oz/A rate simply because the 6 fl oz/A rate provided approximately 4 fl oz/A of Quadris.  However, by reducing the overall application rate of the pre-mix product to achieve a 4 fl oz/A strobilurin rate, the rate of the triazole component is also reduced.  Each fungicide, at the appropriate labeled rate, contains a certain amount of strobilurin and a certain amount of triazole/SDHI (Effective product in combination).  For example, when applying 14 fl oz/A of Quilt (Quadris + Tilt) the resulting rate combination will be 4.2 fl oz/A of Quadris + 4.0 fl oz/A of Tilt.  Applying a reduced rate of the pre-mix fungicide simply reduces the rate of both of the active ingredients contained within the pre-mix fungicide.

Rate and development of fungicide resistance

Increasing the application rate of a stand-alone strobilurin fungicide product, or doubling the rate in a situation where resistance to the strobilurin fungicides has already been documented will not solve the problem.  Over the next year we will continue to observe strobilurin resistance within the greater frogeye leaf spot fungal community.  Since the initial detection of fungicide resistant C. sojina in MS (as documented by Dr. Carl Bradley from the University of Illinois) a graduate student (Mr. Jeff Standish) has been added to screen fungal isolates that were recovered during the 2013 season from throughout MS.  During 2013 frogeye leaf spot infected leaves were collected from 49 counties in MS (Frogeye survey map 2013b).  The green shaded counties include the counties where frogeye leaf spot infected leaf material was collected.  The red color indicates the intensity of the soybean crop throughout MS.  The specific project has been funded by the MS Soybean Promotion Board and to date has yielded some outstanding results.

Part III of this series on resistance will include information regarding the presence of strobilurin-resistant frogeye leaf spot isolates from the MS soybean production system.