Automatic Soybean Fungicide Applications: Timing, Product Choice, Rates in Product Combination

Numerous decisions regarding automatic fungicide applications at R3/R4 have been the topic of conversation for the better part of the past two weeks.  Frogeye leaf spot (FLS) is clearly driving decisions for specific fungicide products, especially at the automatic fungicide timing.  Following a two year survey of soybean fields from throughout the state it was determined that 73 counties contained strobilurin-resistant FLS.  Essentially, from this point forward, in fields where FLS is observed we should assume the fungal population is resistant to the strobilurin class of fungicides (e.g., Aproach, Evito, Gem, Headline, Quadris, and associated azoxystrobin containing generics).  Increasing the rate of fungicide or making multiple applications with a stand-alone strobilurin on FLS susceptible soybean varieties will not manage the disease.  In addition, remember that the fungicide application will not make the disease go away if it was present at the time of application.  Moreover, once a fungicide is applied and the residual has worn off the disease can still occur.  Generally speaking, a stand-alone strobilurin provides 21 days of protection while a triazole (demethylation inhibitor; DMI) provides 14 days of protection.  Put most simply, applications made as early as R3/R4 do not provide season long protection against any fungal disease.

Soybean variety

Choose a fungicide based on the variety planted.  If the variety is FLS susceptible, then the fungicide application should contain at least two modes of action to manage FLS at the automatic timing.  For the past two to three years the general suggestion for automatic fungicide applications at the R3/R4 timing has remained:

-frogeye leaf spot susceptible soybean varieties = pre-mix or tank mix of a product that contains two modes of action

-frogeye leaf spot resistant soybean varieties = a stand-alone strobilurin fungicide product (if available)

If you do not know the specific response of the varieties planted in a given field refer to the MSU soybean variety trial ratings that can be found:

For the 2014 Maturity Group IV trials go to: https://www.mississippi-crops.com/2014/12/02/2014-soybean-variety-trial-disease-ratings-part-ii-maturity-group-iv-varieties/

For the 2014 Maturity Group V trials go to: https://www.mississippi-crops.com/2014/12/29/2014-soybean-variety-trial-disease-ratings-part-iii-maturity-group-v-varieties/

For the 2014 Conventional and LibertyLink trials go to: https://www.mississippi-crops.com/2014/11/25/2014-soybean-variety-trial-disease-ratings-part-i-conventional-and-libertylink-varieties/

Application timing

Making a fungicide application at the R3/R4 growth stage has been an important management practice for many years.  Many people will refer to this specific timed application as the “yield bump”.  Over the past 15 years research at MSU has indicated that 50-60% of the time the fungicide application provides a positive yield benefit when the fungicide is applied in a high-yield situation.  A high-yielding soybean environment is termed one whereby soybean has been the continuous crop and the field is irrigated.  Double-crop soybean following wheat is not considered to be a continuous soybean situation.

Fungicide application rate

Much is made on an annual basis regarding the specific rate of fungicide to be applied at R3/R4 or any other growth stage.  Full label rates perform best when attempting to manage disease.  However, oftentimes two applications of a reduced fungicide rate (e.g., 2 fl oz/a @ R3 followed by 2 fl oz/a @ R5 of a stand-alone strobilurin fungicide) are applied to provide a 4 fl oz/a of a product in a single season.  Making a reduced rate application of a fungicide increases the risk of resistance developing if a disease is present and also greatly reduces the residual period of protection.  For example, making a 4 fl oz/a application of a stand-alone strobilurin fungicide will generally reduce the residual effectiveness of the fungicide by as much as 7 days compared to the 6 fl oz/a rate.

Effective rates of product in combination

With the current fungicide products you can’t simply compare apples to apples.  The specific percentage of each active ingredient differs between the pre-mixes (see: https://www.mississippi-crops.com/2013/03/19/the-fungicides/ for more related information).  Rates are based on the percentage of the active ingredient within each product or the pounds of active ingredient contained within one gallon of product.  For example, the more typical labeled application rate of Headline or Quadris (or Aproach) is 6 fl oz/a, but the percentage of the active ingredient within each of the products is on the order of 23% and there is typically 2.08 pounds of active ingredient/gallon.  Compare this to other strobilurin fungicides, such as Evito (or Aftershock) or Gem, which contain on the order of 40% active ingredient or a greater amount of pounds of a.i./gallon.  The application rate for each of these products is Evito 2 – 5.7 fl oz/a and Gem 3 – 3.5 fl oz/a.  The same goes for pre-mix products with different application rates: for example, Quadris Top is an 8 – 14 fl oz/a while Stratego YLD is a 4 – 4.65 fl oz/a application rate.  If you split the products up into the effective rate of the active ingredients for each of the products contained in the pre-mix you will see that the rates of the two active ingredients in each product will be different, but are on the order of each of the product rates as applied separately.  For example:

Quadris Top = azoxystrobin (Quadris) + difenoconazole (no soybean product)

-making an 8 fl oz/a application of Quadris Top applies 6.4 fl oz/a of Quadris and 4 fl

oz/a of difenoconazole

Stratego YLD = trifloxystrobin (Gem) + prothioconazole (Proline)

-making a 4 fl oz/a application of Stratego YLD applies 3 fl oz/a of Gem and 1.1 fl oz/a

of Proline

For the full table, with most if not all of the associated fungicide pre-mix products see 2015 Effective product in combination.

Response of frogeye leaf spot to a fungicide application

Applying a fungicide to a frogeye leaf spot (FLS) susceptible soybean variety will not make the disease go away.  At best, regardless of fungicide product or the specific modes of action of the products contained within a fungicide, the application will slow the disease.  Fungicides do not generally kill the fungus involved and in most cases the disease will observationally appear worse following the fungicide application.  During 2014 a fungicide trial was conducted on a frogeye leaf spot susceptible soybean variety (Armor DK 4744) to provide information regarding the observable disease following fungicide application.  Essentially, regardless of the fungicide applied at R3 in the presence of FLS, the disease continued to progress post-application.  In fact, on average, and by the time the plants had reached R6+, the sprayed plots had just as much FLS as the nontreated plots (7.5 vs 8) on a 0-9 scale with 0=no disease and 9=severe FLS.  Even though the figure at left is busy, look at the disease progression over time from time 0 (R3 on August 17) through 45 days post-application (October 1).  Several single mode of action triazole (DMI) products were applied alone and in combination with Priaxor at their full label rate and compared to Quadris Top.