Preventive Soybean Fungicide Applications: Products, Rate Selection, Timing

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 15, 2012 11:24 Updated

Over the past few weeks I can’t count the number of calls I’ve received regarding fungicide application strategies in soybean.  Not only have I received calls regarding product selection but lately I’m receiving calls regarding specific fungicide timings that we are not accustomed to in MS.  I’ve outlined several different scenarios below with specific information pertaining to each situation.  Keep in mind that the environment is the single most important component of the disease triangle.  At present, few if any diseases are occurring in soybean throughout MS.  However, with the recent rainfall received throughout parts of the state this could change.

Product selection

The bulk of the research trials that have been conducted in MS suggest that the use of a strobilurin is the best choice when making an R3/R4 fungicide application.  While I realize there are a tremendous number of pre-mix and tank-mix combinations the choice to use one of these products that either contain a triazole (or an application that only contains a triazole) should most typically be based on the presence of disease (e.g. frogeye).  The available triazole products, and I’m only speaking about the triazole products on their own (e.g. flutriafol, propicanazole, tebuconazole, tetraconazole), are excellent products when used in the proper situation.  Plant pathologists absolutely hate to refer to a fungicide as having “curative” properties.  However, triazoles are best utilized in situations where low levels of a disease are detected.  With that in mind, over the past few years I’ve heard an increased number of situations where a triazole was applied at the R3/R4 timing and the farmers felt it didn’t perform near as well as a strobilurin.  Again, triazoles should be placed in situations where disease has been detected.  The triazole products themselves have wholly different modes-of-action than the strobilurin fungicides alone or a strobilurin + triazole pre-mix or tank-mix.

The R3/R4 application timing is strategic for several different reasons.  First and foremost, the application timing is usually targeted when disease is not present in the field.  The application is made to prevent disease, particularly diseases that occur after the R3/R4 timing.  Oftentimes I’ve heard “well, I don’t have disease in the field now, so I don’t think I need to make the R3/R4 application” OR “it has been hot and dry and since there is no disease in the soybean crop I’m not going to apply an R3/R4 fungicide.”  As I said above, the timing is preventive and made in the absence of disease.  If you are expecting to see a potential yield benefit from a fungicide application the vast majority of the data from MS suggests that a fungicide applied at R3/R4 will most likely provide the best advantage.  However, placement should be targeted in those situations where:

a)      Continuous beans have been planted (keep in mind, if you had wheat in the winter, and beans last year, and are planting beans again, this is not a continuous soybean situation)

b)      Irrigated fields

c)      High-yielding environments, so most typically early planted soybean in a continuous situation with irrigation

Last year a large plot trial with aerial application in the Tchula, MS area was conducted in response to several questions regarding fungicide placement.  Replicated and randomized plots were arranged in a field of Pioneer 94Y70 in a high yielding environment (40” row spacing, furrow irrigated, continuous soybean production).  Fungicides were applied in 5 gal/A of water at the R3 growth stage with 0.25% NIS.  Disease was not detected in the plots either before or after the application.  Several different products were applied and while a strobilurin fungicide in one case did increase yield compared to the untreated plots, in 2011 the best product choice at this particular location was 4 oz of trifloxystrobin + prothioconazole, a strobilurin + triazole product.  However, comparisons were made with a triazole and as stated above, the option that is likely the most beneficial in high-yielding environments is to apply a strobilurin-based fungicide, especially at the R3/R4 timing in the absence of disease.  Even though the strobilurin + triazole was a better option at this particular location, the larger set data set from MS trials suggests that a strobilurin alone is the most consistent choice.Yield table 2011 large plot fungicide trial

Product rate at the R3/R4 timing (4 versus 6 oz)

Everyone has an opinion on the particular product rate to be applied.  The greater MS data set suggests that 4 fl oz/A of a strobilurin is just as good as 6 fl oz/Acre if you are only accounting for the harvested yield regardless of the fungicide application being made at R3 or R4.  But, in years when soybean rust could become a threat – and I realize we are still trying to determine exactly what that year may be – I personally would prefer to see farmers use a 6 fl oz/A rate of a strobilurin product.  The extra 2 fl oz/A (from 4 to 6) of a strobilurin can be expected to provide 7 additional days of prevention/protection from disease.  Ideally that could mean you’d reach something around R5.7 if you held off a little past R3 and made the application at R4.  If soybean rust were to come into the picture at that point (> R5.7) you might not need a second fungicide application.  However, as with all plant disease situations this will greatly depend on the environment encountered following the fungicide application.  Remember, the most important part of the R3/R4 timing is the fungicide is applied in the absence of disease.

Reduced product rates (less than 4 oz)

On an annual basis I regularly receive telephone calls, or hear stories, where someone is applying 2 oz of a strobilurin (e.g. azoxystrobin or pyraclostrobin) at R3/R4 and coming back with a second shot of 2 oz at R5.  To my knowledge, data to support this type of strategy doesn’t exist in MS; but will at the end of this season.  In addition, application strategies such as a split shot of reduced rates of a fungicide goes against IPM strategies and could present a situation where resistance within a particular fungus could develop if the organism is present.  Occasionally I’ll receive calls about a 1 oz application of a strobilurin fungicide for whatever reason, typically for an application on dryland soybean.  As a plant pathologist, I doubt that a reduced rate fungicide application (less than 4 oz of a strobilurin or a cut rate of a stand-alone triazole) is providing much benefit from a yield loss strategy as it relates to plant diseases.  Also, keep in mind, there are some fungicide products that have different labeled rates.  Most of the information I’ve included in this is based on a 6 fl oz/A application rate of either azoxystrobin or pyraclostrobin.  Some of the newer fungicides (e.g. Evito, Stratego YLD) have different application rates.

Situations in other states

Lately I’ve heard several radio advertisements and had numerous telephone calls dealing with fungicide placement in:

a)      Situations where strobilurin-resistant frogeye has been detected

b)      Early fungicide applications (predominantly R1) to manage late-season Cercospora (Cercospora blight)

c)      Multiple applications of a fungicide at different timings similar to what is suggested by a group from LA

Put quite simply, we have not detected strobilurin-resistant frogeye in MS.  However, if you do find frogeye please call (662-402-9995) as I would like to send samples off for resistance screening purposes.  At present, strobilurin-resistant frogeye has been detected in isolated cases in IL, KY, LA, MO, and TN.  If, and I can’t stress IF enough in this statement, a strobilurin fungicide has been applied in response to frogeye infection (meaning this season), and a second application is warranted then we need to discuss some options.  Frankly, this is where the triazole products should be used.  Keep in mind there are resistant soybean varieties available that can be planted in situations where frogeye has been a consistent, historical problem.  Ideally, in some situations, a fungicide should be considered as a second option.  I realize in the past that a strobilurin fungicide program has been suggested as a management option when frogeye has threatened.  However, with the potential of resistance to the strobilurin chemistries developing within the fungal population I think at this point a triazole-based program would be the best yield loss prevention of frogeye is detected.

Intense Cercospora blight as observed in some parts of Louisiana.

Fungicide application strategies used in Lousiana may not apply to our specific soybean management strategies in MS.  More specifically, the majority of the fungicide trials that have been conducted whereby an R1 fungicide application provided a benefit in reducing the observable levels of Cercospora blight were planted late (> June 20).  Data from trials conducted on late soybean plots should not be applied to full-season, early planted soybean fields in MS.  In addition, LA has a tremendous amount of Cercospora blight, in fact, annually more of the disease than we do in MS (see photo).  I have seen this issue first hand and I know they consider Cercospora blight their number 1 disease.  I’d put Cercospora blight in our top 3 diseases annually, and say last year we likely had more than any other recent year.  Expecting a particular fungicide strategy that works in LA to work in MS may not happen.  The fungus that causes Cercospora blight, Cercospora kikuchii, has a tremendous knack for developing different strains.  Researchers in the past have suggested this could differ on a field-by-field basis.  More specifically, what I’m suggesting here is that the strains of the fungus in LA may be more aggressive than the ones in MS.  However, at best, fungicides (at least the commercially available products) are not benefiting farmers when applied to manage Cercospora blight by either utilizing a fungicide application prior to observing symptoms or in response to symptoms.  If I had to speculate I believe we can say that the R3/R4 application timing is providing a little benefit by reducing the yield losses attributed to Cercospora blight but this timing is certainly not reducing the observable symptoms of the disease.  However, since resistant soybean varieties are not available we have no way to confidently test this theory.

Multiple applications (R3/R4 followed by R5)

In short, while sequential fungicide applications may seem like an outstanding option, our MS data set suggests little if any yield benefit from making multiple fungicide applications in a single season.  As a portion of a MS Soybean Promotion Board funded project over the past 3 seasons we conducted fungicide and insecticide trials to attempt to reduce seed rot by applying a fungicide.  Soybean growing conditions were outstanding during 2010 and 2011 so the data set has provided some additional insight into fungicide timing strategies when considering the yield response to the fungicide.  Over the 3 year period, 13 different locations received azoxystrobin at R3, R5, R3 fb R5, and 9 of those locations in 2 years received an azoxystrobin application at R1 fb R3 fb R5 fb R6.  All trials were conducted with Pioneer 94B73, a gray pubescent soybean variety with a history of poor weathering in harsh environmental conditions.  Generally speaking, as stated above, the azoxystrobin application at R3 yielded a 2.8 bu/A greater yield response compared to the untreated plots.  However, of greater interest is the response from the sequential R3 fb R5 application that yielded 2.4 bu/A greater than the untreated.  Even though the sequential application timing might be considered by some to be more beneficial, a single fungicide application made at R3 was more beneficial.  The 4 sequential application treatment yielded less than the untreated plots.Yield of soybean project – the Quadris component over 3 years

R5+ applications made for seed rot prevention

At this time, fungicide applications specifically to manage seed rot are not necessary since little of the soybean crop has reached the R5 stages and seed rot is not a threat at the present time.  Managing see rot will likely be a topic later in the season.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist June 15, 2012 11:24 Updated
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