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  • Writer's pictureRachel

Foliar Fungicides: Take 2. Soybeans.

Updated: Jul 18, 2018

A couple weeks ago we took a stab at foliar fungicides in corn, including the thresholds for application and economics. Today we are going to take a go at foliar fungicides in soybeans. Our results will cover the yield advantage of foliar fungicide applications, and since we often lump them together, we are also going to chat about insecticides in soybeans. Both of these applications can go a long ways toward improving plant health. Read along to learn more about the decision-making process for this aspect of the growing season.

Diseases causing a Yield Impact

Only fungal diseases can be controlled with a foliar fungicide. These include common diseases to our area like cercospora leaf blight, frogeye leaf spot, Septoria brown spot, and soybean rust. A foliar fungicide will not control any bacterial based diseases such as bacterial blight or viral diseases such as soybean vein necrosis. Fungal diseases that are infected in the roots are also not controlled by foliar fungicides. These include diseases like charcoal rot and sudden death syndrome. Finally, there are several fungal diseases that will be controlled by a fungicide but rarely have a yield impact. Consequently, treatment of these diseases is not generally recommended. Downy mildew would be an example of this. Because of this variability in disease response, it is essential to scout your beans regularly and be able to correctly identify different diseases to know which diseases will respond positively to treatment. Generally, when diseases do result in a loss in yield, it is because photosynthetic area has been reduced from lesions on leaves or premature leaf drop.

For a refresher on various foliar diseases, check out our disease ID gallery under the photos menu of our website.

Disease Infection in Plants

Infection of these diseases depends on several important factors. First, the weather plays a huge factor on disease development. Wet and humid conditions that keep leaves wet for extended periods of time can favor infection. This can include overcast days where dew remains on leaves longer into the morning. Dry and hot conditions are not conducive to fungal disease development. While this is similar to the current conditions we have been having, we have had several mornings where dew has set on leaves for several hours, which would be favorable for development.

Foliar diseases overwinter well in soybean stubble. In fields that are soybean after soybean, the disease pressure may be higher. Rotation of crops is a good way to break up the pest cycle.

Finally, how susceptible a variety is to disease plays a huge role in infection. Different varieties will respond differently to each disease. Information on disease ratings is available to look up in product guides.

Types of Fungicides

Quinone outside inhibitor, also known as strobilurins, and triazole fungicides are the two most commonly used fungicides for treatment of foliar diseases. Strobilurins are generally more effective when applied in early stages of disease infection and are often touted as providing an increased level of plant health. Triazole fungicides can be applied later during disease development but generally work the best when applied early in the disease infection window.

Foliar fungicides often are attributed to an increase in overall plant health and physiological benefits. Some studies show foliar fungicides provide increased water use efficiency, increased flowering and pod fill, and a delay in leaf loss late in the growing season.

Triggering Application

It is important to remember when deciding to apply to look at the current conditions as well as what future conditions may be. Is there an extended period of cool, wet conditions that will further increase the current disease levels?

How often do we need to apply? Research from the University of Illinois shows that an application at R3 is just as effective at controlling diseases as two applications at R3 and R5.

Additionally, if passes of fungicide and insecticide will be completed together, defoliation percentage may take precedence over disease levels for triggering application.

Local Results

It is important to note that in all of our local studies and in many national studies, fungicide was not tested alone. Often time applications of fungicide and insecticide are completed in conjunction making it difficult to compare fungicide alone.

In Pioneer trials across the Midwest comparing a fungicide vs no treatment, the average yield response was 2.6 bushels per acre. These trials did not have an insecticide included. The yield results increased to 5.3 bushels per acre when an insecticide was included in the treatment. Additionally, in some small plot studies, four different fungicides resulted in a yield response of 2.9 to 5.5 bushels per acre.

Locally across the last three growing seasons, we’ve seen a yield increase of 2.4-10.9 bushels per acre by using fungicide and insecticide at R3. Over all three years, the average was 5.9 bushels per acre. We’ve also analyzed the difference between several different products: Priaxor and Approach Prima. In yield environments above 50 bushels per acre, Priaxor showed a 1.4 bushel advantage to Approach Prima. In yield environments below 50 bushels, the results between the products were very similar.

When we consider the economics of this, with soybean prices hovering below 8 dollars, we would need to see a yield advantage around 3.5 bushels. Our average the last three years has been 5.9 bushels per acre. In a year with lower disease pressure, our average response was 2.4 bushels per acre. It is possible in years like this year, depending on disease pressure, we could see a response somewhere between those values. In that case, we are hovering around that breakeven mark. Remember, this is going to be very field specific, so in cases where we are beginning to see moderate pressure, it is likely that we may see a response somewhere closer to that average 6 bpa mark. Additionally, our local values also contained an insecticide application, so some of this response could be covering insect pressures. There is a lot to take into consideration, but if doing a combined application, be sure to consider both disease levels and percent defoliation when making the decision to spray.

Wrapping Up

In most cases, a fungicide and insecticide application will be applied together. While it’s important to consider disease levels when timing treatment, in climatic conditions we are experiencing this year, with low disease pressure, timing the application based on defoliation percentage will likely take precedence. Look for an upcoming blog post on insecticide applications and defoliation in beans for more information. We'd love to hear what your yield response and experience with foliar fungicide in soybeans has been. Leave us a comment below sharing your experience!

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