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Treating indeterminate soybeans for Asian rust

Dear Farm Industry News,

We enjoy your magazine the most of all the magazines we receive. You get to the issues.

In regard to treatment for Asian soybean rust, everybody is talking about how to prevent or treat for Asian rust in Brazil. I believe most varieties in Brazil are determinate soybean varieties (you can check). My concern is that no one has told us if Brazil has indeterminate soybeans in the southern regions of Brazil and if they have any experience in spraying indeterminate soybeans for Asian rust.

Indeterminate soybeans have a much different growth pattern than do the determinate soybeans. Indeterminate soybeans continue to grow and put on new leaves most of the summer. Determinate soybeans flower in a short period and do not put on a lot of new leaves after a certain period. If you think about spraying a preventative foliar fungicide for the Asian rust on an indeterminate soybean variety, the new leaves that it put on every few days will be unprotected. This then leads you to the question, what effect will spraying fungicide on indeterminate soybeans have on yields in the Midwest?

I believe that most soybeans that are grown north of Missouri latitude are indeterminate varieties and south of Iowa has determinate soybeans. A lot of acres in the Midwest are indeterminate soybeans.

In regard to foliar fungicide treatments, farmers need a chart that guides them in choosing a fungicide that not only tells them the fungicide to use for Asian rust but also tells what other fungi (if they do not have rust on their farm) a certain fungicide will protect the plant from.

Best regards,
Denny Winterboer

Hi Denny,
You pose an excellent question. As you point out, indeterminate soybeans will require more vigilant scouting and potentially more spraying to protect the new leaves and pods.

Mississippi State University says, "For indeterminate soybeans, timing of applications in relation to growth stage is much more difficult because the plants continue growing and putting out new leaves and flowers. Thus, indeterminate soybeans may require more sprays with shorter intervals between sprays, because new leaves may be exposed to infection every week."

Read the full report at

Also, Brazil does grow both determinate and indeterminate soybeans. A soybean rust trial in Brazil conducted by Bayer Crop Science, for example, demonstrates levels of control over time with various fungicides using one application. Read the report at

Syngenta is another company that has extensively studied rust control methods in Brazil. Syngenta states, "Understanding the growth stages of the soybean plant is a very helpful tool in predicting when the crop is most vulnerable to soybean rust and scheduling fungicide applications. The growth stage chart pictured in this document ( discussed in detail at applies equally well to determinate and indeterminate varieties."

Thanks for reading Farm Industry News!

Best regards,
Wayne Wenzel, Senior Editor

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