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Four steps for scouting soybean diseases

Agriculture.com Staff 05/15/2007 @ 8:31am

It's not always possible to control field crop diseases when you find them, says Carl Bradley, University of Illinois Extension plant pathologist.

Still, it pays to scout. Taking notes on disease-stricken fields can help when it comes to variety selection. Although it's too late to curb some diseases once you find them, you can help prevent them the next time you plant soybeans by selecting resistant or tolerant varieties.

Plus, there are some diseases growers can control in-season by applying fungicides. Field scouting and watching weather forecasts can aid growers during the fungicide decision process. Although a few fungicides have some "curative" properties, they all work better when applied at the beginning stages of disease, says Bradley.

Here are four disease scouting tips to keep in mind during field scouting this season.

  1. Check soybean emergence early.
    If stand losses and seedling diseases are a problem, consider using a fungicide seed treatment or choosing a variety with good seedling vigor the next time around. Send diseased seedlings to a plant clinic or diagnostic lab for determination of the causal agent. If Phytophthora root rot is the cause on soybean, plant a variety this next time with a combination of an effective Phytophthora resistance gene (Rps gene) and a good background of "field tolerance" or "partial resistance."
  2. Remember rust.
    As soybeans approach flowering (R1), check with university extension sources, Web sites, dealers, farm magazines and other information sources regarding Asian soybean rust. You can track disease due to the many states that participate in the soybean rust sentinel plot program. One good source in particular to check is USDA's rust Web site, www.sbrusa.net.

    If soybean rust is moving closer to you and your fields are between R1 and R5 (beginning seed) and your area has adequate moisture, consider applying a fungicide. Know the difference in symptoms between soybean rust and some of the common foliar diseases like bacterial blight, downy mildew, frogeye leaf spot, and Septoria brown spot.

    It's important to know what you're trying to control. For example, applying a foliar fungicide application will not control bacterial blight. Not only are you out of money, but applying an unnecessary fungicide application does not follow the principles of integrated pest management (IPM). IPM is a multi-step pest-management strategy of which chemical control is only one part.

  3. Scout for late-season diseases.
    Be aware that some late-season disease symptoms may not appear until soybean plants are in the reproductive stages (beginning at R1). In some cases, the pathogen may have infected the soybean at an earlier time, but symptoms are not observed until later. Examples include:
    • Sudden death syndrome
    • Brown stem rot
    • White mold
    • Stem canker
    • Anthracnose

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