Corn and soybean fungicide: To spray or not to spray – that is the question

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

High market prices for both corn and soybeans this year are once again tempting growers to consider applying fungicides in the absence of disease. Whether or not a yield increase will result continues to be a topic of many lunch counter discussions, and among plant pathologists.

Corn

Fungicides are routinely applied in seed corn fields where there may be little resistance to disease. In a multi-state study where fungicides were applied to field corn, results ranged from instances of yield decrease, to no differences from untreated controls and some instances of increased yield. In the absence of threshold levels of disease, no consistent pattern emerged that that could help predict when applying a fungicide would result in a yield increase. Yields were found to increase with fungicide application where gray leaf spot (Cercospora zeae-maydis) was present at levels above economic thresholds. Gray leaf spot does not occur on a widespread, annual basis in Michigan. When it does occur, it is more likely to be found in susceptible hybrids grown in corn after corn or in no till situations and is favored by high humidity. Hence, irrigated fields would tend to provide a better environment for the disease than non-irrigated fields.

Timing of fungicide application is important in corn. When certain fungicides were applied during the critical period one to two weeks prior to tasseling, particularly in combination with another pesticide or adjuvant, abnormal ear development (“beer can ears”) resulted in significant loss of yield. The cost of fungicides and application needs to be considered to determine whether there will be an economic return from a fungicide application. Run-down resulting from ground application needs to be figured into the equation as well.

Soybeans

In soybeans, there has also been considerable work done to evaluate foliar fungicide applications, partly as a result of looking at fungicides to control soybean rust. Data has been compiled to summarize the results of foliar fungicide application to soybeans across a wide range of soybean varieties and growing conditions in multiple states across multiple years. Again, results ranged from instances of yield decrease, to no differences from untreated controls and some instances of increased yield. In the absence of threshold levels of disease, no consistent pattern emerged that that could help predict when applying a fungicide would result in a yield increase.

Most soybean diseases that foliar fungicides are designed to control do not reach economic levels in Michigan. We have not found soybean rust in Michigan. The most common foliar disease, brown spot (Septoria glycines) has not been shown to cause economic loss. Downy mildew, bacterial pustule and bacterial blight may commonly occur, but rarely affect productivity. These diseases would not be controlled by the fungicides typically applied for foliar diseases and fungicides are not part of the recommendations to manage them.

White mold is another disease of soybean that can cause economic losses, but foliar fungicides do not control it. Frogeye leaf spot (Cercospora sojina) can cause economic losses in susceptible varieties that have shown significant responses to foliar fungicides. Frogeye leaf spot has been more of a problem in states further south, but has occurred in scattered fields in Michigan in the last couple of years. Application of strobilurin fungicides has a greening effect on soybeans that may delay maturity. Growers who wrestled with stems tangling in combines last fall as they harvested soybeans might want to keep that in mind when making a decision to apply strobilurin fungicides.

If you do decide to invest in a fungicide application to your corn or soybeans, why not set up a fair evaluation in your field to see whether or not you realized any benefit? Here are some guidelines for setting up check strips and an evaluation in your field. (From the C.O.R.N. OSU newsletter July 7, 2007, So You Are Trying Fungicides On Corn And Soybeans, How To Evaluate If They Are Worth The Cost? - Anne Dorrance, Pierce Paul, Dennis Mills)

  1. Know what the variety or hybrid is. For both corn and soybeans only the moderately susceptible and highly susceptible hybrids and varieties have demonstrated yield impacts in fungicide applications.
  2. Have more than one check strip, wider than your combine, and space them across the field. Three should be plenty but two is not enough.
  3. In your comparisons, do not include the parts of the field where you have weed escapes or along tree lines, these areas are going to yield less anyway, so this is false data whether it was treated or not. Make sure other variables such as soil types, soil fertility variations and pest issues are not biasing your field experiment.
  4. Approximately three weeks after applications, walk some of the strips. For corn, look at the ear leaf, determine what percent leaf area has got lesions in the treated vs. non-treated. Is gray leaf spot, anthracnose, or northern leaf blight present? For soybeans – look at the upper canopy for frogeye or downy mildew and on the lower canopy look at how much brown spot is present.
  5. Take averages. When you do harvest your fields, take several strips, both untreated and treated. Then take the average of the untreated strips and compare that average to the treated. Fields are not uniform and with our stand issues and unevenness across many fields this year, there is going to be even more variability.

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