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Pest Management like fungus and insects in corn

"Management is all about managing in the short term, while developing the plans for the long term." -Jack Welch

Corn diseases above Left to Right: Physoderma, Gray Leaf Spot, and Southern Rust.

A couple points in corn agronomy to consider before applying fungicide.

  • Corn Hybrid's all have different tolerances to disease pressure. Ask your seed dealer about these characteristics in the products you planted. Not knowing this can result in a wasted app or lost bushels for a much needed application.

  • Beck's PFR (Practical Farm Research) shows maximum ROI when applied at VT-R1 on most years, in fact research shows that corn would have to fall to $3.00/bushel before it would not pay to treat with aircraft application cost included. If disease is not present, I would push the app to between R1 and brown silk. And, a R5 app is typically effective only on continuous corn situations.

  • Time of day is can be an amazing ROI if you can manage it. Like other practices that cost $0.00 in terms of purchasing product, this practice if managed will be 100% take home pay. In PFR studies, this change resulted in 3.6 bpa increase of applying at 8:00 am vs. 3:00 pm on the same day. At $4.00 corn this is over $14.00/acre of pay placed in your personal bank account.

  • Rate of Carrier is another element to consider. Rarely, we use ground rigs in our area, but research across 2 years of multiple Beck's PFR sites in low disease conditions still producing an average 2.8 bpa advantage to apply 15 gallons vs 10 of water with Y Drop ground rig. Since we are typically using aircraft to apply, ask if the pilot can up the rate by a gallon or two. At least upping the rate to 3 gpa will increase your rate by a third.

  • Spraying fungicide early (vegetative stages) on corn after a hailstorm does not typically prevent disease. Many of the infections that come from foliar wounds are diseases that are not treatable with fungicide. The thought that floats around that treating hail damaged corn will return a positive ROI assumes the benefits of reducing or stopping disease. However, most likely any perceived advantage is not from disease prevention. Any improvement that comes from the fungicide may be from helping the plant tolerate high stress. This reduces ethylene production and may increase photosynthesis as the plant hangs on to more green leaves. Nevertheless, the jury is still out on this being a payback.

Figure 2: Scouting Corn at R1 Stage

Now, a few words on scouting:

  • For corn planted in April, we are approaching a time where weekly scouting may be needed. Corn is changing rapidly every few days, and a lack of disease one week is not indicative of next week's corn conditions. With the cool wet weather we may see Northern Corn Leaf Blight an issue as well. For corn that is planted late we need to keep an eye on southern rust as we get late in the year.

  • Look low, disease found on the lower leaves will gives an idea of what the ear leaf and the upper leaves will look like down the road. If the disease level is above threshold on the lower leaves, then this will be a problem as the spores will travel upward. As fungicide is prevents disease more than cure, we must be ahead of this curve.

Figure 3. Purdue University Extension: Corn & Soybean Field Guide
  • Thresholds to apply. I am going to say that this year of all years will be beneficial to apply treatment due to higher prices and wet humid conditions. Basing disease threshold is hard as weather, hybrids, and economical situation of that field all change where the crop is at on the risk ladder. However, I have heard of 5-10% damage on half of the plants as being a minimum threshold. Note the picture in Figure 3. for reference.

Finally, when making a fungicide pass, I recommend using an insecticide in the tank mix. However, no fertilizers and obviously no herbicides at this sensitive time period.


July 13, 2021 -Edit for clarification on identifying Southern Rust

A quick word on Southern Rust symptoms vs Common Rust. Southern tends to be more orange in color and Common more brownish. The easiest way however is to look at the pustules on the leaves. Find a pustule on the leaf's upper surface, then turn it over and examine the same area on the lower surface. If the pustule has physically broken through the tissue, it is most likely Common Rust, however; if it has not then it is most likely Southern. You may see colorations from Southern when examining the bottom side of the leaf, however it will be smooth to the touch when rubbing with your finger.

Works and People cited to create this article:

Beck's Hybrids Agronomist for Missouri, Alex Long

Beck's Hybrids Practical Farm Research sites and teams:




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