Examples of Northern Corn Leaf Blight

Don’t wait till the planes are flying to decide on a fungicide strategy

By Harold Watters, Ohio State University Extension

Nothing creates a buzz in the coffee shop like an aerial applicator flying across corn and soybean fields in the county. The questions of, “What are they spraying?” “Why are they spraying?” will quickly turn into, “Did I miss something?” and “Should they be spraying for me?” Oh, yes that last question starts the mind rolling and makes for sleepless nights. Doing a little preparation now, by reviewing disease packages of hybrids/varieties planted on your farm plus knowing a little about common disease is the best way to avoid a panic buy as crops move toward reproductive growth stages. 

There is no doubt disease can cost yield in susceptible varieties/hybrids and fungicides are very effective at controlling target diseases. You will be glad you sprayed a susceptible variety/hybrid at the threshold level of disease. Did you notice the key word in those last two sentences “susceptible” variety or hybrid. 

Now you do need to think back to last fall’s conversation with your seedsman. You talked replant package, yield, standability, dry down, stress tolerance, root strength and disease resistance. Now is when you need to know what foliar disease resistance you paid for in your seed purchase. Is there a hole in that disease package you may need to cover with a fungicide?

Let’s use a corn example so you know what I mean. What you know now from your seed catalog is that 3 of the 4 hybrids you planted have a good resistance package for gray leaf spot. If the planes are flying because gray leaf spot is increasing in your region, then it may be worth spraying that one hybrid that does not have that good disease package.

You probably notice the weasel word “may” in the last sentence. Why did I have to do add that word? Planes are flying the county treating corn for gray leaf spot, so I must have it in my fields, right? The funny thing about diseases is weather conditions are so critical in their life cycle. Date of planting, rainfall amounts, humidity, and even storm front wind direction impact the amount of disease that may be in a field. Before deciding to spray a susceptible variety/hybrid it is worth a half a day to do some scouting, so you know that the disease is there. Better yet is when you can scout a few fields on a consistent basis every couple of weeks. Just because planes are flying in the north part of the county does not mean your field in the southern part of the county, that had less rainfall, has a problem.

Scouting can be time consuming but can be manageable if we target when we scout and what we are looking for. The when for corn scouting is around growth stage V14, just prior to tasseling. The when for soybeans scouting will be growth stages R1 to R3, beginning flower to beginning pod. 

A 2020 article in Plant Health Progress by plant pathologists from across the U.S. and Canada, including our own Pierce Paul, can be helpful to answer what to scout for in corn for our region. They identified the 10 most destructive diseases each year from 2016-2019. The foliar disease that topped the list was gray leaf spot with mentions of Northern corn leaf blight and Southern rust. These are foliar diseases that can be controlled by fungicides. The other disease in the top 10 are primarily stalk and ear rots. Based on recent experiences with soybean, the number one foliar disease has been frogeye leaf spot. Table 1 lists these common diseases, conditions where the disease grows quickly, and identification.

Table 1. Conditions, time of year and identification of common foliar disease for corn and soybean in Ohio.

DiseaseConditionsTime of YearIdentification
Gray Leaf SpotProlonged wet, relative humidity >90%75-850 FJuly-OctRectangular lesions with straight margins, tan to gray with no boarders
Northern Corn Leaf BlightWet/humid weather 64-810 FJune-OctLarge lesion 1-6 inches long, 0.5-1 inch wide, cigar shaped brown to tan
Southern RustHumid77-820 FAugust-OctReddish orange pustules on upper leaf surface
Frogeye Leaf SpotRelative humidity >90%75-850 FJuly-AugustCircular to irregular lesions, gray in center surrounded by a deep purple ring

By doing a little preparation you can be ready to know what your strategy is when the planes start flying. 

  • First identify and know where you planted susceptible varieties/hybrid to our common diseases. 
  • Next do some scouting, just prior to tassel in corn or early reproductive stage of soybean. 

With commodity prices where they are we certainly do no want to miss yield due to untreated crop disease. Yet if we have already covered the high priority disease in our seed purchase, why add a fungicide treatment that has no return on investment.

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