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The FMC Upside Insights

Weather is a Wildcard, but Tar Spot isn’t

Originally published June 29, 2023

croplife magazine


With hot and dry conditions across the Midwest, disease is the last thing on growers’ minds right now. But don’t let these conditions fool you. Weather can turn on a dime to create an opening for disease threats, especially tar spot.

Tar spot continued its march across the Corn Belt in 2022, causing 116.8 million bushels in yield loss, according to the Crop Protection Network. While some growers had their first experience with the disease in 2022, others dealt with it for their second, third or more years in a row. Regardless, it’s important for growers to prepare for the disease to reappear this year, no matter the weather conditions.

Corn crop facing early season setbacks

To put it bluntly, the 2023 corn crop is getting beat up right out of the gate. Growers faced two scenarios at planting that affected the early developmental stages with season-long ramifications.  

  1. Earlier planted fields contended with rains, resulting in shallow planted crops that stunted root development and led to general stress on the plants at emergence.
  2. On the other end of the spectrum, growers who got in the field after the rain stopped experienced emergence issues due to dry soils or severe crusting. The lack of water has limited nutrient uptake for corn plants and created a deficiency in potassium.

Add plant stress from postemergent herbicide applications into the mix, and we have a corn crop in pockets of the Corn Belt that may not be able to stand up as well to tar spot once it rears its ugly head in July and August.

On the lookout for tar spot

Given the current state of the corn crop, it can be tempting to write off this year’s crop. But growers need to be prepared for the weather to turn and with it, the emergence of tar spot.

If you and your growers planned for tar spot, you need to stick with the plan that was made. In most cases, a corn plant is infected long before symptoms of tar spot are visible. With a 14 to 20 day latent period, tar spot can easily sneak up on you. Left untreated, tar spot negatively affects the plant’s photosynthetic capacity, causing leaves to brown and die early, which can result in reduced yields. On the low end, growers can lose 15 bushels an acre and, in areas of high disease pressure, growers can lose as much as 100 bushels an acre in yield losses.  

Pockets of tar spot lesions can start to pop up in fields around the 4th of July, as a result of infections that started in June. Once growers begin seeing these lesions, that’s when they need to make those critical decisions for fungicide applications. Though it might not seem like it right now given the growing conditions, it is essential to get at least one fungicide application on corn fields that could see tar spot in 2023.

Proactive planning pays

If growers are planning to just make one fungicide application this season, the best return on their investment is going to be a fungicide application when corn is at the VT to R1 growth stage. To take it a step further, it’s imperative they focus their application on the fields with the highest risk for tar spot.

To determine your growers’ risk level for tar spot, I always ask these questions when evaluating the situation with a grower: 

  1. Was this corn field affected by tar spot in the past?
  2. Is the field corn-on-corn?
  3. Does this field have reduced tillage or no-till implemented, causing the crop residue to serve as a host for tar spot inoculum to overwinter?
  4. Is the field near a river bottom, low-lying area or region around the Great Lakes with high soil moisture?

Taking careful steps when selecting a foliar fungicide can pay significant dividends. Better tar spot management results can be observed when growers use a fungicide with at least two or three modes of action. If they’re in an area with high disease pressure or planted a susceptible hybrid, it may warrant three modes of action that provides longer residual protection for the crop. This level of control provided by three modes of action may, in some cases, reduce the need for a second fungicide application later in the season.

Expect tar spot to be as unpredictable as ever this season. It can and will make an unwelcome appearance at any time this season. What we can do to remove the uncertainty of the disease is to have a plan and stick to it. Being prepared for tar spot is the key to the fight because once it’s here, it’s here to stay.

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