Disease Outlook 2020: Some New, Some Old Threats to Corn and Soybeans

“It’s important to remember that 2019 had excess rainfall and high humidity. So, it was a prime environment for a lot of diseases, and with the increase in disease, we have a lot of inoculum in the soil,” says Randy Kool with Syngenta. ( Lindsey Benne )

As Syngenta regional agronomist Randy Kool looks ahead to 2020, he’s ready to be proactive on three diseases in corn and three diseases in soybeans. 

“It’s important to remember that 2019 had excess rainfall and high humidity. So, it was a prime environment for a lot of diseases, and with the increase in disease, we have a lot of inoculum in the soil,” he says. “That sets up 2020 so that if we have the right environment, we could have an explosion in disease.” 

Kool’s geographic area of responsibility is the western half of Iowa. And for his area, he’s got his eye on three main corn diseases: tar spot, gray leaf spot, and northern corn leaf blight. 

  1. Tar Spot: “In Syngenta trials, we’ve seen that 10% to 15% in disease severity can be a 10 to 15 bu yield loss,” Kool says. He notes the identification of this new disease has been rapid from when it was first identified in Iowa in 2018, and in 2019 it was found in 75 to 99 counties.
    Kool encourages retailers and growers to watch for improved and more advanced hybrid ratings for tar spot disease tolerance. And he says to not let this disease steal yield by being aware, scout, apply fungicide when needed, and make sure the fungicide is labeled for tar spot.
    “Timing is critical for the retailer and the grower when managing tar spot. Start scouting at V6,” he advises.  
  2. Gray leaf spot: Kool estimates 35% of corn fields in his area were sprayed for gray leaf spot control in 2019, including all seed corn fields. 
    “That said, the inoculum level will be high,” Kool says. “And farmers should be mindful of hybrid rating when placing hybrids. And that means we also still have to scout because we can still get surprised by the level of severity overall.” 
    He highlights that diseases have latent periods where they are in the plants without any visible expressions. Because of that, he encourages retailers to stay on top of pressures and apply as early as the disease is identified. 
  3.  Northern corn leaf blight: While retailers and farmers have familiarity with this disease, it can still surprise them. 
    “We’ve had years where northern corn leaf blight essentially exploded and it can become quite severe,” Kool says.  

For soybean diseases, here are the three Kool is paying attention to the most for 2020: 

  1. Frogeye leaf spot: “This is the newest soybean disease to Iowa and has increased its presence in the last two years,” Kool says. “We’ve also found most of the frogeye leaf spot is strobilurin-resistant, so we need to be sure we know the active ingredients in a fungicide so it’s effective.” 
  2. Septoria leaf spot/brown spot: “We usually see this disease on the lower leaf from splashing off the residue on the ground,” Kool says. “And while it’s common to see this disease every year, what’s new is that it also has demonstrated strobilurin resistance.” 
    The resistance exhibited by these diseases has Kool focusing a lot of time on educating about active ingredients. And while most fungicides have two active ingredients, he’s a big believer in the new Miravis Neo fungicide which has three to provide preventative and curative control.  
  3. White mold: The first step in managing white mold is knowing the field history, and then second look at varieties in terms of plant type (tall, bushy, etc). 
    “We were surprised by the white mold levels in 2019, but it was because a lot of growers tried to compensate for late planting by increasing plant densities,” Kool says. “And if you have high levels of infestation, the best recommendation is two applications of fungicide.” 

Kool concludes that while he’s watching for individual disease events, it’s very common to have more than one disease in a field. 

“We need to be looking at our disease complex,” he says. “If you add all the diseases together, our pressures are higher and our yield losses are a lot greater.” 

Fungicides and technology can be used together to fight the increased risk of disease in 2020. 

“Fungicides have been improved dramatically over the last two to three years. We can manage the diseases better, and we are also seeing more plant-health benefits.  Results aren’t like what they’ve been in the past. We can easily see 5 to 7 bu increase with the new types of fungicides vs what’s five plus years old,” he says. 

He also encourages retailers and farmers to check out Syngenta’s disease modeling program and app as well as University of Wisconsin’s SporeCaster app. 
 

Related Article: 

“New” Corn and Soybean Diseases Present Seed Selection Challenges

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