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2024 Tar Spot

BY Dairyland Seed Agronomy Team
Although it seems early, there are confirmed cases of tar spot in the Dairyland Seed footprint. Tar Spot is a relatively new disease to our area, and we continue to learn more each season about its behavior as well as control measures.

Small black spots appear on corn leaves when the corn is in V10+ growth stage. The corn plant is between 4 and 5.5 feet tall at this stage. Areas where infection is present have corn that meets these criteria. High humidity and prolonged leaf wetness seem to be the cause of rapid spread of tar spot infection. Fields under irrigation are often where identification of the disease occurs first in a county or region.  

Questions of tar spot and where we see infection are common throughout the growing season. To better alert folks in areas impacted by tar spot, the agronomy team will update a map weekly identifying where tar spot has been confirmed through lab analysis as well as where we have identified suspected tar spot infections. We will update this map throughout the week and publish it in this newsletter for your reference. Counties highlighted in red are areas where tar spot has been confirmed through tissue sample analysis. Counties highlighted in orange are where Dairyland Seed agronomists have identified tar spot and are waiting for lab analysis to confirm infection.
 
Identification 
  • Small, black, circular, raised lesions called stromata appear on leaves, husks and tassels 
  • Stromata cannot be removed from leaf tissue with water, rubbing or scraping with fingernail
  • Commonly misdiagnosed with frass (insect poop), rotten or aged pollen, rust pustules, these can be removed by rubbing the leaf or scraping with your fingernail.
  • Tar Spot is a polycyclic disease, this means that the disease will continue to spread in the plant as long as environmental conditions remain favorable (refer to disease triangle).
 
Disease Triangle 
The three components of the disease triangle are Host, Pathogen and Environment. All three of these must be present for the infection to occur. The component that varies the most is the environment. Humid conditions with extended periods of leaf wetness favor tar spot infection. 
 
 
Treatment
  • Application is warranted if:
          › Infection is found
          › Suspectable hybrid is planted in an area  favoring infection (corn on corn, irrigation etc.)
          › Corn is V10+ in an area of confirmed high  disease pressure
  • Fungicide residual control can last from 14-21 days (about2-3 weeks) depending on rates and products used. If a treatment is made early, pre tassel, a follow up or second fungicide application is likely to be necessary after the residual control period of the first fungicide application has been exhausted.
  • Fungicides containing both a Group 3 (DMI Triazole) and Group 11 (Strobilurin) modes of action are most effective with Tar Spot. Specific product efficacy can be found at the Crop Protection Network website.
  • Treatments after R4 have not shown to increase yield when applying for Tar Spot control. R4 (dough stage) is the cut off timing for applications of fungicide made for Tar Spot control in corn grown for grain.  
  • Applications of fungicide in corn grown for silage should consider the preharvest interval of the fungicide being applied. The preharvest interval is the amount of time that must pass between application and harvest of the crop, most fungicides have a 14-21 day PHI. 

 

Brian Weller
Brian Weller
Western Region
507.456.3034
Rod Moran
Rod Moran
Western Region
507.456.3034
Dan Ritter
Dan Ritter
Central Region
219.863.0583
Branden Furseth
Branden Furseth
Northern Region
608.513.4265
Mark Gibson
Mark Gibson
Eastern Region
260.330.8968
Amanda Goffnett
Amanda Goffnett
Eastern Region
989.400.3793
Ryan Mueller
Ryan Mueller
Eastern Region
989.400.3793
Enjoying our Agronomy Updates? Suggestions for topics you'd like us to weigh in on? Drop us an email at dairylandseed@dairylandseed.com. We'd love to hear from you!
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