Tar Spot Threatens Corn Crop
Published: Friday, March 1, 2019
The following is from John Woodmansee, Extension educator, Agriculture/Natural Resources.
A relatively new disease of corn, tar spot, was first confirmed in northern Indiana and north-central Illinois in 2015. Data by Midwest researchers indicate that severe tar spot outbreaks can reduce yield by more than 30 bushels per acre—so it is something to look out for in 2019.
Midwest land-grant University researchers have published a new free publication on the Crop Protection Network website entitled, "Tar Spot," number CPN-2012. Purdue University's Darcy Telenko is a contributing author.
"Tar spot has been present in Indiana annually since first identified in 2015; we believe the weather conditions this year [2018] were just the perfect storm for this disease to spread and increase in severity," said Telenko. "In those fields that were hardest hit the corn experienced a rapid and premature senescence."
Telenko offered these recommendations based on initial observations:
• This disease is here and likely to spread.
• Some hybrids are more tolerant than others.
• Fungicide application can reduce tar spot severity, but timing is going to be extremely important. It will probably be an episodic disease similar to white mold and Fusarium head blight – bad one year and low activity the next. Scout fields and pay attention to weather and Extension reports.
The authors of the collaborative publication wrote, "Tar spot is a foliar disease of corn that commonly occurs throughout Mexico, Central America, South America and the Caribbean." It has now been confirmed in most northern Indiana counties, including Whitley County. Circled areas on the graphic (northern Indiana and east central Indiana) are those that had highest amount of disease and yield loss, according to Telenko.
Tar spot is a fungal disease, caused by the fungus Phyllachora maydis.
"The fungus produces small (.2-.8 inch), round to semi-circular, raised black structures called stromata," said the authors. "The structures form on both the upper and lower surfaces of corn leaves." They add that in severe cases, the structures can also be observed on leaf sheaths and husks.
The best way I know how to describe this in more common terms is that these structures literally look like small, tar-like spots that can't be rubbed off. They are a bit raised from the leaf surface, so if you run your finger over the leaf it will feel bumpy.
"Occasionally, tan to brown lesions with dark borders can develop that surround the stromata," said the authors. "These are known as fisheye lesions because of their appearance."
The authors said that cooler temperatures (60-70°F) and high relative humidity (greater than 75 percent) favor tar spot development. They added that tar spot has been observed most often during mid- to late grain fill (growth stages R3-R6), and usually on leaves below or near the ear leaf.
A couple of other common diseases may be confused with tar spot. The rust diseases of Common and Southern Rust in later stages of the disease may have lesions that began as bright orange or red, but later became dark brown or black. The authors said that rust pustules erupt through leaf surfaces, and you can rub off the spores. Physoderma brown spot forms purple-to-brown lesions along the midrib. The lesions are embedded in leaf tissue, while tar spots are raised.
The authors suggested four main management practices to help reduce tar spot: manage residue, rotate to other crops, avoid highly susceptible hybrids and investigate fungicides.
To find the new publication go to https://cropprotectionnetwork.org/library/. If you suspect the disease in your corn this year, samples may be sent to the Purdue Plant and Pest Diagnostic Laboratory. Find information on the Purdue lab at www.ppdl.purdue.edu. Follow Telenko on Twitter @DTelenko or at https://extension.purdue.edu/fieldcroppathology/. She and her colleagues will be posting weekly seasonal updates about this and other field crop disease issues.
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