First noticed in northwest Indiana, tar spot has crept across the Corn Belt over the past eight years. Today, the disease is present from Iowa to Pennsylvania and has the potential to decimate corn yields.
With few resistant seed varieties available, corn farmers rely on perfectly timed fungicide applications to keep the disease at bay.
“It’s huge,” says Ruhiyyih Dyrdahl-Young, a corn fungicide product manager for BASF. “You can lose 15 bushels, on the conservative side, to as much as 100 bushels an acre. This generation of corn farmers has never seen a disease like this before.”
While the disease was discovered in the United States in 2015, it wasn’t until 2018 that farmers truly saw the impact tar spot could have on corn yields.
“After the first detection in 2015, tar spot didn’t initially cause a problem,” says Darcy Telenko, a field crops pathologist at Purdue University. “Over time, inoculum continued to build until the 2018 epidemic when favorable conditions caused the disease to impact fields around Lake Michigan. Every year we continue to have high risk because that inoculum is present and continues to build.”
While the disease remains a threat each year, several factors will influence the severity of impact.
A FAST FOE
An obligate parasite, tar spot is difficult to grow and study in a lab culture, making it harder for scientists to understand the disease’s biology. While tar spot originated in Mexico, it has adapted easily to the Midwest climate, able to overwinter even in harsh conditions.
Early signs point to greater tar spot infections in high humidity and cooler temperatures. The disease can spread quickly after just seven hours of leaf wetness.
“It’s very fast moving,” says Steven Schany, western agronomy manager with AgriGold. “It can be blown in through wind and splashed in through rain, so very aggressive disease management is required.”
Scouting, especially when tar spot has been located nearby, is key to getting ahead of the disease before it explodes.
“With the nature of the disease and how aggressive it is, this is truly something all farmers need to be scouting for throughout the year,” Schany says. “You can see it as early as V8 in some fields, but I would say the most critical time is going to be in the reproductive stage.”
IDENTIFYING TAR SPOT
The disease will appear as black spots that do not rub or wash off. The spots will start on the lower canopy and move up the plant.
A long incubation period means if the first signs of tar spot are missed, it may be too late to save yields. In favorable conditions, tar spot can quickly take over the entire field.
“Have fungicides ready to go and a plan in place to be proactive against it,” Schany says. “You’ve got one to two weeks, if conditions are right, before it explodes, so be prepared to make the decision to spray your field with a fungicide. Then be diligent at re-scouting because fungicide is only going to protect for so long with that type of aggressive disease.”
Fungicide applications at the VT/R1 stage provide the best return on investment, Dyrdahl-Young says. Depending on how severe the outbreak is, several applications of a quality fungicide may be necessary.
“Generally, all our products labeled for corn can control tar spot to some level, but there’s some differences between product efficacy and longevity,” Telenko says. “That being said, our fungicides have a window of protection. If we get three to four weeks of protection, but go back and check six weeks later, you’re going to think the fungicide didn’t work. The problem is the product ran out of its activity, and tar spot is going to find any opportunity to continue to infect green leaves.”
Working with retailers and trusted advisers before seeing problems in fields can help farmers get products faster and make more timely applications.
“We think there’s some benefit to putting fungicide out there to help control disease in general, but a lot of folks are going to wait to take action until they see something,” says Jeff Moon, a market development specialist with Corteva. “At least have those conversations and a plan in place to know what you’re going to do if tar spot pops up. Having those conversations allows your retailer to make sure it has the products it needs on hand to execute that plan.”
Once tar spot is found in a field, it’s there to stay. Even rotating to a nonhost crop doesn’t seem to reduce risk in later years. When making planting decisions after a tar spot infection, seed choice can help bolster a field’s tolerance.
“Start managing tar spot by selecting the right hybrids,” Schany says. “Industry-wide, there’s a lot of susceptible hybrids on the market. It will take time to develop tar spot resistant corn hybrids, but we can offer farmers products with a high level of tolerance.”
TAR SPOT IS TOP OF MIND
In a 2022 survey commissioned by AgReliant Genetics, more than half the farmer respondents cited tar spot as a potential disease that may impact their area. One-third of those respondents listed tar spot as their first concern.
Source: agriculture.com
Photo Credit: gettyimages-songdech17
Categories: Pennsylvania, Crops, Corn