Commodity Update: Tar spot, weather affect corn yields in Michigan and Wisconsin 
GreenStone tar spot on corn


It’s harvest time in Michigan and Wisconsin, and while high yields are being forecasted, disease and weather may affect individual farms. 


Tar spot is a corn disease that was identified in the United States in 2015, in Illinois and Indiana. Tar spot has also been confirmed in Wisconsin and Michigan, as well as in Florida, Iowa, Minnesota, Missouri, Ohio.  


It’s caused by the fungus Phyllachora maydis, and it produces small, round, raised spots called stromata. They form on the leaves and in more severe cases, on the leaf sheaths, husks, and tassels. The disease affects the growth and can cause major yield loss. For instance, during a tar spot outbreak in Indiana in 2018, farmers lost 20-60 bushels per acre. 


Integrity in question 
“We are hearing from producers that are starting to take the corn early,” said Jim Zook, executive director at Michigan Corn Growers. “They’ve been checking the integrity of the stalk and finding it’s gone. So, they’re harvesting the crop earlier than normal.” 

However, even though it’s earlier in the season, farmers are still seeing fairly good yields. 

“When they’re getting it in, they’re pleasantly surprised by the moisture content. They are not surprised that the test weight has decreased, as is expected with tar spot,” he said. 

According to the U.S. Department of Agriculture’s National Ag Statistics Service’s recent crop forecast, Michigan's average corn yield is expected to be 169 bushels per acre, which is 15 bushels above last year. If this is reached, it will be a record high by 7 bushels. NASS is also forecasting record high yields in California, Illinois, Indiana, Kentucky, New York, North Carolina, Ohio, Oklahoma, and Pennsylvania. 

Corn harvest times will vary this year due to the wide variety of planting dates, but working to get it out earlier is a goal.   

“Tar spot started in the southern end of Michigan, and everybody else is paying attention as it creeps up to the middle part,” Jim said. “Once word gets out that guys are getting corn harvested now, you’ll see more people start getting it out of the field.” 

Tar spot makes harvesting corn difficult, if not impossible. 

“What will happen is because the stalks have been weakened, as you push on it, it snaps close to the ground,” Jim said. “If we get continued strong winds, it will snap over and be a bear to harvest. Another factor is the longer it stays out there, the more chance that the disease will affect it. If the plant is still growing, it’s going to use all of its energy to survive, and that will pull out of the ear and reduce yield and test weights even more.” 

Zook said he hasn’t seen tar spot this severe in Michigan before this year.  

“Early on, we thought we were safe with the dry weather, but when the rains came in, they came in with a vengeance,” he said. “There are a number of growers that are doing their own type of test plots with products that help alleviate the impact of tar spot, so it will be interesting to see the data on the timing, and all of the different factors from across the state.” 

Jim’s advice is to harvest the corn as soon as possible, to avoid even more damage. 

“Go get the corn when you can, get it dried down, in your bin or hauled off to the market,” he said. “It’s going to be fragile, and we’ve seen in years past when corn has stood in the field and gotten too much moisture, then we have the problem of vomitoxin.” 

The USDA National Ag Statistics Service estimates Wisconsin farmers will see an average yield of 172 bushels of corn per acre, which is 2 bushels lower than last year. Total corn production is forecast at 506 million bushels across the state, about two percent lower than the total in 2020. 

However, while southeastern Wisconsin started with drought conditions, it received adequate rain in July and August. Unfortunately, the rain combined with cooler temperatures produced the perfect conditions for tar spot in areas of the state. 


“We have a section north of Madison along I-94 that has a lot of tar spot, and people are starting to get in the field to get it out,” said Nicole Wagner, executive director at Wisconsin Corn. “Tar spot is moving farther north. It used to stay very southern, and we only saw it south of Madison, but now it’s moved north of Madison.” 

Wisconsin farmers generally start combining corn 10-14 days after chopping silage.  


“The northern area of the state near River Falls and Chippewa Falls started at the end of September this year,” Nicole said. “Everything is starting to dry down nicely. We’re in a good spot.” 


Wisconsin also has varied harvest times and quality due to rain. The U.S. Drought Monitor shows southeastern and northwestern Wisconsin are currently under severe drought, while southern and northwest Wisconsin are experiencing abnormally dry or moderate drought conditions. 


“Though some areas only had drought early, others had a severe drought all season long,” Nicole said. “We’re seeing that effect on the crops.” 


Wisconsin Corn is working after harvest to look at how the ag statistics are monitored and reported, since they vary greatly from week to week. They are also looking at changing how they do production research. 


“We’re going to be working with UW-Madison to discuss more about what growers are looking for from researchers,” Nicole said. 


Impact to your crop insurance policy 
David Moll, GreenStone crop insurance manager, predicts that even with the challenges of this year, farmers will still see above average yields. 


“The farmers I work with in the southern part of Michigan are concerned about it, but most customers believe their above average yields with higher prices will exceed their guarantees,” David said. “Per the policy, if enough bushels don’t make it in the bin, and if Mother Nature is causing the problem, then it is a covered cause of loss.” 



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