Tar Spot Found to Thrive on High-Humidity Days

Before planting even starts, the inoculums for in-season diseases are likely already on residue covering the soil. That includes the fungus tar spot. 

Showing up as black speckled dots on plant leaves, tar spot has the potential of severely reducing yield, with some reported losses of 20 bu. to up to 100 bu, per acre. The fungus zaps plant health ultimately reducing ear size and kernel fill as it increases stalk rot and lodging. 

Spot Progression

Farm Journal Field Agronomist Missy Bauer, and her husband, Bill, said 2021 was much worse than 2022 in terms of tar spot.

Take a look at this video to see how an infected field evolved from the first of symptoms of the disease on Aug. 20, 2021. 

About 12 days later, you can see how the plants began suffering. By the end of September, the field was completely dead. 

What to Watch

Bauer says while many think rainfall and temperatures are a key driver, there’s another indicator to watch. 

“Universities have found when average daily humidity is above 75%, crops are at risk," Bauer says. 

Purdue University reported in June 2022 that Tar spot pressure in corn is fueled by:

•   Cool conditions (60-70 degrees F)
•   Humid conditions (>75% relative humidity)
•   Prolonged leaf wetness (>7 hours)

"Tar spot pressure is typically higher in areas such as those closer in proximity to the great lakes (e.g., Northwest Indiana), river bottoms, and irrigated corn acres," Purdue researchers said in the report. 

Get Ahead of the Game

Due to tar spot's ability to overwinter and reappear in following years, Bauer points to a proactive approach. She recommends:

•    Scouting during the key development stages in mid-June through July. 
•    Selecting naturally resistant hybrids in high pressure areas.
•    Using newer fungicide technology or products with multiple modes of action.
•    Planning ahead to make sure treatment products are available.

Bauer offers further material on corn diseases, including everything from tar spot to leaf blight on her site, B&M Crop Consulting.