The farmer was perplexed. Every irrigated field in his lineup yielded about 30 to 40 bushels per acre off his normal average for irrigated corn, barely topping dryland fields.

“We might as well have not irrigated at all — it was a waste of money,” he complained to his wife.

Then he got to his last irrigated cornfield. Maybe it would be better. No, as it turns out, it was worse! Yields were 20 to 30 bushels per acre lower than any irrigated field so far. And to add insult to injury, dry corners in this field ran 40 bushels per acre higher than irrigated sections.

How can that be? It left him scratching his head — until he did some research.

Can you solve the mystery? You really only need one clue: tar spot. By this point in the season, stalks were deteriorating and beginning to go down in the least-productive parts of the field. Tar spot was so thick in crop residue that it painted the combine black.

What experience says

“My father learned this unfortunate truth a few years back,” explains Dan Quinn, Purdue Extension corn specialist. “He farms in Michigan and has irrigation. He discovered that tar spot can actually contribute to lower yields in irrigated corn versus dryland corn.”

In the farmer’s case this year, he didn’t spray for tar spot. Concerned about investing too much in the crop with corn prices tumbling, he asked a local crops consulting firm to scout the field in early August instead. Not finding much disease, they recommended leaving off the fungicide.

Related:How can you prepare for tar spot in ’25?

Here’s another lesson learned: Don’t rely on a single scouting trip. If you’re going up against tar spot, you need to continue scouting throughout grain fill until the crop reaches maturity, Quinn says.

What science says

Darcy Telenko wasn’t surprised to learn this farmer’s irrigated corn yielded less in a year when tar spot was rampant. Telenko, a Purdue Extension plant pathologist, has heard the story before — just a different chapter and verse.

“This is exactly what we see in northern Indiana when there is a favorable year for tar spot,” Telenko says. “Areas under irrigation actually can be worse than the dry corners.”

Why? Because periods of leaf wetness created by irrigation when corn needs moisture most are also the exact conditions needed for tar spot to thrive. High-moisture conditions are favorable for tar spot, Telenko explains. The more often these conditions occur, the more likely disease will increase if it is a concern in the field.

“It will be a balancing act of providing the crop-needed moisture but also understanding that by adding irrigation, we also increase the disease risk,” Telenko says. “That happens since irrigation increases the number of moisture events during the summer.”

Some suggest running irrigation rigs more slowly and less frequently during the summer, Telenko adds. However you slice it, managing irrigation becomes tougher if tar spot is also a concern.