“Tar spot really wiped out stalks, especially along the edge of the field where the aerial applicator couldn’t spray fungicide,” a farmer told his neighbor. “By harvest, it looked like stalks were just falling apart.”

The neighbor responded: “I had a heavy tar spot infection in a field I did not spray, and I saw up to 70% lodging when I did the push test. I pushed on 100 plants down the row, and less than 30 snapped back.

“But it wasn’t due to tar spot — I heard it just affects leaves. I split open stalks and found signs of stalk rot. That’s what took my stalks over, not tar spot. Yes, the tar spot stressed plants, but then stalk rots took over.”

“No, I think tar spot wiped out stalks by themselves,” the first farmer insisted.

Both right, more or less

These two neighbors took their dilemma to Dan Quinn, Purdue Extension corn specialist. Does tar spot affect more than leaves? Does it attack stalks, too? Or does it just weaken plants so stalk rot invades and takes over stalks?

“Basically, both guys are right,” Quinn says. “True, tar spot attacks leaves. But if the infection is full blown and leaves can no longer produce sugars from photosynthesis, corn plants seek nutrients elsewhere to continue filling ears.

“One of those places where plants begin to draw down nutrients from are stalks. The result is that stalks lose integrity and become hollowed out. Technically, tar spot fungi don’t invade stalks, but instead, by destroying leaf tissue, it initiates the process which leads to the plant itself pulling nutrients out of the stalk, weakening it.”

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

However, that’s only half the story, Quinn says. “Once stalks are weakened, they are prime targets for disease organisms to invade,” he explains. “In fact, pathogens that cause various stalk rots can get into stalks more easily because they are hollowed out inside, as plants attempt to finish filling corn kernels on the ear with nutrients from stalks. Once inside the stalk, the stalk rot organisms can multiply and develop infection rapidly.”

Quinn says this is what’s likely to happen in a field with a severe tar spot infection: Tar spot kills leaves, forcing plants to seek nutrients to finish filling ears by mobilizing nutrients in stalks, starting at lower nodes. After this process hollows out stalks, bona fide stalk rot organisms invade, further weakening stalks. Soon, they can’t withstand mild windstorms or even a gentle push from a farmer doing a push test to check for lodging potential.

Pinkish discoloration inside the stem is often indicative of infection by fusarium stalk rot, Quinn says. Blackish specks or discoloration on the outside of the stem or in the pith, especially at nodes, may signify that anthracnose stalk rot is at work.

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