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Determine cause of stalk rot

Anthracnose stalk rot
SEE AND SMELL: Notice the brownish tissue around the node where the cornstalk is split open. The stalk also emitted an odor that goes along with rot and decay. These are signs of anthracnose stalk rot.
Corn Watch: Anthracnose stalk rot is detected in the field before harvest.

Late in the season, on his last scouting trip before harvest, Dave Nanda does the “push test” on cornstalks. He walks down the row and counts off 100 plants. Then he walks back up the row, giving each one of the 100 stalks a strong push. If they snap back, they still have stalk integrity and likely aren’t affected by disease. If they fall over or fail to snap back, it’s time for a closer look.

“They are likely infected by stalk rot at that point,” says Nanda, a former plant breeder who is now an independent crop consultant based in Indianapolis.

When Nanda performed his test at a couple of locations on both hybrids in the Corn Watch ’18 field, he found 4% stalk rot for one hybrid and 2% for the other. In the Corn Watch ’17 field, the numbers were considerably higher before harvest. Even so, when stalk rot reaches 4%, Nanda believes it’s time to mark the field for harvest to avoid possible losses due to lodging and ears that the corn head can’t pick up.

Seed Genetics-Direct, Washington Court House, Ohio, sponsors Corn Watch ’18.

Identify diseases
Nanda says whenever you find a disease in the field, you ought to identify it, if possible. That allows you to consider options for next year, even though it’s perhaps too late to make any changes this year. You can use a simple pocket guide, such as the Purdue University Corn and Soybean Field Guide, or the smartphone or tablet versions of the Purdue guide to make identifications in the field.

In this case, the stalk rot that caused a few plants to fall over was easy to identify, Nanda says. The stalk rot version of anthracnose forms a shoe-polish effect on the outside of the stalk, usually starting near the base. Look for black specks on the outside.

SHOE POLISH LOOK: Check out the blackish color and presence of black specks on the stalk. These are outward signs of anthracnose stalk rot.

The next step is to slice the stalk open with a knife, Nanda says. Look for discoloration. In this case he typically found light-brown tissue that was beginning to deteriorate near nodes inside the stalks. There was also a slight rotten odor, Nanda notes.

Anthracnose can overwinter in corn residue. There is also a leaf blight phase of the fungus. This year, however, gray leaf spot and northern corn leaf blight were more common leaf diseases. A considerable amount of both diseases was present in the Corn Watch ’18 field late in the season. It was sprayed with a fungicide around tasseling, and the fungicide application appeared to slow down disease development during pollination and the early part of grain fill.

If you know you have a fungus in a field, you may want to pay attention to disease resistance of hybrid choices when you come back with corn, Nanda concludes.

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