Cornfields at the beginning of harvest should be scouted for signs of stalk rot. If a field has 10% or more of the plants with stalk rot, the risk of significant lodging is high enough to justify harvesting the field on the early side. Especially if a big storm with wind comes along, you don’t want stalks and ears falling to the ground.

Over the past couple of weeks Iowa State University Extension field agronomists have been doing stalk rot assessments at several of the ISU Research Farms around the state. They include the Southeast Research and Demonstration Farm at Crawfordsville, the McNay Research Farm at Chariton, and the ISU farm at Ames. While the corn plants seemed to be standing well, minus where the raccoons had fun in one of the trials, it was not uncommon to find stalk rots in the plants the agronomists sliced open to evaluate.

Initial symptoms of rot inside stalk
“This is a good reminder that initial symptoms of stalk rots are not easily observed,” says Rebecca Vittetoe, ISU Extension field agronomist covering southeast and south-central Iowa. “Unfortunately, we typically don’t notice stalk rots until either the exterior stalk tissue is affected, or lodging or snapping of the stalks has occurred.”

ANTHRACNOSE: The shiny black blotches on this stalk rind are characteristic of anthracnose stalk rot. Photo was taken Sept. 22 at the ISU Southeast Research and Demonstration Farm. 

Vittetoe and Alison Robertson, ISU Extension plant pathologist, recommend farmers check cornfields now for signs of stalk rot, to prioritize which fields should be harvested first. Rotted stalks are good candidates for lodging and yield loss. The ISU specialists offer the following observations and suggestions for farmers and crop scouts to use when evaluating fields for presence of stalk rot.

Use ‘pinch test’ to check for stalk rot
Vittetoe says, “We encourage you to get out and take some time to evaluate your fields for stalk rots. Check the stalk firmness by pinching the lower internodes — the pinch test. If the stalk crushes easily by hand, then the integrity of that stalk has been reduced by stalk rot and the risk of lodging increases.”

An alternative to the pinch test is the “push test.” With the push test, you push the plant tops approximately 30 degrees from vertical. If the plant fails to snap back to vertical, that means the stalk has been compromised by stalk rot. Either method works fine.

With both tests, you should randomly select a minimum of 100 plants in the field.

Robertson and Vittetoe suggest as you are walking through the field, stop at five different spots and evaluate 20 plants. “You want to get a good representation of the field,” notes Robertson. “If more than 10% of plants in a field exhibit stalk rot symptoms, that field should be one of the first or next fields harvested to reduce the potential for plant lodging and for yield loss.”

Several kinds of stalk rot can infect corn
Anthracnose, gibberella, diplodia and fusarium are the usual culprits in Iowa. Anthracnose is the most common stalk rot faced by corn growers worldwide, with yield losses reaching as high as 40%, as a result of reduced ear size and stalk lodging. Infection is favored by warm temperatures and high humidity. Anthracnose has both a leaf phase and a stalk phase in corn. Splitting open the stalk reveals degenerated pith tissue, usually dark gray to brown in color.

For more information, read this article: Towards a Successful Harvest: Stalk Rots and Standability Issues.

CHECK INSIDE: Discoloration and rotting of the pith (inside of stalk) are caused by stalk rot. Photo was taken Sept. 22 at the ISU Southeast Research and Demonstration Farm.