Southwest Michigan Field Crops Update – August 21, 2025

Aug 25, 2025

By Nicolle Ritchie and Angie Gradiz

Weather

Much-needed precipitation fell early this week, ranging from a little over half an inch to two inches or more. Daytime temperatures continued to be relatively high, but nighttime temperatures were lower than they have been most of the summer. This upcoming week will bring a cooldown in daytime and nighttime temperatures, although little precipitation is expected for southwest Michigan.

Crops and pests

Soybean pod fill continued. Most soybeans have seeds of 1/8 inch or larger (R5), and some are close to full pod fill (R6). Watch for sudden death syndrome, brown stem rot and red crown rot, all of which have similar symptoms. Red crown rot is a disease new to the Midwest, and it has not been confirmed in Michigan yet. Several insects contribute to soybean defoliation at this time of year, including grasshoppers, Japanese beetles, bean leaf beetle and more. During R3 to R5, the action threshold is 10% insect defoliation. The article “Soybean Defoliation: It Takes a lot to Really Matter!” from the Ohio State University Extension has helpful images to evaluate defoliation and even a quiz to test and train yourself.

To evaluate soybean defoliation, first find a representative plant. Then remove the two trifoliates with the most defoliation and the two trifoliates with the least defoliation. Remove the best and worst leaflets from each of those trifoliates. This should leave you with one representative leaflet from each trifoliate and four leaflets in total. Finally, rate the defoliation of the remaining leaflets to come up with overall defoliation. In most cases, soybean defoliation is below threshold and does not require action. However, check regularly to make sure that significant feeding is not occurring on the pods.

Grain fill continued in corn. More acres entered R5 (dent), at which point indentations are visible on the outer tips of kernels. During this time, liquidy white parts of kernels are converting to starchier, more yellow parts. The visible delineation between these two parts of the kernel is referred to as the milk line. Male row destruction continued in seed corn, and defoliation sprays on some early planted acres are probably not far out.

Tar spot is present in most of the counties in southwest Michigan. The Crop Protection Network tar spot of corn map contains a helpful table for determining if a tar spot fungicide application is beneficial at various crop stages. As more fields enter R4 and R5, the benefit of applying a tar spot fungicide decreases. Southern rust was confirmed in St. Joseph County this past week. For more information on southern rust and how to manage it, check out the article “Southern rust an occasional threat to Michigan corn” by Michigan State University Extension field crops pathologist Martin Chilvers.

Source : msu.edu

Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.