Under surveillance

Written by Jason Worthington on .

Toward the end of the growing season, it can be easy for soybean growers to assume the only thing needed until harvest is favorable weather. Often, this thought occurs about the time the soybean plants canopy over the rows and weed control is all but ensured. However, MFA’s Crop-Trak consultants know that their work is just beginning.

Reproductive stages of soybean growth and development are called the critical period for a reason. Lost leaf area is tougher to replace than during vegetative stages. Aborted blooms will never develop into pods. Pods clipped by insects will not produce grain. Stress from any source can impact the size and quality of the grain. And, this is when foliar diseases will impact the crop the most.

Crop-Trak consultants continue thorough field surveillance before and during critical growth stages to ensure fungal disease pressure is kept in check. Depending on the disease, late-season pressure is something that can be mitigated by fungicide applications, but critical to success are early detection, proper identification and correct product selection. These three factors are the tenants of Crop-Trak’s disease control recommendations.

Early detection

Preventative treatments of fungicides are much more popular than they once were. Growers often see benefits in terms of yield and disease control when making a fungicide application at early pod set. This application is often made without intensive scouting to determine the presence of a disease, and even then, gains can be realized.

If yield benefits are often realized without scouting for the disease, why is thorough scouting so important? Because it is possible or even likely for many diseases to start showing up before the onset of pod development. By the time visible symptoms of a disease appear, the plant is already infected. The goal is to detect these symptoms when a small number of leaves are first infected and then treat to prevent further infection. Regardless of whether the fungicide is a “curative” or “preventative” type of fungicide, an application will only prevent further plant tissue damage, not repair damage that is already done. This is the reason for weekly Crop-Trak field inspections up to and all the way through critical reproductive periods. Diseases don’t always follow the calendar, and growers must be ready for an early arrival.

Product selection

Proper identification of soybean diseases not only helps determine whether a field should be sprayed, but also ensures the proper product is selected to remedy the problem. This is important because not all fungicides are the same. Most foliar-applied fungicides fall into one of three families or modes of actions: strobilurin, triazole and SDHI (succinate dehydrogenase inhibitor). All three families have strengths and weaknesses compared to the other two. As an example, strobilurins are excellent against many diseases, but they are ineffective against frogeye leaf spot in areas where it has developed resistance. That means two or three modes of action are preferable to fight resistance development.

Beyond using the correct family of fungicides, the specific product can be very important. For instance, only a limited number of products have activity on white mold, and they must be applied in a specific window.

Selecting the proper foliar fungicide and applying at the correct timing are not only good stewardship practices but also ensure that yield is protected. Even when growers have a planned late-season application, its success can only be ensured by intensive and timely scouting. Growers can expect just that from the weekly field visits provided by the Crop-Trak program.

Proper Disease ID

When it comes to identifying diseases in soybean fields, it’s important to know what diseases are problematic versus ones that have more limited yield impact. Because the differences from one disease to the next can be subtle, it often takes a trained eye to identify them. Crop-Trak’s consultants and agronomists have extensive experience developed by walking soybean fields every year. Their collective expertise provides the resources critical for proper disease identification and control. These pictures show several diseases broken into three categories:

Bacterial Diseases

These seldom have an economic impact on soybean yields. Even if the disease were severe enough to cause economic damage, there are few control options. Bacterial infections are much more commonly found after rain or irrigation. For example, disease can be spread by rain splashing bacteria onto plant tissue.

Large angular reddish brown lesions with yellow halos are indicative of bacterial blight.

Pale green spots with brown centers and a yellow halo are symptoms of bacterial pustule. Spots may contain a blister on the lower leaf surface.

Fungal Diseases of Minor Economic Importance

Though fungicides will have activity on most of these diseases, rarely are these issues of economic importance in Missouri and surrounding states. Making a fungicide application based solely on finding these diseases may be a mistake.

When Asian Soybean Rust first arrived in North America, it was a disease of great concern, but the threat never materialized. This disease prefers tropical environments like those in parts of South America, where it is a very devastating disease. Easily confused with bacterial diseases, ASR lesions lack the yellow halos but contain small, volcano-like pustules.

Downy mildew is a common sight in soybeans. The upper leaf surface will exhibit yellow spots while the corresponding underside will have small, cotton-like growths. The disease can impact seed quality but is rarely of economic importance.

An extremely common disease of soybeans, septoria generally only infects the lower canopy where small brown lesions will develop and can progress and run together. Infected leaves tend to drop prematurely, but economic returns for treating septoria alone are uncommon since the disease usually remains in the lower canopy.

Fungal Diseases of Significant Economic Importance

Proper identification of these diseases and quick response with appropriate control measures such as foliar fungicides will pay dividends for growers. Preventative measures or treating at the first sign of disease is vital to get the highest economic return.

Appearing first in the upper canopy, cercospora leaf blight gives the leaf a leathery, purple appearance. Cercospora may also lead to a condition in seed known as purple seed stain.

One of the most important diseases in terms of economic importance, frogeye leaf spot is a frequent concern to Midwestern soybean growers. It appears on leaves as grey lesions with a red border. In southeast Missouri and throughout the Delta, frogeye is known to be resistant to strobilurin fungicides, making product selection important.

While uncommon in Missouri, white mold is the most damaging late-season soybean disease in other parts of the Midwest. It is indicated first by white growth on infected stems, followed by development of black, hard fungal bodies and leaf death.

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