Crops

Common Soybean Crop Diseases and Their Management

Published:

September 30, 2024

Key Takeaways:

Soybeans are generally easy to grow but are susceptible to several types of diseases causing yield loss if untreated.
The most common causes of soybean diseases are fungi, viruses, nematodes, and bacteria.
Bacterial infections and nematodes affecting soybean fields cause billion-dollar losses annually.
Disease management commonly includes the use of fungicides, crop rotation, and beetle control.

In the 2020s, soybeans are a big business in America. Soy has been one of the fastest-rising cash crops in the US, nearly doubling in 20 years: from 922 million bushels in 2004 to 1,789 million bushels in 2023. Soy is popular around the world, and the demand is likely to continue to rise. It's also a great rotational crop since it fixes nitrogen and reduces reliance on synthetic fertilizers.

While soybeans are generally easy to grow, they are fairly susceptible to disease. Estimates are that US farmers lose over $4.5 billion dollars annually due to soybean blights. Considering the agriculture industry as a whole — processing, packaging, retail, etc. — the overall national economic loss approaches $100 billion, just from diseases alone.

This isn't just costing the ag industry money; it also represents a substantial loss of food that the world increasingly needs to keep the growing human population fed. So, let's look at soybean disease symptoms, as well as strategies farmers can use to reduce crop vulnerability.

Know Your Soybean Plant Diseases

Soybeans are susceptible to a wide variety of diseases from different sources, be it seedling diseases, leaf diseases, stem rot, and others. The key causes include fungi, bacteria, viruses, and nematode (roundworm) infestations.

Fungal Diseases

Soybean Rust. Originating in Asia, soybean rust made its way to America in the early 2000s and has caused significant problems since. As the name suggests, this fungal outbreak causes small red/brown spots across the leaves — like rust on metal — making it easy to identify. Left unchecked, it can ruin entire crops. Soybean rust disease is primarily seen in warmer regions, as hard freezes will kill it. Currently, as with many of the diseases here, fungicides are the most common solution.
Frogeye Leaf Spot. This fungus causes large, easily seen tan-brown spots surrounded by a reddish or even purple ring. As the infection expands, so does the ring. Eventually, the inner areas will begin to host small black spots, which are the spores. Like soybean rust, frogeye needs warm temperatures to thrive and is primarily seen in the south. The spores can survive in crop waste from previous seasons, and can also infect a variety of other leafy crops. Prevention is better here: rotate crops away from species susceptible to frogeye fungus. Fungicides can be used, but use them sparingly. Resistant strains of the fungus have begun to appear.
Septoria Brown Spot. Brown spot is another common soybean leaf disease, starting as small brown spots that steadily grow. As the fungus consumes the leaf, it turns a sickly yellow on one side. Brown spot soybean disease prefers warm, wet conditions, and can be transmitted between crops. The good news is that unless the outbreak is exceptionally bad, it typically will not affect the yield. Once again, crop rotation and fungicides are your best options for fighting it.
Sclerotinia Stem Rot (aka White Mold). White mold is among the more pernicious soybean stem diseases, capable of ruining entire harvests. It begins as white spots on the soybeans' stems, which then turn into fluffy growths that continue to spread. White mold needs cool temperatures and moisture. Frequent damp conditions, along with too much cover shade, can encourage its growth. Worse, the spores can survive for years in the soil, after an outbreak occurs. Increased plant spacing can reduce the spread. Otherwise, fungicides are called for.

Viral Diseases

Viruses are among the biggest threats to soybean crops, capable of widespread devastation. As with viruses in humans, direct treatment is impossible, so proactive management is a must. In particular, watch out for:

Soybean Mosaic Virus (SMV). Possibly the single biggest threat to soybeans worldwide, outbreaks of SMV have been reported to destroy up to 94% of harvests, although a more typical range is 8-35%. Symptoms can vary depending on the strain of the virus but typically involve a mosaic of light and dark green patterns. In addition, the leaves will often shrivel and curl downwards, almost looking like cooked greens. The virus is most commonly transmitted through infected seeds, so be vigilant in sourcing your seed. No SMV-resistant soy species exist, and nor does a cure. Drastic action may be required if a field becomes infected.
Bean Pod Mottle Virus (BPMV). Another killer, BPMV, has been attributed to killing up to half the crops in some fields. Symptoms are similar to SMV — dark/light green mosaics and shriveled leaves. And, as with SMV, there are no resistant soybeans or chemical solutions. The difference is that BPMV is spread by a soybean pest known as the bean leaf beetle. It spreads the virus as it feeds on leaves in a field. Control the beetle, with either pesticides or natural remedies like insectivore birds, and you'll reduce your crop’s susceptibility to BPMV substantially.

Nematode Diseases

Nematodes — more commonly known as roundworms — are the other major threat to soybeans, causing soybean root diseases. These microscopic pests can be very hard to eliminate, once they've taken hold.

Soybean Cyst Nematode. Named for the cysts that form when eggs are laid on soybean roots, soybean cyst nematodes have been credited with over a billion dollars in economic loss alone. The parasites eat their way into the soybeans' roots, potentially destroying the entire plant. It's almost impossible to spot a cyst nematode infection without digging up roots until the plant has already suffered considerable damage. Look for stunted, wilted, or yellowing plants without any clear microbial cause. Soil sampling, looking for the worms' eggs, is usually required to confirm a case. Containment is a combination of crop rotation and soil treatment with nematicides. Resistant varieties of soybeans are under development.
Root-Knot Nematodes. Another roundworm that attacks soybean roots, root-knot nematodes produce similar symptoms to cyst nematodes — inexplicably stunted, withered, or yellowed crops. If an infection is suspected, visual inspection of the roots may reveal severe galling. These are the eggs of the nematode. Most chemical methods of containing an outbreak are ineffective. If one is discovered, rotating crops to those not affected by root-knot nematodes — so they die in the soil — is one of the only viable solutions.

When you have questions, we'll help you find the answers you need to take control of your fields and the harvest you care about. Contact Cropler to discover more about our products and the ways we help you proactively protect your operations from the effects of crop diseases. Discover More

Bacterial Diseases

Bacterial infections can also occur in soybeans, although they're less common than fungal infections. The two most common bacterial diseases are bacterial blight and bacterial pustule, both soybean foliar diseases. Both have symptoms very similar-looking to soybean rust, although the spots and lesions created are more angular and jagged, compared to the smooth rounded spots of soybean rust.

We aren't giving them a deep discussion here because these diseases rarely affect harvest yield. They look unsightly, but shouldn't harm your bottom line. Treating them is usually more trouble than it's worth.

Looking to the Future of Soybean Disease Management

Soybean disease management is multi-faceted, encompassing the development of resistant soybean plants, better chemical solutions, as well as the adoption of smarter technology-based detection methods.

Cropler’s revolutionary combination of weather-resistant agri-cameras and smart online dashboards allow you to monitor your fields in detail, 24/7/365 from anywhere in the world. This allows you to quickly spot signs of disease, without having to constantly monitor the fields in person, keep track of their spread, or, for example, monitor fungicide efficacy for control of soybean diseases.