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Crop Disease Field Guide


One of the most stressful factors of the growing season is learning how to navigate crop diseases. There are countless diseases that take over fields and lower yield potential—hurting your bottom line. Here is a quick rundown of some of the most common diseases to be on the lookout for in your corn and soybean fields this summer.


Tar spot is a fungal disease making its way across the Corn Belt. There are still many unanswered questions about the fungus, but researchers have found that cooler temps, around 60-70ºF at night, and high humidity create the ideal environment for tar spot disease to grow.

Tar Spot Disease

If tar spot disease hits your field, the biggest impact you will see is yield loss (and no one wants that). Tar spot will turn the leaf brown and make it die early. It affects corn in the beginning R stages (reproductive) with a reduced ear size and, if affected later, can be linked to poor kernel fill and ear weight.

Reports show an average 30 bushel per acre yield loss when tar spot is in fields and left untreated. There have been some growers with severe infestation that resulted in around 60 bushel per acre losses. Observations also suggest that stalk rot and lodging are increased when tar spot severity is high. Severe tar spot can also reduce forage quality.

Leaves with tar spots have small, raised, round or irregular-shaped black spots, which are fungal structures called stromata. Occasionally, tan to brown lesions with dark borders can develop, known as fisheye lesions because of their appearance. Tar spot can be identified visually, however, a laboratory diagnosis is required to distinguish it correctly from other pathogens as it can get confused or mistaken for rust.

Soybean Cyst Nematode

Soybean Cyst Nematode

Although it toes the line between disease and pest, soybean cyst nematode (SCN) is one of the most devastating infestations that can hit a soybean field. This pathogen causes yield losses of more than 30 percent, so it’s important to be prepared for it.

Because juvenile nematodes are microscopic, it can be difficult to spot the individual organisms, but as they grow, you may notice the small, white bodies of adult female nematodes on soybean roots. The Soybean Research and Information Center from the Soybean Check-Off explains that many farmers don’t know their fields are infested with soybean cyst nematode, as you often can’t tell SCN is present from looking at the field.

While soybean cyst nematode is nearly impossible to eliminate, you can manage the pathogen by implementing a crop rotation program or intermixing non-host plants into your soybean fields that are resistant to the nematodes to slow reproduction.

Common Rust of Corn

Common Rust of Corn

Common rust of corn is a fungal disease caused by Puccinia sorghi. It is one of the most widespread and economically important diseases affecting corn crops worldwide. The disease is favored by warm, humid weather and can cause significant yield losses if left unmanaged.

If you see small, circular to elongated, reddish-brown to tan lesions on the leaves, leaf sheaths and husks of the corn plants, you may have common rust. These lesions can be scattered or clustered and may have a yellow halo surrounding them.

As the disease progresses, the lesions may become more numerous and merge, causing significant damage to the leaves. Severely infected leaves may become twisted, curled, and may prematurely die, reducing the plant's photosynthetic capacity. In extreme cases, common rust of corn can also affect the ear, causing yield losses.

Soybean Rust

Soybean Rust

Similar to common rust of corn, soybean rust is another major disease to look out for in soybean fields. Soybean rust is favored by the same warm, wet condition as common rust of corn. While it can occur during any stage of soybean development, it is most common after plants begin flowering and developing pods. Soybean rust can cause yield losses of more than 50 percent.

Soybean rust resembles common rust of corn in appearance, with tan or reddish-brown lesions that develop beginning on the underside of leaves. The lesions develop small pustules, or blisters, which release masses of tan spores when they break open on the leaf. These spots may also extend to pods or stems as the disease progresses.

Root Rot

Root Rot

Corn and soybean plants are both susceptible to root rot, caused by a variety of fungi. Root rot is common in fields with poor drainage and high moisture levels. This disease damages and kills seedlings and plants throughout the growing season, hurting yields at harvest. Root rot can also progress into stalk/stem rot.

If root rot hits fields early in the season, it may kill seedlings in the ground or just after emergence. If the disease is delayed, yellowing, wilted leaves can signal potential root rot in your fields. When you see these signs, pull a couple of plants and examine their roots to confirm the diagnosis. Rotted roots are mushy and deteriorated, and they may have dark brown lesions.

For all of these diseases, prevention and treatment can be difficult, but the main ways to protect your crops are to select disease-resistant seed varieties, apply fungicides at the proper time and strengthen your plants’ stress mitigation and overall health with BW Fusion’s biological crop nutrition products.