Southern corn rust found its way north

The 2025 corn crop is generally favorable across the Midwest, despite its share of challenges. Warm, humid conditions have created a favorable environment for pathogen growth in the fields. Foliar diseases — such as gray leaf spot and northern corn leaf blight — are nothing new for most corn growers.

However, for some of us in south-central Minnesota, it was our first experience of walking in the corn fields in August and exiting with an orange tint on our arms. You may have also noticed an orange cloud of dust following a chopper as it moved through the field at harvest. Although not widespread, southern corn rust defied its name and made its way north into some Minnesota and South Dakota corn fields. To no surprise, reports were more prevalent in the southernmost counties of these states, along with Iowa, Wisconsin and Illinois.

What is corn rust?

Corn rust is a fast-developing fungal leaf disease that can spread rapidly in corn fields when wet weather patterns persist for an extended period. Unlike other major foliar diseases of corn in North America, the rusts do not overwinter in the Midwest Corn Belt. All rust pathogens are obligate parasites, which means they can only survive on a living host. Consequently, each new growing season starts with a clean slate when it comes to southern rust. Infections in this region result from prevailing weather systems carrying spores northward from the Southern states.

Two kinds of rust can affect corn in North America: common rust (Puccinia sorghi) and southern rust (Puccinia polysora). Although these rusts have similar lifecycles, their impact on the crop can be very different. Southern rust is generally much more damaging to corn than common rust due to its ability to rapidly develop and spread under favorable conditions. Southern rust can cause substantial yield losses in corn plants under severe infestation of the fungi.

Rust outbreaks generally occur during the ear-fill period of corn growth. However, in recent growing seasons, southern rust has occurred further north and earlier in the season than is typical for this disease. Several regional agronomists have shared that the southern rust is more detrimental to corn yields than common rust this season across the Midwest, especially as we move further south.

Impact on the corn plant

Yield loss due to rust depends on the timing of infection, the amount of leaf area damaged and the location of damaged leaves on the plant. Yield losses will be higher if significant damage to upper leaves occurs early in the life of the plant. If damage is confined to lower leaves of the corn plant or occurs in the later reproductive stages of development, little economic loss would be expected. Generally, the latest-planted corn in an area is at higher risk for yield loss due to leaf diseases. Although this column focuses on southern rust in corn silage, early reports on the impact of corn grain yields in infected fields have been devastating.

Corn stalk integrity can be greatly impacted by leaf diseases. Rust lesions reduce the viable surface area and thus the degree of photosynthesis by the leaves. When the demand for sugars by developing kernels exceeds that produced by the leaves, the plant takes structural carbohydrates from the stalk to meet the need. This weakens the stalk and makes it more susceptible to invading pathogens that cause stalk rot.

Feeding challenges

There have been several questions about chopping and feeding corn silage from corn infested with southern rust. The most common questions I’ve received are about forage quality and potential animal health risks.

Severe rust infestations may reduce forage quality primarily because of the early death of the plant. Increased silage dry matter will generally equate to lower fiber digestibility. Because southern rust coverage reduces the photosynthetic area of the leaf, grain fill is often hindered, leading to lower energy and protein content in the silage.

There are no known toxic effects from feeding rust-infected corn silage. If harvested at the proper DM for effective fermentation (30%-38% DM), the ensiling process generally creates enough heat and acids to kill the fungus and detoxify the forage. In addition, the sugars and other byproducts that are produced during the ensiling process should overwhelm any unpalatable tastes that the rust may impart. As a precaution, feeds can be tested for mycotoxin levels prior to feeding.

Monitor cow performance and work with your nutritionist if you are concerned about the adverse effects of harvesting corn silage with southern rust. In addition, work with your agronomist to discuss timing and effectiveness of fungicide applications in the future.

Barry Visser is a nutritionist for Vita Plus.