Southern Rust Identification and Management in Corn

Southern rust is a common disease of corn that many growers may face on an annual basis. The disease is caused by the fungal pathogen Puccinia polysora and can cause severe yield loss and reduced standability.

Disease Identification

Initial southern rust pustules appear as orange to brown raised lesions on the top side of the leaf. When an open pustule is touched, the spores can rub off and leave an orange to brown mark on your fingers. This is an important distinction as common rust appears very similar, but common rust pustules can be found on both sides of the leaf and are typically less abundant than southern rust. Common rust is a less impactful disease; therefore, this identifying characteristic is important.

Some articles reference color differences in the lesions where common rust is a darker brown and southern rust is a brighter orange color. Unfortunately, those differences are not always apparent. During early lesion development, it may be best to send samples to a plant diagnostic lab for specific identification between these two diseases. It is easy to misidentify southern versus common rust and considering the difference in impact the two diseases can have, it is very important to ensure accurate identification.

Damage from the Disease

Southern rust can reduce the amount of photosynthetic leaf area the plant has which can negatively impact yield depending on timing of infection and disease severity. Severe infection early in the plant’s life increases the potential impact on grain yield. Heavy disease infestation in the Midwest is unusual; however, in southern growing areas, 25 bu/acre yield reductions are common on susceptible corn products.¹ Even when the disease occurs later and has a smaller impact on grain yield, it can still reduce plant standability by causing the plant to cannibalize itself to fill grain.

Disease Life Cycle

The pathogen does not survive Midwest winters and can only survive in warm tropical climates. This means that disease inoculum must move-up from the tropics into the southern United States before arriving in the Midwest. This is different from many of the foliar corn diseases that overwinter in residue and are consistently present in the Midwest. Southern rust pressure across the Midwest varies from one year to the next depending on when the inoculum arrives from the south and the environment. Corn planting date can have a direct impact on the disease’s impact. If the inoculum arrives late and corn is planted late, the plants have the potential to be infected during a great portion of grain fill compared to corn planted earlier. Once the disease is here, it prefers 77 to 82 °F and high humidity for infection. Generally, July and August weather in the Midwest produces many days that are favorable for initial disease development and spread.

Disease Management

Unfortunately, many of the cultural control methods such as tillage and crop rotation used to manage other corn diseases are not useful for southern rust because it doesn’t overwinter. Thankfully, there are a couple other options that can help control the disease.

The first is selecting corn products with greater southern rust tolerance. Some products have higher natural resistance to the disease than others, so if southern rust is a concern, picking those products with greater resistance can help reduce loss.

In high pressure situations, a foliar fungicide may be needed. There are several foliar fungicides labeled for corn that can help reduce the spread of the disease. Optimum application timing is dependent on when the disease arrives, but a good rule of thumb is a VT to R1 growth stage application which can help give residual control until around the R3 to R4 growth stages. If the disease isn’t present at those early stages, the application can be applied later to help control the disease even later into the plant’s lifecycle. When deciding which foliar fungicide to use, select products with multiple effective modes of action. Many products on the market today have a group 11, 7, and 3 modes of action to help provide control of southern rust.

Summary

Southern rust can be a detrimental corn disease in years where disease development is early in the plant’s life cycle and the environment is favorable. The disease can reduce yield potential and stalk quality. The disease can be managed by utilizing a combination of more tolerant products and foliar fungicides. If pressure is high, foliar fungicides may be the best method of control.

Channel Agronomist
Cody Hornaday

Sources:

¹Wise, K. 2010. Common and southern rusts. Diseases of corn. BP-82-W. Purdue Extension. Purdue University. https://www.extension.purdue.edu/extmedia/bp/bp-82-w.pdf

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