Distinguishing between common and southern rust is important for proper corn management. Rusts are fungal diseases that affect a wide range of crops worldwide. In the southern United States, two rusts, common rust (Puccinia sorghi) and southern rust (Puccinia polysora), are known to affect corn. Although both diseases can cause yield reductions, common rust is much less aggressive and must be present at high levels to cause a yield loss. Southern rust can lead to dramatic yield reductions if present at critical growth stages due to a loss in the photosynthetic capacity of the leaves.
Symptoms and Signs
Both common and southern rust begin as small lesions on the leaves that develop a pale green to yellow halo. As these lesions mature, the fungi erupt through the epidermal tissue to form raised spore-producing structures called pustules, which produce reddish or rust colored spores.
Common rust pustules are typically brown to red in color, elongated in shape and appear on the upper and lower leaf surfaces. Southern rust pustules are orange to tan in color, with pustules having a circular to oval appearance and are typically only present on the upper leaf surface. In severe cases, southern rust pustules can also be found infecting husks and the leaf tissue that wraps around stalks in addition to leaves.
In South Carolina, common rust infection typically occurs prior to tasseling because of cool and moist environmental conditions favoring disease development, with optimum temperature ranging from 61 to 77 degrees Fahrenheit. Hot, dry conditions can limit common rust development, resulting in pustules becoming inactive.
In contrast, southern rust typically occurs later in the growing season after tasseling, when temperatures reach 77 to 82 degrees and high relative humidity is present. Both diseases can infect the host plant after approximately six hours of leaf wetness, typically a heavy dew. However, periods of frequent rainfall can promote severe disease development, especially southern rust.
Rusts require living host tissue to survive and complete their life cycle. In South Carolina, these hosts produce limited amounts of primary inoculum the following spring. Each year, wind currents from tropical climates deliver common and southern rust spores into corn-producing regions of the United States. Once the corn plant is infected, these pathogens can develop spore-producing pustules in seven to 14 days, which can lead to secondary infections.
Typically, common rust has a limited impact on yield, with normal levels of common rust not requiring fungicide sprays. Resistant hybrids are available and provide good control for no added expense. In cases where large amounts of inoculum are present and environmental conditions are favorable, fungicides may be necessary to achieve effective control.
Management of southern rust begins with planting a resistant hybrid and being prepared to apply a fungicide if needed. Scouting to detect infection early in the growing season is essential.
Fungicide applications should be applied at or around the tasseling or silking growth stage, or VT/R1, as a preventative spray.
Using tools such as the corn integrated pest management website, corn.ipmpipe.org/southerncornrust, which tracks southern rust observations throughout the growing season, can be helpful in making informed management decisions. For a complete list of suggested fungicides to control foliar diseases on corn, review the Crop Protection Network’s Fungicide Efficacy for Control of Corn Diseases.
Growers should be careful when utilizing any fungicide to be aware of the mode of action. Fungicides with multiple MOAs are often more effective in providing disease control and help with fungicide resistance management but can also be more expensive. CS