The fungi that cause common (Puccinia sorghi) and southern rust (Puccinia polysora) generally require living plant tissue to survive. The fungi produce fungal spores (urediospore) in structures called pustules which are raised masses of colored spores on leaf surfaces.  Common and southern rust do not overwinter in northern areas, so spores are blown north on wind currents from tropical areas. Both rust fungi require a short period of leaf wetness (about 6 hours) for infection and disease development. Dry weather limits disease development. Common rust prefers cooler weather (61-77°F), while southern rust is favored by warmer temperatures (77-82°F). Due to this temperature difference, common rust symptoms may be observed earlier than southern rust, depending on how quickly spores arrive from the south. Common rust can be damaging in seed corn, sweet corn and popcorn production but it is rarely a concern for hybrid corn. Southern rust is more problematic in the southern United States and occurs less frequently in the Midwest, Great Plains and Ontario. However high inoculum levels of southern rust coupled with late planting can result in economic damage in the Midwest as most hybrids are susceptible.

Symptoms of common rust appear as powdery, brown to brick-red pustules that break through the upper and lower leaf surface. The pustules may be surrounded by light green to yellow halos (Image 1). Pustules may also appear on husks, leaf sheaths and stalks. Southern rust pustules are orange to light brown, circular or oval in shape that appear primarily on the upper leaf surface (Image 2). Rust spores can be rubbed off the leaves and leave an orange/red stain on fingers or clothing. The location, color and shape of the pustules may help distinguish common from southern rust, but visual diagnosis is complicated as mixed infections can be found on the same plant. A single pustule on an infected leaf can contain more than 5,000 spores. These spores can cause secondary infections resulting in increased disease severity. Late-planted corn may be at greater risk for infection if rust spores are blown in at the right time and weather is ideal for disease development.

Rust diseases can siphon nutrients necessary for plant growth and the pustules on leaf surface rupture epidermal tissue which may interfere with regulation of water loss by stomata. These effects can result in yield losses and contribute to drought stress, stalk rot and lodging. Southern rust has a higher potential for yield damage as most corn hybrids are susceptible to it. In 2008, a new genetic race of southern rust was discovered in Georgia, able to cause disease on some of the resistant hybrids available.  

Most corn hybrids are fairly resistant to common rust but some inbred lines are quite susceptible and may require fungicide applications under favorable conditions for disease development. Resistant hybrids are not 100% immune to infection so a few pustules might still be observed on resistant corn plants but severity of disease will remain low. Always check the resistance ratings of your corn hybrid or consult with your seed dealer for more information. In the case of southern rust, most corn hybrids are susceptible, so timely fungicide applications can aid in managing this disease. Young leaves are more susceptible to rust infection than mature leaves (age-related resistance), so late plantings may be more at risk for damage.

Common rust infections will not require fungicide applications unless the conditions are exceptionally ideal for development and susceptible hybrids or specialty corn (seed or sweet corn) are the target. However, fungicide applications are a great tool to manage southern rust as resistant hybrids are limited. Application thresholds have not been defined but in general, on specialty corn or susceptible hybrids, fungicides are most effective when sprayed at the onset of the disease. Depending on weather conditions, southern rust can take days to several weeks to develop. Nonetheless, southern rust infections prior to the R3 stage of corn can cause significant yield reductions. Most fungicides will have a 14-21 day residual so in some instances spraying too early may require a second application later in the season if conditions are favorable for disease development. For a list of southern rust fungicides and their efficacy visit: https://crop-protection-network.s3.amazonaws.com/publications/fungicide-efficacy-for-control-of-corn-diseases-filename-2020-03-18-150007.pdf

There are a number of factors that need to be considered when making fungicide decisions. These include the cost of the fungicide, the market value of the crop, hybrid susceptibility, timing of disease infection, the growing environment and corn growth stage at the time of disease onset and development. Evaluating all these factors will help protect the profitability of your corn acres. 

Because rust pathogens do not overwinter in our area, but are blown up north with storms, cultural practices like rotation and tillage are not effective tools to manage rust diseases. One cultural practice that can potentially reduce the yield damaging potential of rust infections is early planting. All corn plants have age-related resistance to rust so planting as early as possible may guarantee more mature and less sensitive corn leaves at the time of rust infection. Nonetheless, scouting can help us make timely chemical decisions and protect the yield potential of our corn hybrids.