Defend Your Yield from Southern Rust

• Last year’s weather conditions led to a breakout of Southern rust in corn.
• If left untreated, the disease can lead to major yield losses.
• The best defense against Southern rust is applying fungicides at certain stages of corn plant growth, if the disease is identified.

Conditions that favor Southern rust development are high relative humidity and temperatures around 80 to 90 degrees Fahrenheit — especially at night.

Q: Why was 2021 a breakout year for Southern rust in corn? What do you expect to see in 2022?

A. Phil Krieg, Illinois agronomy service representative at Syngenta: 2021 was a breakout year for Southern rust because of the near-perfect weather conditions that we had for the disease to establish, infect and spread north throughout the Corn Belt. When conditions favor rust development, the infection cycle continually repeats itself — spreading disease throughout the plant and helping it to readily move from plant to plant. Because Southern rust does not overwinter in the Midwest, it’s always a wait-and-see game as to whether the disease presents itself in any given year, so we have no idea if we’ll see it in 2022.

A. Travis Faske, Ph.D., professor and extension plant pathologist at the University of Arkansas: Southern corn rust is an annual occurrence in the mid-Southern United States, and severity depends on environmental conditions and the corn growth stage when rust is first detected. In 2021, several factors contributed to the severity of Southern rust development. First, widespread rainfall delayed planting, which contributed to a wide range of corn growth stages when Southern rust was reintroduced in the region. Second, when rust arrived in July, the weather conditions were very good for rust development. Third, hybrids were susceptible to Southern rust. This year, rust will return to the region, but arrival time and summer weather conditions are the unknown factors.

Q: How can growers identify Southern rust in corn? What environmental conditions foster the disease, and how does it spread?

A. Krieg: Southern rust pustules are orange to tan, circular or oval, and about 1/16 inch in diameter. Most pustules develop on upper leaf surfaces. The fungus that causes Southern rust can infect a plant after approximately six hours of leaf wetness. Dew usually provides enough moisture to cause infection, but frequent rainfall can promote severe disease development. Conditions that favor Southern rust development are high relative humidity and temperatures around 80 to 90 degrees Fahrenheit — especially at night. Each year, wind currents from southern, more tropical areas carry rust spores north and begin new infections. The weather conditions outlined above take over after the spores arrive and infect the plant, determining the spread and severity of the disease.

A. Faske: Southern rust produces orange-colored pustules that rupture through the upper leaf surface. Pustules are often surrounded by a light green halo and clustered near the first pustule from the initial infection. These pustules are commonly detected in the mid-to-upper canopy. The most common misdiagnosis is common rust. Common rust produces dark-red-colored pustules that can rupture through the lower leaf but are more common on the upper leaf surface. The environmental conditions that favor Southern rust are frequent rainfall causing four to eight hours of leaf wetness, high relative humidity, and warm temperatures (82 to 88 degrees Fahrenheit). Wind spreads the rust spores to infect nearby plants. During the cropping season, windblown spores move progressively northward — infecting new fields.

Q. What is the yield impact of Southern rust in corn when left untreated?

A. Krieg: The Southern rust fungus uses a plant’s nutrients for growth and reproduction, which affects grain fill and ultimately reduces yields. Rust pustules also rupture leaf epidermal tissue, which can interfere with the regulation of water loss by stomata. Consequently, severe rust outbreaks make it harder for plants to use water efficiently, so infected plants may exhibit symptoms of mild to severe drought stress. In severe cases, these infections may predispose plants to secondary infections by stalk rot pathogens, which leads to lodging and yield loss. Yield losses of up to 45% have been reported with severe disease.

A. Faske: Grain yield losses have ranged from 20% to 40% in Arkansas when Southern rust occurred at tassel and the severity and percent of leaf area (ear and nearby leaves) affected at dent was above 40%. However, grain yield losses ranged from 5% to 10% when Southern rust started later at milk, with a similar degree of severity at dent.

Q. What fungicide strategies do you recommend to control Southern rust in corn?

A. Krieg: Applying Trivapro^® or Miravis^® Neo fungicides before the disease establishes in corn helps give the best return on investment (ROI) and yield protection. In 2021, two-pass fungicide applications helped provide the best ROI and yield response due to the early onset of the disease and the extended period of infection throughout the region.

A. Faske: Scout corn at tassel and later reproductive stages for Southern rust. If detected and conditions favor disease development — and corn growth stages are tassel, silk, blister or milk — protect yield potential with a fungicide. Good coverage is important to protect leaves at mid-canopy. Several fungicides have good efficacy against Southern rust. These fungicides contain a strobilurin fungicide plus at least one other class of fungicide.