An Overview of Southern Rust

Southern rust of corn is caused by the fungus Puccinia polysora. Although generally considered a “tropical disease,” southern rust can occur in important corn production areas of the United States and Canada. This publication describes the symptoms and signs of southern rust, how to differentiate southern rust from other common diseases, environmental conditions that favor southern rust, and practices you can use to manage this disease.

Symptoms and Signs

The southern rust fungus produces raised structures called pustules which rupture the outer leaf tissue (epidermal tissue). The pustules contain masses of colored spores called urediniospores.

Southern rust pustules are orange to tan, circular or oval, and about 1/16 inch (1.5 millimeters) in diameter (Figure 1). The majority of these pustules develop on upper leaf surfaces. Farmers can rub rust spores off leaves, which will leave orange to tan streaks of spores on fingers or clothing. Late in the season, the fungus may form brown to black pustules known as telia (which contain teliospores) (Figure 2).

Pustules are usually first observed in the middle or upper plant canopy in isolated “hot spots” of rust activity. These hot spots are often at the ends of rows along field borders. Secondary infections near initial infections are numerous and densely clustered on leaves. Pustules are often surrounded by light green to yellow (chlorotic) halos, which are evident on the undersideof a leaf (Figure 3). In severe cases, pustules may also develop on husks, leaf sheaths, and ear shanks (Figure 4).

Disease Cycle

Puccinia polysora requires living host plant tissue to survive, so when corn matures or is no longer green, the southern rust fungus can no longer survive. Corn is the major host for the southern rust fungus, so the fungus does not overwinter in northern areas. Each year, wind currents from tropical areas carry rust spores (urediniospores) north and begin new infections (Figure 5). When conditions favor rust development, the infection cycle continually repeats and causes secondary infections. The period between when a plant is infected and when the fungus develops pustules and spores can be as short as nine days when conditions are favorable. Each pustule can produce spores for up to eight days, distributing thousands of spores that can cause secondary infections.

When conditions that favor disease development last for a prolonged period, southern rust severity can quickly reach epidemic levels. Because favorable conditions for southern rust are more common in the southern United States, the disease is typically more problematic in these areas. Each year, spores produced on corn in southern states move north during the cropping season. Southern rust’s severity and impact in the major U.S. Corn Belt states and Ontario depend on when it develops in the region and on how favorable conditions are for the disease.

Conditions that Favor Disease

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. Southern rust is favored by high relative humidity and temperatures around 80°F (27°C). The rate at which southern rust reaches damaging levels depends on:

• The crop’s development stage at the onset of infection

• The hybrid’s susceptibility

• How long favorable conditions last

Young leaves are more susceptible to infection than mature leaves, and late-planted or double-crop corn may be at greater risk for yield loss if environmental conditions favor disease development.

Yield Losses and Impact

The southern rust fungus uses the 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 (microscopic openings on the surface of leaves). Consequently, severe rust outbreaks make it harder for plants to use water efficiently, so infected plants may exhibit symptoms of mild drought stress. In severe cases, these infections may predispose plants to secondary infections by stalk rot pathogens, which leads to lodging. Yield losses up to 45 percent have been reported with severe disease.

Diagnosis

Southern rust can be confused with several diseases and disorders (see Diseases, Disorders, and Injury with Similar Symptoms below). A plant diagnostic laboratory can distinguish southern rust from common rust and other issues by examining leaves under a microscope and looking for the fungal spores of Puccinia polysora. A trained diagnostician can differentiate southern rust spores from the spores of other plant pathogenic fungi (Figure 6).

Diseases, Disorders, and Injury with Similar Symptoms: Diseases

Common Rust (Puccinia sorghi)

Common rust produces brick-red to brown elongated pustules that break through the leaf surface and produce dusty spores (Figure 7). Pustules appear on the upper or lower leaf surfaces in the low- to mid-canopy. Pustules are often scattered across leaf surfaces.

Most yellow dent corn hybrids have moderate to high resistance to common rust, so fungicides for managing common rust are generally unnecessary (sweet corn, seed corn inbreds, and other specialty corn types may be exceptions). Therefore, it is important to correctly diagnose the disease, because foliar fungicides are more likely to be warranted when southern rust is present. Both rust fungi can infect the same plant, which can complicate diagnosis.

How to distinguish common rust from southern rust: Common rust pustules appear on both upper and lower leaf surfaces, tend to be spread out, and are not densely clustered. Conversely, southern rust pustules typically form on upper leaf surfaces, and are densely clustered.

Physoderma Brown Spot (Physoderma maydis)

Physoderma brown spot produces small, round, yellow-brown lesions that often occur in bands across the leaf (Figure 8). Dark purple to black circular lesions occur on the midrib of the leaf.

How to distinguish Physoderma brown spot from southern rust: Physoderma brown spot leaf lesions do not have spores that can be brushed off the leaf like southern rust has. Purple to black lesions on the midrib also distinguish Physoderma brown spot from southern rust.

Eyespot (Aureobasidium zeae)

Eyespot lesions are tiny (1/8-1/4 inch, 3-6mm), clustered, circular spots with tan/brown centers (Figure 9). Each lesion forms a translucent yellow halo around the margin, which can easily be observed if the leaf is held to the sun. Leaf blighting may occur when these lesions join, which kills large portions of leaf tissue.

How to distinguish eyespot from southern rust: The fungus that causes eyespot does not produce the raised, orange pustules that develop on the leaf surfaces of plants infected with southern rust.

Curvularia Leaf Spot (Curvularia spp.)

Curvularia leaf spot produces faint, tan to yellow halos on leaves (Figure 10). These spots are most frequently observed in the upper canopy.

How to distinguish Curvularia leaf spot from southern rust: In general, you can observe Curvularia leaf spot lesions by looking up through the crop canopy from the ground level. Check a lesion with magnification. Curvularia lesions will not sporulate.

Tar Spot (Phyllachora maydis)

Leaves with tar spot have small, raised, round, and shiny black fungal structures that can be surrounded by narrow tan halos (Figure 11). These structures (ascomata) are raised and feel bumpy to the touch. Leaves may have a few ascomata, or ascomata can densely cover the leaf.

How to distinguish tar spot from southern rust: Tar spot ascomata appear smooth and raised, but they cannot be rubbed off the plant tissue like southern rust fungus spores. Since tar spot ascomata are easily confused with southern rust telia (dark pustules) late in the season, a laboratory may need to confirm the diagnosis.

Diseases, Disorders, and Injury with Similar Symptoms: Other Conditions

Genetic disorders

Genetic disorders (such as genetic leaf flecking or spotting) affect all leaves on a single plant or on a few randomly scattered plants in the field and do not spread to other plants (Figure 12).

How to distinguish genetic disorders from southern rust: Most genetic disorders occur only on a few scattered plants in a field. Southern rust tends to occur on many plants that can be in localized or widespread areas of a field. Genetic disorders will not produce raised pustules or spores. Unlike southern rust, genetic disorders do not spread from plant to plant.

Spider mite injury

Spider mites pierce leaf cells and feed on the contents. The upper surfaces of affected leaves can have white or yellow spots (sandblasting) due to feeding injury (Figure 13).

How to distinguish spider mite injury from southern rust: When spider mites are present, you can observe a fine webbing on the underside of the leaf and you may see very small black moving dots. You can shake spider mites off the leaf onto a white piece of paper where you can see them move around. Magnification is not necessary. Spider mites are more likely to cause injury when conditions are dry, whereas, southern rust will be more prevalent when conditions are moist.

Table 1. Symptom distribution and expression for diseases and disorders with symptoms similar to southern rust.

Management

Tracking Southern Rust Movement

The Corn ipmPIPE tracks southern rust movement during the growing season. Note that the map may display “gaps” where active monitoring and scouting for southern rust may not be occurring.

Southern Corn Rust Movement

Resistant Hybrids

Most hybrids are susceptible to southern rust, but a few resistant hybrids may be available. Resistant hybrids may contain a specific type of gene (known as an Rpp gene) that confers resistance to southern rust. Some moderately-susceptible hybrids may be available that do not contain specific Rpp genes. Such hybrids may use multiple genes that slow down southern rust development. Many inbred lines for hybrid production, sweet corn, popcorn, and specialty corn hybrids are highly susceptible to the disease. In 2008, researchers discovered a new strain (race or pathotype) of the southern rust fungus in Georgia that was able to cause disease on hybrids that utilized the Rpp9 resistance gene. This new race of the southern rust fungus has increased the importance of the disease in southern areas, and scouting programs have been implemented in the region to monitor fields for southern rust more carefully.

Foliar Fungicides

Fungicides are effective at protecting uninfected leaf tissue from southern rust; however, there currently is no economic threshold for fungicide applications. Factors to consider include corn stage of development, yield potential, threat or observation of southern rust in the field or region, type of irrigation, and environmental conditions that favor disease development. Applying fungicides between the silking (R1) and milk (R3) stages when southern rust has been detected are most beneficial at protecting corn yield potential. However, additional applications may be needed for seasonlong crop protection, depending on the timing of disease onset (Table 2).

Applying a fungicide to field corn within two weeks (50 percent starch line) of physiological maturity (black layer) is unlikely to provide an economic benefit.

The Corn Disease Working Group updates foliar fungicide efficacy guides each year that contain more specific information about fungicide modes of action and commercial availability. For a current fungicide list, see Fungicide Efficacy for Control of Corn Diseases, available from the Crop Protection Network.

Table 2. Possible benefits (by growth stage) from applying fungicides to protect against southern rust.

Cultural Practices

Cultural practices do not influence southern rust development, because the fungus does not survive in crop residue. However, planting date may influence southern rust development, because the risk of yield- limiting disease levels increases when corn is planted late (Table 2).

Find Out More

Other publications in the Corn Disease Management series are available from the Crop Protection Network. More information about the new strain of southern rust is available in the journal Plant Disease:

Dolezal, W., Tiwari, K., Kemerait, R., Kichler, J., Sapp, P., and Pataky, J. 2009. An unusual occurrence of southern rust caused by Rpp9-virulent Puccinia polysora, on corn in southwestern Georgia. Plant Disease 93:676. Article / Google Scholar

Acknowledgments

Authors

Carl Bradley, University of Kentucky; Tom Allen, Mississippi State University; Travis Faske, University of Arkansas; Tom Isakeit, Texas A&M University; Tamra Jackson-Ziems, University of Nebraska-Lincoln; Kelsey Mehl, University of Kentucky; Daren Mueller, Iowa State University; Adam Sisson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Jafe Weems, University of Kentucky; and Kiersten Wise, University of Kentucky.

Reviewers

Gary Bergstrom, Cornell University; Emmanuel Byamukama, South Dakota State University; Martin Chilvers, Michigan State University; Andrew Friskop, North Dakota State University; Austin Hagan, Auburn University; Doug Jardine, Kansas State University; Heather Kelly, University of Tennessee; Nathan Kleczewski, University of Illinois; Dean Malvick, University of Minnesota; Hillary Mehl, Virginia Tech; John Mueller, Clemson University; Alison Robertson, Iowa State University; Damon Smith, University of Wisconsin-Madison; and Lindsey Thiessen, North Carolina State University.

All photos were provided by and are the property of the authors and contributors except Figure 12, provided by Gary Munkvold, Iowa State University; and Figure 11B, provided by Ed Zaworski, Iowa State University.