CPN-2009-W — August 2017
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.
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. You 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 underside of a leaf (Figure 3). In severe cases, pustules may also develop on husks, leaf sheaths, and ear shanks (Figure 4).
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, which distributes 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. 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, which distributes 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.
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.
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.
Southern rust can be confused with several diseases and disorders (see Diseases and Disorders 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).
The Integrated Pest Information Platform for Extension and Education (iPiPE) helps track the movement of southern rust throughout the season (Figure 14). It is available online at ext.ipipe.org.
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.
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 Diseases of Corn: Fungicide Efficacy for Control of Foliar Corn Diseases (Purdue Extension publication BP-160-W), available from the Education Store, edustore.purdue.edu.
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).
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.
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 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.
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 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 you can easily observe if you hold the leaf to the sun. Leaf blighting may occur when these lesions join, which kills large portions of leaf tissue.
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 produces faint, tan to yellow halos on leaves (Figure 10). These spots are most frequently observed in the upper canopy.
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.
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.
Tar spot ascomata appear smooth and raised, but they cannot be rubbed off the plant tissue like southern rust fungus spores can. 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.
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).
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 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).
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.
Other publications in the Corn Disease Management series are available on the Crop Protection Network website (cropprotectionnetwork.org). More information about the new strain of southern rust is available in the following publication:
Dolezal, W., K. Tiwari, R. Kemerait, J. Kichler, P. Sapp, J. Pataky. 2009. An unusual occurrence of
southern rust caused by Rpp9-virulent Puccinia polysora, on corn in southwestern Georgia. Plant Disease 93:676.
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
Kiersten Wise, Purdue University
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 Delaware
Dean Malvick, University of Minnesota
Hillary Mehl, Virginia Tech
John Mueller, Clemson University
Alison Robertson, Iowa State University
Damon Smith, University of Wisconsin-Madison
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.
This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel.
This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2015-68004-23179.
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. Design and production by Purdue Agricultural Communication.
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
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