Southern Rust in Corn
Corn Disease Information Note 2
Steve Koenning – Plant Pathology Extension Corn Specialist


[General Information] [Symptoms] [Management] [Before You Apply Fungicide]
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General Information

Southern Rust Fig. 1Southern rust of corn is caused by the fungus Puccinia polysora. The rust is considered to be tropical or subtropical in distribution. It does not overwinter in North Carolina. Spores travel on wind from more tropical regions depending on weather patterns. Typically sweet corn in Florida is sprayed with fungicides for this disease. The optimum conditions for rust development are temperatures from 80 to 90 degrees F and high humidity. Ordinarily, southern rust of corn is of little concern to North Carolina growers because spores arrive so late in the season that little or no impact on yield occurs. On average, southern rust is a problem about one in five years. Yield loss due to southern rust, however, can be severe especially when corn is planted late and environmental conditions favorable for disease development persist in late summer.

Symptoms

Southern Rust Fig. 2 The symptoms of southern rust are orange to brown masses of spores (urediospores) that erupt through the upper leaf surface (Figs. 1, and 2). Leaves, stalks, and the husks on ears may be infected. Southern rust typically sporulates profusely on the upper leaf surface and only sparsely on the lower leaf surface. In contrast, common rust (caused by Puccinia sorghi) produces spores on both surfaces, often in streaks.

Management

Southern Rust Fig. 3Resistance to southern rust is controlled by specific genes available in some corn hybrids. Hybrids with high levels of specific resistance to southern rust, however, have lower yield potential than many susceptible hybrids. Some corn hybrids have a more general resistance or tolerance to infection by this fungus. Because corn hybrids change so frequently, it best to consult with seedsmen for information on hybrid resistance to various diseases.

The fungicides TILT® and QUADRIS® (Syngenta) are registered for use on corn to control rusts and may also provide control of gray leaf spot. Fungicide sprays on corn are not generally recommended for North Carolina. Applications of TILT or QUADRIS may prove beneficial when 3-5 % of the leaf surface have rust pustules, corn is four or more weeks from black layer, and there is prospect of continued high humidity and rainfall. TILT sprayed field corn should not be harvested within 30 days after the last application, and should not be sprayed after silking. QUADRIS may be sprayed within 7 days of harvest. We have no thresholds for southern rust on corn or data on the efficacy of fungicides for controlling this disease. The following guidelines are suggestions for factors that need to be considered in making a decision to apply a fungicide.

Before You Apply Fungicide

  1. Field corn within two weeks of black layer is unlikely to benefit.
  2. Estimate yield potential – spraying corn with a yield potential of less than 150 bushels/ acre is unlikely to provide a profitable return.
  3. If more than 10% of the leaf area is affected, spraying may increase yields.
  4. Continued warm humid weather may cause disease to accelerate in susceptible hybrids.
  5. Cost of fungicide and application is likely to be $15.00 to $20.00/acre including application costs, thus a 10 bushel increase is needed to pay for treatment costs.

Other Resources

Corn Disease Notes
Plant Disease Information Notes Home Page

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service Office. Outside North Carolina, look for your state extension service partners.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies

Published by the North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last update to information: July 2005
Last checked by author: July 2005
Web page last updated on July 2005