Southern Rust on Corn in North Carolina in 2003

Dr. Steve Koenning - Plant Pathology Extension Specialist and Dr. Ronnie Heiniger - Crop Science Extension Corn Specialist
North Carolina State University

Introduction

Persistent rainfall and high humidity have created ideal conditions for Southern Rust (caused by Puccinia polysora) in Corn. Several growers across eastern North Carolina have reported finding this disease at levels that may require treatment. Southern Rust can be recognized by the bright orange or golden brown, circular to oval pustules that give leaves a rusty appearance. The pustules are about the size of a pin head and are filled with powdery masses of orange spores that are readily dislodged and blown in the wind. Thanks to these spores, Southern Rust can spread quickly. 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. In 2003, however, there is some concern because much of the corn was planted late and many hybrids have inadequate resistance to this disease.

When and How Should I Consider Treating Corn for this Disease

Fungicide sprays on corn are not generally recommended for use in North Carolina. Therefore, we have no thresholds for Southern Rust on corn or data on the efficacy of fungicides for controlling this disease. Because some corn was planted late, applications of TILT or QUADRIS may prove beneficial this year, especially with the prospect of continued high humidity and rainfall. The following guidelines are suggestions for factors that need to be considered in making a decision to apply a fungicide.

1. Corn within two weeks of black layer is unlikely to benefit.

2. Estimate yield potential - spraying corn with a yield potential of less than 100 bushels/ acre is unlikely to provide a profitable return.

3. If more than 1-2% of the leaf area is affected, spraying may increase yields.

4. Continued warm humid weather may cause disease to accelerate in susceptible hybrids.

5. The labeled rate for control of rusts for TILT is 4 fl oz. /acre and for QUADRIS is 6.2-9.2 fl oz./acre.

6. Cost of fungicide and application is likely to be $15.00 to $20.00/acre, thus a 10 bushel increase is needed to pay for treatment costs. The fungicides TILT® or QUADRIS® (Syngenta) are registered for use on corn to control rusts and may also provide control of gray leaf spot. 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. Preliminary results from tests conducted in Georgia indicate that Quadris may have performed better than Tilt. The 4 fl oz. TILT/acre rate should also provide some control of gray leaf spot, but higher rates of QUADRIS are recommended for control of this disease. Gray leaf spot is generally more of a problem in no-till corn that is not rotated.

For more information on corn diseases, visit the following websites or contact your county extension staff. http://ipm.ncsu.edu/corn/diseases/corn_diseases.html