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NC State Extension

Southern Rust on Corn in North Carolina

Dr. Lindsey Thiessen- Plant Pathology Extension Specialist and Dr. Ronnie Heiniger – Crop Science Extension Corn Specialist 

Introduction

Southern Rust is a disease that can cause severe yield losses for North Carolina corn growers. Tropical storms that cause persistent rainfall and high humidity are ideal conditions for Southern Rust (caused by Puccinia polysora). Over the years corn growers across 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. However, in conditions where corn was planted late and frequent storms occur that bring the spores from further south this disease can infect corn during the grain fill period resulting in conditions where growers must consider treating the crop with a fungicide.

When and How Should I Consider Treating Corn for this Disease

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. 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. Fungicides are generally less effective in controlling Southern Rust and they must be applied at maximum rates.
  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.

Several fungicides are labeled for control of Southern Rust. However, some are more effective than others. For instance, fungicides that have two modes of action are generally more effective than products with a single active ingredient. For more information on fungicides for Southern Rust and recommended rates or for help in identifying corn diseases, contact your county extension staff.