If you are considering applying fungicide to protect corn from Southern Rust, it is imperative to properly identify the type of rust fungus present--especially because Common Rust is often mistaken for Southern Rust.
Common Rust Identification
Common Rust pictured above
Common Rust rarely impacts corn yields. Southern Rust, on the other hand, can have a large impact on corn yields depending on the time of infection. Neither disease overwinters on residue or soils in the central corn belt. Initial infections of both diseases form when spores of the growing fungus are carried in the winds from the southern states, Mexico, or the Caribbean Islands. Once the fungus has established itself on the corn plants, it will sporulate if left untreated and further spread the disease.
Common Rust, which is present at some level in most years, prefers high humidity and cool temperatures up to 60-75 degrees Fahrenheit. It generally develops in the early season of corn growth, before daytime temperatures reach 80 degrees Fahrenheit or more. High temperatures will slow or stop the disease progression. The rust pustules are cinnamon-brown, small round and rectangular in shape. They occur only on corn leaves. Sporulation with brick-red spores occurs on both top and bottom of the corn leaves. Spores are brick-red in color. By rubbing a finger over the pustules, the brick-red residue (spores) will be evident.
Southern Rust Identification
Southern Rust also likes high humidity but, unlike Common Rust, Southern Rust needs warmer daytime temperatures (75-85 degrees Fahrenheit) to develop. The Southern Rust pustules are small and round with reddish-orange spores. The pustules of Southern Rust occur only on the surface of corn leaves and may also be found on corn husks. Rubbing a finger on pustules will reveal a reddish-orange residue (spores).
The earlier Southern Rust develops on the corn plants, the larger the yield loss potential. Southern Rust can quickly spread from plant to plant and reduce yield in several ways. 1) Reduce sugar transfer from the leaves to grain. 2) Cannibalize the stalk and reduce stalk integrity causing plant lodging. 3) Provide entry for stalk rots in corn plants which may decrease standability. 4) Cause premature death of corn plants before the grain fill period is complete.
Southern Rust pictured above
Treatment of Southern Rust
Once Southern Rust has been positively identified, a decision for treatment can be made. If the corn in question can be harvested early, treatment may not justified. Scout your fields and make a management decision if Southern Rust is present. With the corn price near $3 per bushel, cost of control may not be economical. If treatment is justified, there are several fungicides to choose from that are effective in control of Southern Rust. Select a fungicide that has a combination of both curative (trizole) and preventive (strobilurin) for the best results.