Rust in corn

  • Published: Friday, July 23, 2021

Rust in Corn
By Andy Luke, field specialist in agronomy

Rust diseases are showing up in Missouri. Scout your fields now to prevent yield loss from these fungal pathogens.

There are two main types of rust that we commonly see in Missouri cornfields, southern and common rust. Both of these fungi require a living host plant to survive, so are unable to overwinter in Missouri. They typically survive in southern states and tropical areas and spores are carried north on wind currents during the season. They require short periods of leaf wetness for infection, with approximately six hours of dew usually being enough for disease development.

Common rust occurs in most cornfields every year, but rarely causes yield loss. It prefers cooler temperatures for infection and development, but will become inactive in hot, dry weather. Common rust pustules will appear on both the upper and lower leaf surfaces.

Southern rust is relatively common in Missouri and can cause significant yield loss if infection is severe. Southern rust favors temperatures around 80 degrees with high relative humidity for infection. It is generally found on leaves in the mid to upper canopy. In favorable conditions, the rust pustules can produce new spores for up to eight days, distributing thousands of new spores that lead to secondary infections. Southern rust pustules will appear primarily on the upper leaf surface in dense clusters.

If southern rust is present, fungicide applications to prevent new infections are beneficial up to the R3 or milk stage. Take into consideration the current distribution of rust in the area, growth stage of crop and environmental conditions before making a decision to apply fungicides. At this time, southern rust has been confirmed in 3 Missouri counties and one in southeast Nebraska. In scouting, remember that common rust is found on both the upper and lower leaf surfaces, but southern rust is primarily on the upper surface. Both rust diseases are orange to tan in color and can be easily scraped off the leaf surface with a fingernail. Tar spot, another fungal disease, can look similar but can’t be scraped off the leaf surface.

Writer: Andy Luke

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