Southern Corn Rust: What to expect in 2022?

by T. R. Faske, Extension Plant Pathologist - July 19, 2022

Southern rust has arrived, but conditions are not favorable for disease development, so expect slower than normal rust spread across the Mid-South. Southern rust was confirmed in Drew County, Arkansas on July 13, 2022. Rust incidence and severity were low (Fig. 1) on corn at the milk growth stage (R3). The dry condition has slowed rust development and spread, which will likely be slower than in previous years. To monitor rust detection, see the Corn ipmPIPE website (Fig. 2) and access the interactive map. Rust has been detected in North Mississippi; thus, it is likely to find southern rust in other Arkansas Counties. This update provides some guidelines for the management of southern corn rust.  

See the “Southern Rust” article by the Crop Protection Network for great information on southern rust identification and diagnosis.

Figure 1. Southern rust pustules on the lower and upper leaf surface.

Figure 2.  Distribution of southern rust as of July 18, 2022. Red = positive, Yellow = probable, and green = scouted, but not found.

Dry conditions will suppress the spread of southern rust as free moisture (dew or light rain) is necessary for spore germination and infection. When conditions favor disease, symptoms appear about 3 to 6 days after infection. After 7 to 10 days, the pustules rupture to release rust spores. 

Conditions that favor disease include warm/hot temperatures (morning low of 75°F and daytime high of 93°F + 4-6 hr of consecutive leaf wetness) and extended periods of light rain or heavy dew. When these conditions are not met, as in 2022, disease development is much slower. Given the recent forecast, most of the corn crop will likely “outrun” the disease. Fungicides applied in the absence of the disease are less likely to protect yield potential compared to years when a foliar disease is present, and conditions favor disease development.  

Fungicides are effective at protecting corn yield potential, but fungicide use should be made on a field-by-field basis. The “Fungicide Timing for Southern Rust” (Table 1) is a guideline on the benefit of a fungicide to protect yield potential at various corn growth stages with the assumption that southern rust is present (first detection) in the field and conditions favor disease development. See the MP 154 for fungicides efficacy tables to control southern rust in Arkansas.

Table 1. Fungicide timing for Southern Rust. Guide to protect corn yield potential with a fungicide when southern rust is first detected, and conditions favor disease development. 

Please contact tfaske@uada.edu for any comments or questions.