Both common rust and Southern rust are being found in corn fields across the Midwest. Of the two fungi, southern rust is historically the most damaging of the diseases. In fields of late planted (or replanted) commercial field corn, common rust is becoming more noticeable than in years past.
The onset of these pathogens and the spread of the diseases are a concern due to the many acres of later planted corn resulting from the delays in planting this spring from various weather events. Moving forward, the weather will play a huge part in whether one, or both, of these diseases becomes economically damaging.
Southern rust can develop and spread more quickly if the weather is hot, (mid-80s and above), humid, and there are heavy dews and rainfall, Conversely, disease development is slowed by cooler, less humid weather. Damage from southern rust can be moderate to severe, and foliar fungicide applications between tassel (VT) and milk (R3) can help protect plants from infection and disease development.
The recent weather has favored development of common rust, but fungicide applications for common rust in hybrid corn are unlikely to be economically beneficial.
Here are several technical bulletins that have been written by agronomists from LG Seeds regarding both southern rust and common rust:
Tech_341 - Southern Corn Rust
Tech_128 - Common Rust in Corn updated