Southern Rust Continues to Develop Across Iowa

By Alison Robertson and Rebecca Vittetoe

Southern rust has continued to spread across Iowa and increase in severity since the disease was first reported in mid-July. This is not surprising considering the weather we have been having. July was warm and extremely wet across the state, and the precipitation has continued through this first 10 days of August.

Favorable conditions for southern rust development

Southern rust thrives in warm (77-82°F), wet (dew, rain) conditions. At least 6 hours of leaf wetness is required for infection to occur. Under these conditions, the time between a spore infecting a leaf to the production of a pustule filled with new spores is 7 to 10 days. Compare this to tar spot (14-21 days) and gray leaf spot (14 days).  Consequently, southern rust epidemics can develop very quickly (Figure 1). Once we get into the cooler days of fall, southern rust development should slow.

Is it too late to apply a fungicide? 

According to this resource from the Crop Protection Network, applying a fungicide through R3 (milk) is likely to be beneficial. Only with severe disease pressure would a fungicide application be recommended at R4 (dough). I would consider severe southern rust disease pressure to be southern rust present on the ear leaf or in the upper canopy on most plants in the field at approximately 1% disease severity.  I’ve heard lots of chatter that southern rust is severe in the lower canopy.  Remember that the ear leaf and canopy above are filling grain. These are the leaves we need to apply a fungicide to and protect yield.