Under highly disease-favorable weather, which doesn’t always occur in Kentucky, the disease can go from undetected in the field to substantial damage in 5-6 weeks.

If the disease is present more or less throughout the field even at very low levels in all levels of the canopy, it can cause substantial damage in 2-3 weeks.

Detection of the disease in crops in the kernel milk stage (R3) or earlier suggests a significant risk to yield and stalk health, if weather permits continued rapid disease development. Detection in later crop stages is where decisions get more difficult.  The onset of the kernel dough stage (R4) is still roughly 4-5 weeks from maturity; and the onset of the kernel dent stage (R5) is roughly 3-4 weeks away from maturity. That is enough time for crop damage to occur, but only if the disease is established throughout the field and conditions are highly favorable for disease progress. 

Southern rust activity late in grain-fill can take the form of yield loss or, more commonly, widespread stalk rot.

In considering whether a fungicide application is justified, consider the following:

Has southern rust been detected in the area?  It may not even be present. A few days ago, I did a quick tour through a western Kentucky county and found no southern rust, so it is not yet a widespread threat. 

If the disease is present, is it a field with high yield potential?  What is the weather forecast?

How widely established is the disease?  Is it on just a few leaves, or throughout the field? 

Commercial fungicides to use against southern rust are available at http://www.uky.edu/Ag/GrainCrops/Briefs/CDWGCornFungicideEfficacy_Table_2013_FINAL.pdf