Southern rust was confirmed the first week of August on corn in Caldwell County, Kentucky.
Based on photos from an industry representative, it also appears to be present in Logan County (lab confirmation pending). The disease has been developing in southern Georgia and Mississippi throughout July, and it also has been detected recently in Nebraska, Arkansas, Kansas and Illinois. I suspect that southern rust is at low levels in several locations in Kentucky, although scouting thus far suggests it is not yet widely prevalent. In other words, there is no need for “panic.” Just need watchful eyes.
Common rust is found at low levels in many fields in most years, including this year. Adequate levels of resistance to common rust are found in the large majority of hybrids adapted to Kentucky. Southern rust is much more sporadic in Kentucky, and very often comes in too late to cause damage. If it shows up early enough, it is capable of causing significant crop damage.
Southern rust is always a concern whenever it appears in Kentucky this early because many corn hybrids adapted to Kentucky conditions are susceptible, and it can be a very fast-moving disease.
Southern rust poses the greatest risk to late-planted crops, and we have many of those this year. I definitely suggest scouting for this disease in late-plant crops.
We don’t have well-researched spray guidelines for southern rust. Here are some thoughts that may help with spray decisions:
• Detection of a few infected leaves does not mean that the field must be sprayed. How much damage the disease causes depends on how fast disease progress occurs, which is highly weather-dependent. Kentucky conditions can sometimes be favorable for rapid disease progress, but not always. If we get into a pattern of dry weather with cool nights, disease progress can slow down substantially.
Southern rust moves fast
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