By Dewey Lee, Extension Agronomist, University of Georgia
Southern rust has shown up in a corn field in Mitchell County according to Andy Shirley and Dr. Bob Kemerait. It was also seen in Gilchrist Co, Florida earlier this week. Unfortunately, it is most likely to spread over the next several days. The following is a note from Dr. Kemerait, UGA Corn Pathologist.
“Greetings! A hurried e-mail.
Thanks to the efforts of Andy Shirley this morning, southern corn rust is now confirmed from a field in Mitchell County. There was not much there but it WAS there! This is the EARLIEST find of southern corn rust in my 15 years in Georgia. Corn growers in south Georgia should be very much aware NOW that rust is here. Current weather patterns increase the risk.
I believe that any irrigated field corn with good yield expectations and that is at tassel or beyond is a good candidate for treating with a fungicide. Corn approaching tassel is also certainly at risk. NOTE: do not mix an aduvant or crop oil with the fungicide if prior to tassel stage.”
” LET’S AVOID LAST YEAR!”
All right. Now that it is in Georgia we can expect the potential for infection to be widespread as long as the environment favors infection. Remember, southern rust spores erupt from the upper leaf side while commons erupts from both upper and lower. Southern rust spores will form on the lower surface albeit much slower than common. The spores of southern rust tend to be orange/cinnamon to brown. Southern rust is favored by temperatures in the 80’s with high humidity (or rainfall).
Even though this southern rust infection is earlier than usual, most of our corn crop is a little head of schedule. While this might not be much comfort, it does mean we might have saved at least one spraying. Last year, it was extremely difficult to stay a head of southern rust because the infectious time was longer than normal due to favorable conditions for infection. Some corn in the southern areas of the state is as far along as the R3/R4 stage. This makes it easier to control rust and reduce the impact since it is much closer to maturity. Much of the corn crop though, is silking to early ear development (R2/R3) which adds roughly 2 to 3 weeks of time to our potential spraying.
If you have good yield potential (and most irrigated growers do), I would consider spraying a combination of fungicides to provide both a curative and preventative type of action. There are great choices today from lots of sources. You may not have a current infection taking place, but spores are active and an application of a combination of fungicides will be great insurance and likely prevent yield loss. As long as southern rust is active, I would consider staying on a 14 day spray schedule or shorter. This disease can certainly undermine all your efforts this year and significantly reduce corn yields.