Dr. Bob Kemerait, UGA Plant Pathologist reported this morning on two incidences of southern corn rust in Georgia. County Extension agents and crop consultants have verified infections in Jeff Davis and Decatur Counties. The following are Dr. Bob’s quotes from two separate emails:
First email: “Received photo from consultant scouting corn in Decatur County yesterday; southern corn rust has just become established in that field. Our first find of southern corn rust in Georgia in 2020. Certainly more is to come. Grateful that many of our AG consultants try to maintain good relations with you as UGA Extension agents. We have similar but different jobs. Each of you is CRITICAL for the success of our growers.”
Second email: “Greetings – thanks to efforts of a scout and Jennifer Miller, southern corn rust is now confirmed in Jeff Davis County. With detection’s in Decatur and Jeff Davis Counties, Southern corn rust could be lightly scattered anywhere along our southern Coastal Plain. We are early, way too early.”
Fellow growers, this is not good news. I STRONGLY URGE ALL CORN GROWERS TO SCOUT YOUR FIELDS FOR THIS DISEASE! Generally speaking, it is a little early for southern corn rust and given favorable weather for spreading, this disease will hurt corn yields. In this case, I would generally make a recommendation to make a prophylactic fungicide application. This would reduce or prevent the occurrence or infection in an otherwise healthy corn field. However, corn prices are down and everyone is looking at growing this year’s crop as efficiently as possible. In other words, you want the best ROI (return on investment) as you can get. If the current rust infections follow weather patterns of previous years, then we will continue to see it spread and infect more fields. As it infects and produces more spores, then occurrences will increase and more yield can be lost. Previous experience suggests that prophylactic applications give great ROI’s especially when a disease is spotted widespread and infections are earlier than normal, particularly in corn that is VT or R1.
In discussing this development with Dr. Corey Bryant, UGA grain and soybean agronomist, and Dr. Bob, we agree that individual situations and field environments warrant careful consideration on how decisions are made on about spraying. Warm, moist conditions (current weather patterns) favor southern corn rust infection and the best way not to lose yield is to spray a fungicide with mixed modes of actions. See the link to the pest control hand book chapter on Field Corn Disease Control . My expectation of fungicides with mixed modes of action are that they can provide protection for about 3 weeks. If the spore load is continuous over this time and you continue to scout and see new infection, then you certainly will need to spray again. If you choose to use a single mode of action such as a triazole, I would advise that you begin reevaluating two weeks after the initial application for new infections. Corey, Bob and I definitely feel, that IF you are uncertain about making a prophylactic application and you or your consultant have been scouting your corn fields and will continue scouting, then certainly wait but don’t get behind the infection. However, if you only scout on occasion, then consider an application if you are in one of the two counties where it has been identified or in a county that surrounds the county that rust has been identified. If not, then thoroughly scout all your fields for the presence of southern corn rust now, and spray particularly if your corn is just now tasseling and you see a little infection. I encourage you to continue scouting through to black layer.
Our earliest planted corn fields in the southern tier counties are likely in R2 to early R3 growth stage. Heavy rust infection will reduce test weight, undermine grain filling and certainly reduce stalk integrity leading to harvest losses. The earlier the infection begins in kernel growth, the worse the yield loss. As Dr. Bob said, “Respect this disease”. I agree fully.