Southern rust of corn, caused by the fungus Puccinia polysora, was confirmed by the University of Kentucky Plant Disease Diagnostic Laboratory (PDDL) for the first time in 2018 from a sample collected in Logan County. This is the first confirmation of southern rust in Kentucky in 2018. The disease was at very low incidence and severity in the field where it was confirmed.
The good news for Kentucky is that most corn is at a growth stage where fungicide applications will not be needed to manage southern rust in 2018. Previous research from southern states indicates that fungicides are likely not needed once corn is past the milk (R3) growth stage. Also, if fields have already received a fungicide application, they are not likely to need a second application of fungicide in 2018 due to the very low incidence and severity of the disease. More information on timing of fungicide applications for southern rust can be found in Table 2 of the Crop Protection Network publication Southern Rust (CPN-2009-W_Final-1).
Very late-planted fields of high-value corn should be scouted to determine if the disease is present. Fungicide application between tasseling (VT) and milk (R3) growth stages may be beneficial in certain situations, but this should be determined on a case-by-case basis in 2018.
Southern rust is first observed as raised, dusty orange pustules on the upper surface of the leaf (Figure 1). The disease is easily confused with other foliar diseases, such as common rust and gray leaf spot. If you suspect you have southern rust in your field, work with local county Extension agents to submit samples to the PDDL for proper identification. Confirmations will be posted on the Integrated Pest Information Platform for Extension and Education (iPiPE). On the map, red counties/parishes indicate that southern rust has been confirmed by university/Extension personnel (Figure 2).
By Kiersten Wise, Extension Plant Pathologist