Southern Rust Update for Kentucky

Figure 1. Southern rust on corn

Kiersten Wise, Extension Plant Pathologist

Southern rust of corn, caused by the fungus Puccinia polysora, has NOT been confirmed in Kentucky, as of July 16, 2019. However, the disease has been confirmed in southeast Missouri and western Indiana, so it is possible that southern rust is present in Kentucky at low levels. We continue to scout and monitor fields, and encourage others to scout and submit samples to the Plant Disease Diagnostic Laboratory (PDDL) through local County Extension Agents if they suspect they have the disease.

Southern rust is first observed as raised, dusty orange pustules on the upper surface of the leaf (Fig. 1). Pustules will typically be present only on the upper surface of the leaf. The disease is easily confused with common rust, which produces pustules on both sides of the leaf. Common rust (Puccinia sorghi), is present in many Kentucky corn fields and is not economically important to manage, so it is important to distinguish between the two diseases before applying fungicide. If southern rust is suspected, the fastest way to get a diagnosis through the PDDL is to submit samples through County Agents. Confirmations of southern rust will be posted on the cornipmpipe website here: https://corn.ipmpipe.org/southerncornrust/. On the map, red counties/parishes indicate that southern rust has been confirmed by university/Extension personnel.

The potential impact of southern rust in Kentucky in 2019 will depend on the crop growth stage of a field once southern rust is confirmed in an area. 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 once corn reaches the blister (R2) growth stage. More information on timing of fungicide applications for southern rust can be found in Table 2 of the Crop Protection Network publication “Southern Rust” which can be read here: https://cropprotectionnetwork.org/resources/publications/southern-rust.

Very late planted fields of high-value corn should be scouted to determine if the disease is present. Fungicide application may be beneficial in certain situations, but this should be determined on a case-by-case basis in 2019.