Southern Rust Management in a Dry Year

In a year that has had ups and downs for corn farmers, there is good news that corn disease pressure across the majority of the corn producing region of the state is very low. The organisms that cause disease prefer frequent rains and mild/moderate temperatures, which have been uncommon this year across the state. The disease that most farmers have been concerned about over the last week is southern rust, caused by the fungus Puccinia polysora.

Typically, Kentucky has its first confirmation of southern rust in mid-July. However, the disease has progressed more slowly in 2022 because of a lack of tropical storms and dry conditions in states to our south.  Figure 1 shows the typical progression of the disease according to our southern corn rust monitoring system on the cornipmpipe website. On the map, red counties/parishes indicate that southern rust has been confirmed by university/Extension personnel.

Figure 1. Southern corn rust progression in the United States during the third week of July over the last three years.

The potential impact of southern rust in Kentucky 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 may be needed to protect yield while corn is in the tasseling through milk (VT-R3) growth stages. Once corn is past milk (R3), fungicides are likely not needed to manage the disease. If fields have already received a fungicide application this year at tasseling/silking (VT/R1), they are not likely to need a second application of fungicide this year.

This year, it is also important to weigh crop value before applying fungicide. Many areas have limited yield potential due to the hot, dry conditions during critical growth stages in late June and early July. The decision to apply a fungicide for southern rust management in fields that have experienced significant drought stress should be determined on a case-by-case basis and should also involve conversations with crop insurance adjustors.

Southern Rust vs. Common Rust

Southern rust is first observed as raised, dusty orange pustules on the upper surface of leaves (Figure 2). 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), can be found sporadically in Kentucky corn fields and is not economically important to manage, so it is important to distinguish between the two diseases before applying fungicide. Common rust has been rare in 2022, as the causal fungus likes cooler weather than we have experienced in June and July. If southern rust is suspected, the fastest way to get a diagnosis through the Plant Disease Diagnostic Lab is to submit samples through county agents. Confirmations of southern rust will be posted on the cornipmpipe website.

Figure 2. Southern corn rust (Photo: Kiersten Wise, UK)

More information on timing of fungicide applications for southern rust can be found in Table 2 of the Crop Protection Network publication “An Overview of Southern Rust”.

The efficacy of specific fungicide products for southern rust are described in the updated fungicide efficacy table for management of corn diseases, which is developed by the national Corn Disease Working Group and posted on the Crop Protection Network website.

Tar Spot Update

As a final note, my program has been monitoring for the disease tar spot. Tar spot has NOT been confirmed in Kentucky in 2022, to date. We will continue tar spot monitoring, with the help of the Kentucky Corn Growers Association. Tar spot identification and suspected sample submission were discussed in the June 28 issue of KPN.

By Kiersten Wise, Plant Pathology Extension Specialist

Posted in Grains, Uncategorized
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