Southern Rust Found in Central Iowa

July 16, 2012
ICM News

By Alison Robertson, Department of Plant Pathology and Microbiology, and Erika Salaau-Rojas, Plant and Insect Diagnostic Clinic

Southern rust was reported in several fields in Butler and Grundy counties late last week. Leaf samples were received by the ISU Plant Disease and Insect Clinic, and confirmations were made. Because common rust is widespread in cornfields in Iowa, it is important for farmers and agronomists to correctly distinguish between these two rusts, especially if a fungicide decision is to be made.

 Southern rust versus common rust

Southern rust can develop rapidly under favor able conditions (Table 1), and foliar fungicides are often required to protect yield. The earlier during the grain fill period that southern rust occurs, the greater the impact it can have on yield. Although we see southern rust in Iowa in most growing seasons, it is usually only reported in mid- to late August as the crop nears maturity. The outbreak of southern rust in central Iowa is unusually early, grain fill has just started, and, therefore, this outbreak is of concern. Furthermore, weather conditions in central Iowa at this time are very conducive to disease development (Table 1).

Common rust rarely impacts yield because most hybrids have very good tolerance to the disease, and conditions during grain fill are often too warm for common rust disease development (Table 1).  Thus, a fungicide application for common rust is not usually warranted to protect yield.


Hybrids vary in their tolerance to southern rust, so some fields will be more at risk than others. 

Previous cropping history and percent crop residue do not increase risk since this pathogen survives in the south and spores are blown up to Iowa each growing season.

Foliar fungicides are very effective against southern rust, but timing is important, especially if weather is conducive for disease development; applications should be made as soon as possible after southern rust is identified in a field or on susceptible hybrids within the area in which the disease has been reported.

Figure 1.  Common rust pustules are brick red, elongated and sparsely scattered on both leaf surfaces.


Figure 2. Southern rust pustules are bright orange, small and circular, and clustered on the top surface of leaves.

  Update on Goss's wilt and leaf blight

Goss's leaf blight symptoms have been observed in our foliar product efficacy trials in southwest Iowa and central Iowa. Furthermore, ISU Extension and Outreach Field Agronomist Brian Lang reported Goss's leaf blight on drought-stressed corn in northeast Iowa. The ISU PDIC has also received several leaf samples with Goss's leaf blight. Farmers and agronomists are encouraged to continue to scout at risk fields for Goss's. Please see this ICM News article, "Tips for diagnosing Goss's leaf blight."


Jackson, T. A. 2008.  Rust diseases of corn in Nebraska. Nebguide G1680.

Alison Robertson is an associate professor of plant pathology and microbiology with extension and research responsibilities. You can reach her at 515-294-6708 or e-mail Erika Salaau-Rojas is a diagnostician in the Plant and Insect Diagnostic Clinic. You can reach her at 515-294-0581 or e-mail

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 16, 2012. The information contained within may not be the most current and accurate depending on when it is accessed.


Alison Robertson Associate Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is an associate professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received h...