Southern rust plagues corn in central Iowa

Southern rust was reported in several cornfields in Butler and Grundy counties in central Iowa in mid-July. Leaf samples were received by the Iowa State University Plant Disease and Insect Clinic at Ames, and the disease was confirmed.

Southern rust plagues corn in central Iowa

Southern rust was reported in several cornfields in Butler and Grundy counties in central Iowa in mid-July. Leaf samples were received by the Iowa State University Plant Disease and Insect Clinic at Ames, and the disease was confirmed.

Because another type of rust, called common rust, is widespread in cornfields in Iowa, it is important for farmers and agronomists to correctly distinguish between these two rusts (common rust vs. southern rust), especially if a fungicide decision is to be made.

Southern rust can develop rapidly under favorable conditions, and foliar fungicides are often required to be applied to protect yield, says ISU Extension plant pathologist Alison Robertson. 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,” she says.

Key Points

Southern corn rust, a leaf disease, has shown up in some fields this summer.

It’s important to know how to properly identify southern rust vs. common rust.

ISU offers management suggestions to help control various corn leaf diseases.


This year, the outbreak of southern rust in central Iowa was unusually early. Grain fill had just started, and therefore, this outbreak is of concern. Furthermore, weather conditions in central Iowa in mid-July were conducive to disease development.

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

Corn 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 on the soil surface 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, says Robertson. Applications should be made as soon as possible after southern rust is identified in a field or on susceptible corn hybrids within the area in which the disease has been reported.

Update on Goss’s wilt

What’s happening with Goss’s wilt and leaf blight? There are two other corn diseases to watch for this summer in Iowa.

Goss’s leaf blight symptoms have been observed in ISU’s foliar fungicide product efficacy trials in southwest and central Iowa. Also, ISU Extension field agronomist Brian Lang at Decorah has reported Goss’s leaf blight showing up in some fields of drought-stressed corn in northeast Iowa.

The ISU Plant Disease Clinic at Ames has also received some corn leaf samples with Goss’s leaf blight. Farmers and agronomists are encouraged to continue to scout their at-risk fields for Goss’s wilt. For more information, Robertson suggests reading “Tips for diagnosing Goss’s leaf blight” at www.extension.iastate.edu/CropNews/2012/0628robertson.htm.

Source: ISU Extension

1649b.tif

SOUTHERN RUST:
Southern rust pustules are bright-orange, small and circular, and are clustered on the top surface of leaves.

1649a.tif

COMMON RUST: Common rust has pustules that are brick-red, elongated and sparsely scattered on both leaf surfaces.

This article published in the August, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.

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