Corn Disease Update – Southern Rust, Gray Leaf Spot, and Northern Corn Leaf Blight 7-31-15
July 31, 2015
Southern rust (Figure 1) continues to be confirmed in samples from numerous counties in eastern and south central Nebraska, as indicated on the IPM PIPE website. The disease has reportedly been confirmed at low incidence and severity in the fields thus far, but with recent days of warm humid conditions will likely spread and increase in severity. Southern rust is likely already present in low levels in most of the counties in this part of the state, but may not develop in every field.
Frequent scouting of corn fields is critical to determine if or when it develops and to what extent. It can take a few days to several weeks for southern rust to develop and become severe, complicating treatment decisions. The potential for yield-limiting southern rust development is higher this year than in recent years because it developed earlier in the season and a lot of corn was planted later than normal, thus is delayed in development. Periods of rain, high humidity, dew, etc. with day or night temperatures in the 70s and 80s F will likely favor more rapid disease spread and development.Foliar fungicides can be very effective when managing this disease, but the protection they provide can be lost in about 21 days, possibly requiring retreatment if severe disease develops or if the first application was made several weeks ago. Late planted fields that have corn delayed in development are most vulnerable to the effects of southern rust as they have several weeks of grain fill left.
Identification of southern rust can be complicated by the presence of other similar looking diseases and so it is frequently misdiagnosed. Very early symptoms of southern rust or some other diseases may begin as tiny yellow flecks making early diagnoses difficult. The most reliable method for identification of corn rust diseases is based on examination of microscopic spore characteristics. This can be done quickly for samples submitted to the UNL Plant & Pest Diagnostic Clinic. Southern rust spores are usually orange/tan in color, produced mostly on the top side of the leaf, and easily wiped off.
Gray Leaf Spot and Northern Corn Leaf Blight
With the warmer temperatures and high relative humidity during recent weeks, some crop consultants and Extension educators are reporting that northern corn leaf blight seems to be slowing its spread, while gray leaf spot (Figure 2) is becoming increasingly important and moving higher in the plant canopy.
Some hybrids may need treatment with a fungicide for gray leaf spot to protect the upper leaves if it continues to worsen and is in high risk growing conditions, such as susceptible hybrids, favorable warm and humid weather, early disease development, continuous corn, and minimum tillage. Because it takes 14-21 days for gray leaf spot lesions to develop, keep in mind that one to two leaves above the leaf where you find the highest lesion may already be infected.
A list of foliar fungicides labeled for use on corn in Nebraska and their characteristics are summarized in the 2015 Guide for Weed Management with Insecticides and Fungicides.
Results from foliar fungicide trials conducted in Nebraska are in the Corn Plant Disease section of CropWatch. These results were collected from trials with natural infestations of gray leaf spot and sometimes mild southern rust.Other resources linked below can help you identify diseases and track the Nebraska counties where southern rust has been confirmed.
- Map of Confirmed Southern Rust Distribution. Keep in mind that counties highlighted in red are ones where at least one sample was confirmed to have southern rust based on laboratory examination. Red shading does not imply that all fields in the county have southern rust. Southern rust must have been confirmed in the laboratory to highlight a county in red. Likewise, the lack of shading in counties or states does not indicate the disease is not present and fields should be monitored for southern rust development so that treatment decisions can be made in a timely manner for better disease management, as needed.
- Rust Diseases of Corn in Nebraska (UNL G1680)
- Corn Disease Profiles 1: Foliar Diseases (UNL EC1867)
- Corn Diseases: Identifying Rust (UNL Video)
- Corn Diseases: Gray Leaf Spot (UNL Video)
- Corn Diseases: Northern Corn Leaf Blight (UNL Video)
Extension Plant Pathologist