KTIC Radio Extension Corner: Gray Leaf Spot and Southern Rust

Figure 1. Symptoms of southern rust from a late planted field in 2016 near Fremont.

Listen to this week’s KTIC Radio Extension Corner on Gray Leaf Spot and Southern Rust:

“This is Dr. Nathan Mueller, your local agronomist with Nebraska Extension for Dodge and Washington counties. Over the past week two leaf fungal disease have become much more noticeable and may warrant fungicide application. This includes gray leaf spot and southern rust in corn. Let discuss how to identify these diseases in your fields and considerations for fungicide application.

Over the past 10 days, many field went from having no visible symptoms of gray leaf spot to leaves with lesion all the way up to the ear leaf. Visible symptoms of gray leaf spot are tan or gray lesions rectangular in shape and limited by veins on the side and with blunt ends. Symptoms initially appear on lower leaves with lesions growing longer and moving to higher leaves on the plant. Yield losses primarily occur when leaves above the ear have symptoms. Determining economic thresholds is difficult, but the general guidelines are if gray leaf spot lesions add up to the size of a quarter at pollination time on the three leaves, the ear leaf and the leaf above and below it. Fields at higher risk are those later planted irrigated continuous corn fields with more susceptible hybrids, but all fields should be scouted because disease severity is dependent on presence of the disease, environment conditions, and the hybrid susceptibility. Be aware that a new disease called bacterial leaf streak can confused with gray leaf spot and is not controlled with a fungicide application.

The second fungal disease, Southern rust, was confirmed in Dodge County last week near Scribner and as well of many counties to the south in July (see the most updated map of southern rust at CropWatch). Southern rust must blow in every year from the south unlike gray leaf spot that overwinters. The earlier onset of southern rust this year does pose more of a risk. Southern rust pustules typically are a red-orange in color, round to oval shaped, and clustered together on the upper leaf surface (Figure 1 below). The disease is favored by temperatures in the 80s to 90s, so the onset of the cool weather should help. Fields should be scouted for this disease over the next several weeks. There are no good thresholds or recommendations on when to treat for southern rust. Protection from southern rust with fungicides may only last 21 to 28 days. Early planted corn fields that were sprayed with a fungicide application in mid-July at silking are still at risk and will need to be scouted. Be aware that common rust can be confused with southern rust, so please seek assistance if needed.

To listen to this radio message again and to get more information on gray leaf spot and southern rust including pictures, visit our local website at croptechcafe.org or give me a call at 727-2775. Know your crop, know your tech, know your bottom line. This is Dr. Nathan Mueller, your local agronomist for Nebraska Extension on KTIC radio.”

More information on gray leaf spot: