Southern rust of corn has been confirmed in 20 Nebraska counties and has likely spread to others, but samples haven’t been submitted for confirmation, reports Tamra Jackson-Ziems, University of Nebraska-Lincoln Extension Plant Pathologist.
During the weeks since southern rust was first confirmed, severity had not increased much, likely due to the unseasonably cool weather. However, during the last week, farmers, crop consultants, and UNL Extension faculty in eastern and southern Nebraska are reporting a rapid increase of southern rust in some fields. Warmer temperatures forecast for the coming days in combination with high humidity and precipitation could support further disease increase.
Scouting And Management
Unfortunately, the protection provided by fungicides applied more than three weeks ago has likely worn off. Corn in many areas of the state still has several vulnerable weeks of grain fill left. According to the publication, Corn Growth and Development (Abendroth, et al., 2011, Iowa State University Extension), more than 50% of kernel dry matter weight can accumulate during the R5 (dent) stage, making protection of leaf area during this stage critical to maintaining high yield potential.
Scouting is essential in areas where corn is still in the early stages of grain fill and the disease has not been reported or where it has been confirmed and needs to be monitored for severity. Continue to scout and monitor for early evidence that the spores are infecting corn plants, such as the sudden development of new, small yellow flecks on leaves. These flecks can indicate early disease development, such as rusts, and may help anticipate an increase in disease and potential need for fungicide application.
Severe southern rust and other foliar diseases covering substantial leaf area can lead to stalk rot diseases as stalks are cannibalized during grain fill. Stalk rot diseases can reduce standability, leading to lodging and complicating harvest.