Radio advertisements, email blasts, and other media are warning of corn diseases and the need for fungicides. Two months of humid, wet weather has allowed for disease development. It’s important to know what diseases truly are in your field before spraying a fungicide, particularly with today’s economics. Here’s what we’re seeing in fields right now in the Clay, Nuckolls, Thayer, Adams county area. Based on the diseases we’re seeing, we would recommend you scout your fields to know whether you have mostly bacterial or fungal diseases present. Consider disease pressure, where on the plant the disease is occurring, growth stage, and economics. We have had southern rust show up in 10 of the last 11 years I’ve been serving in this area. If you spray a fungicide at tassel, you may not have enough residual to ward off southern rust when it appears later, potentially resulting in the need for a second application. In our area thus far, I’m not seeing enough disease pressure in many fields to warrant a fungicide at tassel; consider delaying an application till later for economic and resistance-management reasons. Ultimately this decision needs to be done on a field by field basis. Please also see this UNL CropWatch article regarding fungicide application and corn growth stage. Although I don’t have a photo of it, I’ve also seen common rust in the mid and lower portions of corn canopies thus far.
Fair week tends to be time for tasseling in corn and considerations for watering and fungicide application are being made. Regarding diseases in corn, there has been confusion about a few diseases, particularly about a disease called physoderma brown spot which some have confused for southern rust. The fungus causing physoderma brown spot feeds on pollen and debris on leaves and does not cause harm to the corn plants themselves. Because the spores of this fungus move via water (it’s closely related to oomycetes), numerous lesions can appear on leaves in bands or areas where water collects. While the lesions may look like early southern rust, there will be no pustules present and often purple colored lesions will also be observed in the midrib, leaf sheath, stalk, and outer husks.
When differentiating between southern rust vs. common rust, there are several criteria to consider and this NebGuide is a great resource. Typically common rust will have brick-red pustules randomly scattered on the upper and lower leaf surfaces that are larger in size. It is common rust that we are currently seeing in our fields.
Southern rust in our area tends to have very small, raised, tan-orange pustules on the upper leaf surface of leaves in localized areas on mid-upper leaves. These pustules are tightly clustered on the leaves. However, color and size are relative as sometimes the two diseases can look alike. Microscopic observation is the best way to differentiate the two diseases. Fungal spores from Puccinia sorghi causing common rust will be near perfect round circles whereas fungal spores fromPuccinia polysora will be oblong in shape.
have a bacterial leaf blight that is affecting quite a bit of leaf tissue on some hybrids. These lesions are long and skinny appearing at first to be limited to the veins. There’s been concern about these lesions being severe gray leaf spot but it’s not and there’s nothing you can do about the bacterial disease. Please don’t mistake this bacterial disease as a fungal one and trigger a fungicide application too early.
Fungicide Application Timing
We tend to see southern rust in our part of the State each year; it’s a matter of time. Triggering a fungicide application too early may result in no residual for when you need it if/when southern rust occurs. Every year some producers make more than one fungicide application due to blanket applications at tassel or shortly after followed by another fungicide application when southern rust occurs later in the year. Consider good fungal resistance management and apply fungicides when disease pressure warrants them in your fields and also consider economics for your situation for proper fungicide application timing.