April 30, 2020
Late June and July are key times to scout for pests. Scouting is the first step to identifying if there are issues and if treatment is warranted. Whether you hire a scout or do the scouting yourself, here are a few things to think about this year.
First, try to cover as much of the field as you can with the least amount of effort. This is easier said than done, but walking your fields in a Z pattern can really make a big difference. (If you hail from Michigan or Wisconsin, it would be an M pattern or W, respectively, but here in Nebraska we will say a Z pattern.) Also try to stop every 10 acres or so to take a closer look.
Second, stay out of the end rows or waterways for a truly accurate representation of your field. Lastly, weeds and insects typically are more straightforward compared to diseases. Having your cellphone or a pocket reference guide can be handy when trying to identify insects and weeds you encounter when scouting your fields. In this article, I will primarily focus on three diseases to look for this growing season while you are scouting.
Don't get "corn"-fused this growing season by lesions in your fields. Bacterial leaf streak and longtime disease-player gray leaf spot typically start making an appearance in early July. Bacterial leaf streak has lesions categorized with wavy margins that when back lit, appear to have a slight yellow halo.
When looking at gray leaf spot, lesions will have blocky edges similar to a rectangle. You can see the side-by-side comparison below of the different lesions. Timely scouting fields for these diseases will be imperative to treatment decisions, so while you are out scouting corn, take a quick trip over to your bean fields.
DISTINGUISHING FEATURES: Gray leaf spot lesions versus bacterial leaf streak lesions. Bacterial leaf streak and gray leaf spot typically start making an appearance in early July. While at first glance the two can appear similar, bacterial leaf streak has lesions categorized with wavy margins that when back lit, appear to have a slight yellow halo. Meanwhile, when looking at gray leaf spot, lesions will have blocky edges similar to a rectangle.
The fields may have eyes this year — frogeye, to be exact. Frogeye leaf spot is a fungal disease that has become more common in Nebraska. Frogeye has been an issue in the eastern Corn Belt for several years and can cause yield losses up to 30% in a susceptible hybrid with extensive lesions.
A good rule of thumb is to start scouting for frogeye around the same time frame as gray leaf spot, and it typically occurs just after soybeans begin to flower on the upper canopy. The lesions are gray-brown with a purple margin. The photo below shows the initial identification of frog eye leaf spot in early July to the blighting of the leaf in early August.
RESISTANCE SPREADING: Frogeye leaf spot progression in a field near Columbus, Neb. As frogeye leaf spot has continued to be more common, resistance also has followed. Fields with frogeye resistance were confirmed in 10 Nebraska counties in 2019.
As frogeye leaf spot has continued to be more common, resistance also has followed. Fields with frogeye resistance have been confirmed in 10 counties in Nebraska from 2019 sampling completed by Tamra Jackson-Ziems, Extension plant pathologist with the University of Nebraska-Lincoln — specifically, resistance to products that contain FRAC Code 11, QoI (Quinone outside inhibitors) or Strobilurins.
If you believe a fungicide application did not slow the spread of this disease, please contact Jackson-Ziems via email at [email protected] or via Twitter at @tjcksn, so a sample can be tested for resistance. Tracking the spread of the resistant populations will be important when making management decisions.
Remember, it is important to confirm the disease before applying treatment. Bacterial leaf streak is not thought to be controlled by fungicides commonly used to treat gray leaf spot. Frogeye leaf spot also can be easily misidentified with other nonyield-threatening diseases. Therefore, application of a fungicide for the wrong disease may be a pricey mistake.
When treating gray leaf spot or frogeye leaf spot, be sure to determine if you are using a curative, preventive or mixed mode-of-action fungicide. There are various cycles of disease throughout one growing season, so using a mixed mode fungicide that contains both preventive and curative ingredients can help protect through multiple cycles of the disease.
It is important to note that "curative" does not mean that the fungicide will cure the plant of the disease, but it will help protect against newly forming infections within the stand. Scouting after the application of pesticides is recommended to determine the effectiveness of the treatment.
At the end of the day, the growing season is a busy time. Whether you hire a scout or cover your own acres, timely scouting can make a difference for management decisions. Prioritizing scouting can get difficult, but taking the time to scout fields or walk in fields can make a difference. If you are finding issues that you are unsure of what they are or how to treat them, contact your agronomist, other farmers or your Extension educator in your area for more information.
Taylor is a Nebraska Extension crops and water educator.
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