Update from the Field: Fungus Among Us
Posted by Dave Dyson, Agronomist on August 08, 2019
This year’s late planting and extreme weather has growers contemplating what inputs to invest in versus cut. Early in the growing season, we experienced wetter-than-normal conditions, and recently, we have been facing higher-than-normal temperatures and humidity during pollination. These conditions have provided a high potential for disease in corn. The easiest way to describe the potential for disease in corn is to take a look at the disease triangle. The disease triangle is comprised of three factors: host, organism, and environment (Figure 1). If one of these factors disappears, the potential for disease will fall to 0%, but if all three factors are present in a field, there is a good chance that disease will play a major role in the yield potential.
Figure 1: An image displaying the disease triangle. Source: 21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci
The early-season high-moisture conditions inoculated corn plants with disease, and now the hotter weather is propagating the disease inside these plants. As you’re making decisions, let’s review some of the major diseases that are affecting corn, potential impacts to yield, and product applications to help the corn overcome these diseases.
Southern rust is a major disease threat in several areas. As the name suggests, it normally stays south of the Corn Belt. In 2017, we saw this disease reach as far north as the Great Lakes. Southern rust pustules generally tend to occur on the upper surface of the leaf and produce chlorotic symptoms on the underside of the leaf. These pustules rupture the leaf surface and are orange to tan in color. They are circular to oval in shape. We are seeing southern rust starting to reach into the Corn Belt already this year (Figure 2). If left untreated, this disease could stress the corn plant so severely that it will not put an ear on the stalk (Figure 3).
Figure 2: Southern rust is advancing north at an uncomfortable rate.
Source: https://corn.ipmpipe.org/southerncornrust/
Figure 3: This picture was taken near Fostoria, Ohio, in 2017, showing the devastating effect southern rust has on ear development.
Tar spot on corn, caused by the fungus Phyllachora maydis, was first confirmed in the United States in 2015. It has its origins in Mexico and Central America, where the disease is prevalent. These areas are favorable for infection and disease development because of the, cool and humid conditions with long periods of leaf moisture.Tar spot appears as raised, black, circular spots on the top side of the corn leaf (Figure 4). The fields infested with tar spot last year experienced severe stalk degradation. As a result of the weakened stalks, they fell to the ground, making for a difficult harvest. As of August 1, tar spot had been confirmed in Indiana, Michigan, Illinois, and Iowa.
Figure 4: Multiple tar spot stroma found in the lower canopy on corn.
(Photo Credit: Tiffanna Ross, Purdue)
Gray leaf spot can be devastating to a crop, as I’ve witnessed yield reduction as much as 80 bu/acre. Hybrid resistance or susceptibility will determine whether the yield is affected or not. As gray leaf spot lesions mature, they elongate into narrow, rectangular, brown to gray spots. Lesions expand parallel to leaf veins and may become 1.5 to 2 inches long (Figure 5). I have been finding gray leaf spot for over two weeks out in the corn fields, and now I’m finding this disease advancing up past the ear leaf. We can protect the yield potential of all hybrids by applying a fungicide at corn’s VT stage.
Figure 5: A lesion from gray leaf spot was identified on this corn ear leaf in a field near Logansport, Indiana, in 2019.
As mentioned, this year’s environment is conducive to a major disease outbreak in corn. Continue to scout fields and be vigilant about the threat of disease as the season continues. I strongly encourage a fungicide application in and around the VT stage. While going over the corn at this stage, OverPass®CF can be included with the fungicide. The slow-release nitrogen and boron in OverPass CF will help promote pollination and tissue repair due to fungal damage. Contact your local Ag Advisor from The Andersons for disease identification and recommendations of products to curb the fungus among us.
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Dave Dyson is a regional agronomist for The Andersons’ Farm Centers which are located throughout Ohio, Michigan, and Indiana. He is an Indiana native and grew up on a dairy farm in Miami County. A graduate of Purdue University with a degree in Crop & Soil Science, Dave has a deep knowledge of various agronomic topics and is committed to helping growers improve their crops. If you have any questions, Dave can be reached at david_dyson@andersonsinc.com.
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