Status of disease pressure in corn and soybeans

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It is middle of August and corn is rapidly progressing toward maturity with many fields in the hard dough to early dent stage of development. At these growth stages, there will be minimal returns to any fungicide applications.

Southern rust

Based on recent scouting trips, there is little, if any, southern rust in the western third of the state at this time. In northeast Kansas, some level of southern rust can be found in many fields, but nothing seen would warrant a fungicide application at this point. Some fields are just silking and scouting should continue in these fields until the late soft dough stage of development.

Gray leaf spot

Late fields not past R2 should be evaluated for gray leaf spot (Figure 1). Some fields have high levels of gray leaf spot, but most of these are at hard dough when fungicide application would not be profitable. Other fields have very little gray leaf spot present. It is not clear if this is because a fungicide was applied, a resistant hybrid was used, or because weather conditions in the field were not conducive to disease development.

Figure 1. Early development of gray leaf spot lesions showing a distinct yellow halo. Photo by Doug Jardine, K-State Research and Extension.

Stalk rot

As corn progresses to maturity, stalk rot is beginning to be identified. In a field near Perry, Kansas, both Fusarium stalk rot and charcoal rot were present. However, in this field, Fusarium was by far the predominant type of stalk rot. Wet springs followed by a period of dry weather, such as we had in July, with a return to frequent rain is an ideal prescription for Fusarium stalk rot. The very hot and dry July would also be conducive to some charcoal rot, especially on drier upland or sandier soils.

Soybean disease update

The soybean crop is amazingly healthy at this point if fields are in the R3 to R5 growth stages. No frogeye leaf spot could be found in northeast Kansas on a recent scouting trip. The predominant disease has been bacterial blight. This disease is associated with big, blowing thunderstorms that spread bacteria around the field. Fungicides will not be effective on bacterial diseases; however, this disease is not known to be yield limiting.

Fungicides for foliar disease problems are not warranted at this point of the season. Growers who had problems in 2018 with seed quality due to pod and stem blight, purple seed stain, and anthracnose may want to consider a fungicide application with the goal of improving seed quality, if not necessarily yield. Beginning pod fill (R5) would be the best time to make this type of application to provide protection longer into the pod-filling stage. Fungicide efficacy ratings for various soybean diseases can be found at:



Doug Jardine, Extension Plant Pathologist

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