Frogeye leaf spot is a common soybean disease that many soybean growers face annually. The disease is caused by the fungal pathogen Cercospora sojina and can cause severe yield loss and poor-quality grain.
Initial frogeye leaf spot lesions appear on soybean leaves as small circular lesions that have a dark halo around a light brown center (Figure 1). As these lesions grow, they can become blocky in appearance, but maintain the dark brown halo (Figure 2). These lesions can be easily confused with another soybean disease, Phyllosticta leaf spot (Figure 3). Early on, it may be necessary to send leaf samples to a lab for identification as Phyllosticta and frogeye lesions can be very difficult to distinguish. As Phyllosticta progresses, black pycnidia (fungal structures) become visible inside the lesion making it distinguishable from frogeye. These lesions are not limited to the leaves. As the disease spreads, the pathogen can cause lesions on both the stem and pods. Stem lesions are brown in appearance and can take many shapes. Pod lesions are also brown to black in appearance with a more circular shape.
Damage from the Disease
Frogeye lesions on plant leaves reduce the amount of photosynthetic area the plant has to produce grain yield. Depending on how late in the plant’s life cycle the lesions form, reduced photosynthesis can cause the plants to abort pods and stems and reduce seed size. In many situations yield loss is often attributed to the foliar symptoms; however, pod lesions can cause a different form of yield loss. Pod lesions often turn into infected grain which has a brown/black and shriveled appearance (Figure 4). In 2018, growers across much of the country saw firsthand the damage frogeye can do to grain quality. These impacts continued to be felt into 2019 as the seedstock was also impacted. Specifically, in the seed production market, frogeye can cause poor quality germination and it also acts as another vector for disease spread as infected seeds spread the disease into the next growing season.
The pathogen survives on infected plant residue and on infected seeds as discussed. Although many soybean fields are in a rotation with corn, there is still enough soybean residue left after a year of corn to spread the disease. In-season, spores are produced and blown by the wind allowing additional disease proliferation in the correct environmental conditions. Generally, this disease takes warm and humid conditions to develop and spread, which a large part of the country exhibits during soybean grain fill.
Thankfully, there are several different management options to aid in controlling this disease. Tillage and crop rotation can help reduce the amount of disease inoculum that is carried over from year to year. Tillage mixes the infected residue with the soil which helps increase residue breakdown and reduces the opportunity for inoculum survival. Similarly, crop rotation gives infected residue more time to breakdown, again not allowing that residue to be present when the next soybean crop is planted.
Selecting the best soybean product for the environment is another management option. If, for example, a grower is planting soybean after soybean in a no- till environment (high risk), selecting a product with better frogeye leaf spot resistance would be very beneficial. Although, in high pressure situations, no product can be completely protected, and a foliar fungicide may be the best option. Finally, chemical control utilizing foliar fungicides has been very effective at reducing the spread of this disease. Optimum application timing has been R3 to R4 growth stage depending on conditions. However, growers need to be careful which fungicide is selected as frogeye leaf spot has been found to be resistant to group 11 (Strobilurins) fungicides. This drives home the importance of selecting fungicides with multiple modes of action. Delaro® Complete fungicide is a great example of a product with both group 11 and group 3 (triazoles) modes of action to help control frogeye and other diseases.
Frogeye leaf spot can be a detrimental soybean disease that some growers face every year. The disease limits the amount of photosynthetic area by causing leaf lesions, reduces yield potential, and reduces grain quality by infecting the pods and grain. The disease can be managed by utilizing a combination of tillage, crop rotation, and foliar fungicides. If pressure is high, foliar fungicides are the best method of in-season control.
Mane, A., Everhart, S., and Jackson-Ziems, T. 2020. Fungicide resistance and management of frogeye leaf spot of soybean in Nebraska. https://cropwatch.unl.edu/2020/fungicide-resistance-and-management-frogeye-leaf-spot-soybean-nebraska.
Westphal, A., Abney, T. S., and Shaner, G. Frogeye leaf spot. Diseases of Soybean. BP-131-W. Purdue Extension. Purdue University. https://www.extension.purdue.edu/extmedia/bp/bp-131-w.pdf.
Hershman. D. E. 2013. Soybean foliar spots and blights. Plant Pathology Fact Sheet. PPFS-AG-S-19. University of Kentucky. http://plantpathology.ca.uky.edu/files/ppfs-ag-s-19.pdf
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