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Do I Need A Fungicide On My Soybeans?

Fungicide use in soybeans is a somewhat controversial subject in Delaware.  Overall, Delaware soybeans do not suffer to a significant degree from fungal diseases such as white mold or even Frogeye leaf spot.  More commonly, we see brown spot and downy mildew on our beans, especially this time of year.  Should you spray for downy mildew and brown spot?  The answer for downy mildew is no.  The answer for brown spot, in the vast amount of cases, is no as well.  Why is this?
 
Downy mildew is a disease that prefers cooler temperatures and persistent humid conditions.  Hot, dry weather stops this pathogen dead in its tracks.  Often growers see a slight flush in downy mildew right around canopy closure.  Then, the disease stops as temperatures increase and we hit summer full on.  Don’t worry about downy mildew.
 
Septoria brown spot is a residue borne disease that also requires very persistent, wet conditions to cause disease.  This often means that the disease is restricted to the lower canopy, which is not contributing to yields later in the season.  Significant defoliation in the lower canopy, upwards of 30%, will likely be required before any yield loss will be detected in fields.  Most varieties are very tolerant to brown spot and the disease often has little to no impacts on yield except if an extremely susceptible variety is planted.
 
The only foliar disease that could occasionally require intervention in our area is Frogeye leaf spot.  Thusfar this year has not been conducive to the development of  Frogeye leaf spot in full season soybeans.  If you do have noticeable levels of Frogeye in a field at R1, an application of a premix or triazole fungicide at R3 may provide some benefit if wet, humid conditions are likely to persist in the coming days or you plan on heavily irrigating your soybeans.
 

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