Using Foliar Fungicides to Manage Soybean Diseases
Applying foliar fungicides to soybeans at about stage R3 to protect yield has become a common practice. However, the following points should be considered before automatically adopting this approach.
• Selecting a soybean variety that has genetic resistance to a particular disease pathogen should be considered as the first line of defense against the targeted disease. This should be the foundation of all disease management plans for soybeans if a variety or varieties with such resistance exists.
• Fungicides should only be considered or used as the first line of defense against any disease if there are no other control options available.
• The primary purpose of applying any foliar fungicide is to control fungal pathogens that pose a known and significant threat to a soybean crop. Thus, choice of product should be based on known efficacy against the target pathogen. Click here for a table of efficacy ratings from the Crop Protection Network [CPN]. The mode of action [MOA] for each product is listed in the table.
• The timing of spraying any foliar fungicide should be based on the soybean growth stage that is thought to be the most vulnerable to a disease infestation. Spraying some products too early may not provide protection that lasts long enough to protect a crop from yield loss resulting from a disease infestation. This means that a second application may be required to protect yield of the crop, thus adding an additional expense that could have been avoided by a more timely application that was closer to the expected time of disease presence. Such multiple fungicide applications will increase costs that will influence return on investment [ROI].
• Fungicide resistance management should always be a prime consideration when selecting a fungicide product to use. This means that the MOA of an applied product should be known and recorded to ensure the rotation of fungicides with different MOA’s.
• Pay particular attention to the residual activity that the applied fungicide has. A longer residual activity will allow an application of a particular fungicide earlier than the usual time. The fungicide Revylok [fungicide from BASF that contains ingredients with preventive and curative properties] is such a fungicide that is purported by the company to have long-lasting residual activity that will allow it to be applied earlier than R3 [e.g. R2] and still control diseases that may appear late in the growing season. This property will allow a fungicide to be applied early enough that it is still present on the leaf to control the target disease or diseases should they appear. Again, the fungicide application should be based on expected or known presence of a disease.
• The application of many foliar fungicides labeled for soybeans is promoted to increase yield. However, yield increase that results from a foliar fungicide application may be minimal and not cover the cost of the product and its application. Thus, ROI should be calculated each year a fungicide is used to determine if it was effective at increasing ROI. It likely will not be if the target disease does not appear or is only minimally present.
• An article titled A machine learning interpretation of the contribution of foliar fungicides to soybean yield in the north-central United States by D.A. Shah et al. provides the following information. 1) More yield benefit will be realized when foliar fungicides are applied to late-planted soybeans, but even then the yield increase will likely be very small. 2) Yield gain due to use of foliar fungicides sufficiently offset the costs that are associated with their use when soybean prices were near-to-above-average. Thus, prophylactic application of foliar fungicides to soybeans will rarely increase profits but will hasten resistance development. 3) The results reported in this article support the importance of scouting for disease presence before applying a foliar fungicide.
• All of the above point to the following important points that should be considered when contemplating the application of foliar fungicides to soybeans. 1) Selecting soybean varieties that have genetic resistance to a particular disease pathogen should be considered as the most cost-effective first line of defense for soybean disease management. 2) A continuous annual prophylactic application of a fungicide or fungicides with the same MOA will hasten the development of disease resistance to those fungicides. 3) Prophylactic applications of foliar fungicides to soybeans will only rarely increase yield enough to positively affect ROI. 4) Producers should take note that foliar fungicide applications to soybeans rarely provide a significant yield increase.
Composed by Larry G. Heatherly, June 2025, larryh91746@gmail.com