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 soybean varieties that have genetic resistance to a particular disease pathogen is the first line of defense against the resulting 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 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 for each product is also 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.

•   Fungicide resistance management should always be a prime consideration when selecting a fungicide product to use. This means that the mode of action of an applied product should be known and recorded to ensure the rotation of fungicides with different modes of action. The mode of action for current foliar fungicide products is shown in the above-linked table from the CPN.

•   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 recommended time of R3. The fungicide Revylok [new 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 any fungicide to be applied early enough that it is present on the leaf to control the target disease or diseases should they appear.

•   The application of many foliar fungicides labeled for soybeans is promoted to increase yield. However, yield increases from their application may be minimal and not cover the cost of the product and its application. Thus, Return-on-Investment [ROI] should be calculated each year of their use to determine if they are effective at increasing ROI. They likely will not be if the target disease does not appear or is only minimally present.

•   This then leads back to the point made in item 1 above. Selecting soybean varieties that have genetic resistance to a particular disease pathogen should be considered as the most cost-effective first line of defense in soybean disease management.

Composed by Larry G. Heatherly, June 2024,