Soybean is fast approaching or is already at the flowering stage in several fields scouted in the state. This is a critical period when disease management decisions are made, specifically, fungicide applications. Fungicides are usually applied between R1 (beginning flowering) and R3 (beginning pod). Most common foliar fungal diseases start to be noticed around the beginning of flowering mainly due to canopy closure, which provides a conducive microclimate for plant pathogens to initiate infection.
Common Soybean Diseases
Some of the most common soybean diseases that can be managed through a foliar fungicide application are Septoria brown spot, Cercospora leaf blight, downy mildew, powdery mildew, frogeye leaf spot, and white mold. Of these, white mold is the most destructive and can result in severe localized yield losses within a soybean field. The majority of the fungal foliar diseases usually cause minimal yield loss in our area unless a very susceptible soybean cultivar is planted, soybean is planted following soybean, there is narrow row spacing, or there is excessive rainy warm weather at the flowing growth stage. Unless warranted, foliar fungicides in soybean seldom result in profitable yield increase.
White Mold Management
The white mold pathogen infects soybean through senescing flowers or petioles but infection is strongly correlated with the prevailing weather and field conditions. The favorable weather for white mold development is rain and a maximum temperature < 85° F at flowering. Field conditions that increase the risk for white mold include; history of white mold, narrow row spacing (< 20”), high soil fertility environment, and landscape (low spots/valley bottoms).
The challenge with managing white mold is that by the time symptoms are seen, it is too late to apply a rescue treatment. Therefore knowing the risk factors (especially field history of white mold and high yield potential/high fertility environment including use of animal manure) is important in deciding the need for a fungicide application for white mold management.
Fungicide Application Considerations
It should be noted that there are fungal diseases that can develop in soybeans at about the same time as white mold that cannot be managed by a foliar fungicide application. These include sudden death syndrome, charcoal rot, Phytophthora stem rot, and brown stem rot. It is important to know the field history of these diseases and to accurately diagnose them.
If a fungicide is necessary, there are a few on the market that are effective against white mold and other foliar fungal pathogens. Products containing boscalid, picoxystrobin, prothioconazole, or tetraconazole active ingredients are ‘good’ against white mold. For more information on fungicide efficacy on various soybean diseases, consult the Soybean Diseases Working Group table. For a list of fungicides registered on soybean in South Dakota, consult the 2016 Pest Management Guide: Soybean.
Source: Emmanuel Byamukama, South Dakota State University